Khronos Data Format Specification

Khronos Data Format Specification License Information

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Revision History
Revision 0.1 Jan 2015 AG
 Initial sharing 
Revision 0.2 Feb 2015 AG
 Added clarification, tables, examples 
Revision 0.3 Feb 2015 AG
 Further cleanup 
Revision 0.4 Apr 2015 AG
 Channel ordering standardized 
Revision 0.5 Apr 2015 AG
 Typos and clarification 
Revision 1.0 rev 1 May 2015 AG
 Submission for 1.0 release 
Revision 1.0 rev 2 Jun 2015 AG
 Clarifications for 1.0 release 
Revision 1.0 rev 3 Jul 2015 AG
 Added KHR_DF_SAMPLE_DATATYPE_LINEAR 
Revision 1.0 rev 4 Jul 2015 AG
 Clarified KHR_DF_SAMPLE_DATATYPE_LINEAR 
Revision 1.0 rev 5 Mar 2019 AG
 Clarification and typography 
Revision 1.1 rev 1 Nov 2015 AG
 Added definitions of compressed texture formats 
Revision 1.1 rev 2 Jan 2016 AG
 Added definitions of floating point formats 
Revision 1.1 rev 3 Feb 2016 AG
 Fixed typo in sRGB conversion (thank you, Tom Grim!) 
Revision 1.1 rev 4 Mar 2016 AG
 Fixed typo/clarified sRGB in ASTC, typographical improvements 
Revision 1.1 rev 5 Mar 2016 AG
 Switch to official Khronos logo, removed scripts, restored title 
Revision 1.1 rev 6 Jun 2016 AG
 ASTC block footprint note, fixed credits/changelog/contents 
Revision 1.1 rev 7 Sep 2016 AG
 ASTC multi-point part and quint decode typo fixes 
Revision 1.1 rev 8 Jun 2017 AG
 ETC2 legibility and table typo fix 
Revision 1.1 rev 9 Mar 2019 AG
 Typo fixes and much reformatting 
Revision 1.2 rev 0 Sep 2017 AG
 Added color conversion formulae and extra options 
Revision 1.2 rev 1 Mar 2019 AG
 Typo fixes and much reformatting 
Revision 1.3 Oct 2019 AG/MC
 Updates for KTX2/glTF. BC6h and ASTC table fixes and typo fixes. More examples. 

Table of Contents

1. Introduction
2. Formats and texel access
2.1. 1-D texel addressing
2.2. Simple 2-D texel addressing
2.3. More complex 2-D texel addressing
2.4. 3-dimensional texel addressing
2.5. Downsampled channels
3. The Khronos Data Format Descriptor overview
3.1. Texel blocks in the Khronos Data Format Descriptor
3.2. Planes in the Khronos Data Format Specification
3.3. Bit pattern interpretation and samples
3.4. Canonical representation
3.5. Related concepts outside the “format”
3.6. Translation to API-specific representations
4. Khronos Data Format Descriptor
4.1. Descriptor block
5. Khronos Basic Data Format Descriptor Block
5.1. vendorId
5.2. descriptorType
5.3. versionNumber
5.4. descriptorBlockSize
5.5. colorModel
5.6. colorModel for compressed formats
5.7. colorPrimaries
5.8. transferFunction
5.9. flags
5.10. texelBlockDimension[0..3]
5.11. bytesPlane[0..7]
5.12. Sample information
5.13. Sample bitOffset
5.14. Sample bitLength
5.15. Sample channelType and qualifiers
5.16. samplePosition[0..3]
5.17. sampleLower and sampleUpper
5.18. Paletted formats
5.19. Unsized formats
5.20. C99 struct mapping (informative)
6. Extension for more complex formats
7. Additional planes descriptor block
8. Additional dimensions descriptor block
9. Frequently Asked Questions
9.1. Why have a binary format rather than a human-readable one?
9.2. Why not use an existing representation such as those on FourCC.org?
9.3. Why have a descriptive format?
9.4. Why describe this standard within Khronos?
9.5. Why should I use this descriptor if I don’t need most of the fields?
9.6. Why not expand each field out to be integer for ease of decoding?
9.7. Can this descriptor be used for text content?
10. Floating-point formats
10.1. 16-bit floating-point numbers
10.2. Unsigned 11-bit floating-point numbers
10.3. Unsigned 10-bit floating-point numbers
10.4. Non-standard floating point formats
11. Example format descriptors
12. Introduction to color conversions
12.1. Color space composition
12.2. Operations in a color conversion
13. Transfer functions
13.1. About transfer functions (informative)
13.2. ITU transfer functions
13.3. sRGB transfer functions
13.4. BT.1886 transfer functions
13.5. BT.2100 HLG transfer functions
13.6. BT.2100 PQ transfer functions
13.7. DCI P3 transfer functions
13.8. Legacy NTSC transfer functions
13.9. Legacy PAL OETF
13.10. Legacy PAL 625-line EOTF
13.11. ST240/SMPTE240M transfer functions
13.12. Adobe RGB (1998) transfer functions
13.13. Sony S-Log transfer functions
13.14. Sony S-Log2 transfer functions
13.15. ACEScc transfer function
13.16. ACEScct transfer function
14. Color primaries
14.1. BT.709 color primaries
14.2. BT.601 625-line color primaries
14.3. BT.601 525-line color primaries
14.4. BT.2020 color primaries
14.5. NTSC 1953 color primaries
14.6. PAL 525-line analog color primaries
14.7. ACES color primaries
14.8. ACEScc color primaries
14.9. Display P3 color primaries
14.10. Adobe RGB (1998) color primaries
14.11. BT.709/BT.601 625-line primary conversion example
14.12. BT.709/BT.2020 primary conversion example
15. Color models
15.1. Y′CBCR color model
15.2. Y′CC′BCC′CR constant luminance color model
15.3. ICTCP constant intensity color model
16. Quantization schemes
16.1. “Narrow range” encoding
16.2. “Full range” encoding
16.3. Legacy “full range” encoding.
17. Compressed Texture Image Formats
17.1. Terminology
18. S3TC Compressed Texture Image Formats
18.1. BC1 with no alpha
18.2. BC1 with alpha
18.3. BC2
18.4. BC3
19. RGTC Compressed Texture Image Formats
19.1. BC4 unsigned
19.2. BC4 signed
19.3. BC5 unsigned
19.4. BC5 signed
20. BPTC Compressed Texture Image Formats
20.1. BC7
20.2. BC6H
21. ETC1 Compressed Texture Image Formats
21.1. ETC1S
22. ETC2 Compressed Texture Image Formats
22.1. Format RGB ETC2
22.2. Format RGB ETC2 with sRGB encoding
22.3. Format RGBA ETC2
22.4. Format RGBA ETC2 with sRGB encoding
22.5. Format Unsigned R11 EAC
22.6. Format Unsigned RG11 EAC
22.7. Format Signed R11 EAC
22.8. Format Signed RG11 EAC
22.9. Format RGB ETC2 with punchthrough alpha
22.10. Format RGB ETC2 with punchthrough alpha and sRGB encoding
23. ASTC Compressed Texture Image Formats
23.1. What is ASTC?
23.2. Design Goals
23.3. Basic Concepts
23.4. Block Encoding
23.5. LDR and HDR Modes
23.6. Configuration Summary
23.7. Decode Procedure
23.8. Block Determination and Bit Rates
23.9. Block Layout
23.10. Block mode
23.11. Color Endpoint Mode
23.12. Integer Sequence Encoding
23.13. Endpoint Unquantization
23.14. LDR Endpoint Decoding
23.15. HDR Endpoint Decoding
23.16. Weight Decoding
23.17. Weight Unquantization
23.18. Weight Infill
23.19. Weight Application
23.20. Dual-Plane Decoding
23.21. Partition Pattern Generation
23.22. Data Size Determination
23.23. Void-Extent Blocks
23.24. Illegal Encodings
23.25. LDR PROFILE SUPPORT
23.26. HDR PROFILE SUPPORT
24. PVRTC Compressed Texture Image Formats
24.1. PVRTC Overview
24.2. Format PVRTC1 4bpp
24.3. Format PVRTC1 2bpp
24.4. Format PVRTC2 4bpp
24.5. Format PVRTC2 2bpp
25. External references
26. Contributors

Abstract

This document describes a data format specification for non-opaque (user-visible) representations of user data to be used by, and shared between, Khronos standards. The intent of this specification is to avoid replication of incompatible format descriptions between standards and to provide a definitive mechanism for describing data that avoids excluding useful information that may be ignored by other standards. Other APIs are expected to map internal formats to this standard scheme, allowing formats to be shared and compared. This document also acts as a reference for the memory layout of a number of common compressed texture formats, and describes conversion between a number of common color spaces.

1. Introduction

Many APIs operate on bulk data — buffers, images, volumes, etc. — each composed of many elements with a fixed and often simple representation. Frequently, multiple alternative representations of data are supported: vertices can be represented with different numbers of dimensions, textures may have different bit depths and channel orders, and so on. Sometimes the representation of the data is highly specific to the application, but there are many types of data that are common to multiple APIs — and these can reasonably be described in a portable manner. In this standard, the term data format describes the representation of data.

It is typical for each API to define its own enumeration of the data formats on which it can operate. This causes a problem when multiple APIs are in use: the representations are likely to be incompatible, even where the capabilities intersect. When additional format-specific capabilities are added to an API which was designed without them, the description of the data representation often becomes inconsistent and disjoint. Concepts that are unimportant to the core design of an API may be represented simplistically or inaccurately, which can be a problem as the API is enhanced or when data is shared.

Some APIs do not have a strict definition of how to interpret their data. For example, a rendering API may treat all color channels of a texture identically, leaving the interpretation of each channel to the user’s choice of convention. This may be true even if color channels are given names that are associated with actual colors — in some APIs, nothing stops the user from storing the blue quantity in the red channel and the red quantity in the blue channel. Without enforcing a single data interpretation on such APIs, it is nonetheless often useful to offer a clear definition of the color interpretation convention that is in force, both for code maintenance and for communication with external APIs which do have a defined interpretation. Should the user wish to use an unconventional interpretation of the data, an appropriate descriptor can be defined that is specific to this choice, in order to simplify automated interpretation of the chosen representation and to provide concise documentation.

Where multiple APIs are in use, relying on an API-specific representation as an intermediary can cause loss of important information. For example, a camera API may associate color space information with a captured image, and a printer API may be able to operate with that color space, but if the data is passed through an intermediate compute API for processing and that API has no concept of a color space, the useful information may be discarded.

The intent of this standard is to provide a common, consistent, machine-readable way to describe those data formats which are amenable to non-proprietary representation. This standard provides a portable means of storing the most common descriptive information associated with data formats, and an extension mechanism that can be used when this common functionality must be supplemented.

While this standard is intended to support the description of many kinds of data, the most common class of bulk data used in Khronos standards represents color information. For this reason, the range of standard color representations used in Khronos standards is diverse, and a significant portion of this specification is devoted to color formats.

Later sections describe some of the common color space conversion operations and provide a description of the memory layout of a number of common texture compression formats.

2. Formats and texel access

This document describes a standard layout for a data structure that can be used to define the representation of simple, portable, bulk data. Using such a data structure has the following benefits:

  • Ensuring a precise description of the portable data
  • Simplifying the writing of generic functionality that acts on many types of data
  • Offering portability of data between APIs

The “bulk data” may be, for example:

  • Pixel/texel data
  • Vertex data
  • A buffer of simple type

The layout of proprietary data structures is beyond the remit of this specification, but the large number of ways to describe colors, vertices and other repeated data makes standardization useful. The widest variety of standard representations and the most common expected use of this API is to describe pixels or texels; as such the terms “texel” and “pixel” are used interchangeably in this specification when referring to elements of data, without intending to imply a restriction in use.

The data structure in this specification describes the elements in the bulk data in memory, not the layout of the whole. For example, it may describe the size, location and interpretation of color channels within a pixel, but is not responsible for determining the mapping between spatial coordinates and the location of pixels in memory. That is, two textures which share the same pixel layout can share the same descriptor as defined in this specification, but may have different sizes, line or plane strides, tiling or dimensionality; in common parlance, two images that describe (for example) color data in the same way but which are of different shapes or sizes are still described as having the same “format”.

An example pixel representation is described in Figure 1: a single 5:6:5-bit pixel composed of a blue channel in the low 5 bits, a green channel in the next 6 bits, and red channel in the top 5 bits of a 16-bit word as laid out in memory on a little-endian machine (see Table 89).

Figure 1. A simple one-texel texel block

images/565pixels.svg

2.1. 1-D texel addressing

In bulk data, each element is interpreted first by addressing it in some form, then by interpreting the addressed values. Texels often represent a color (or other data) as a multi-dimensional set of values, each representing a channel. The bulk-data image or buffer then describes a number of these texels. Taking the simplest case of an array in the C programming language as an example, a developer might define the following structure to represent a color texel:

typedef struct _MyRGB {
  unsigned char red;
  unsigned char green;
  unsigned char blue;
} MyRGB;

MyRGB *myRGBarray = (MyRGB *) malloc(100 * sizeof(MyRGB));

To determine the location of, for example, the tenth element of myRGBarray, the compiler needs to know the base address of the array and sizeof myRGB. Extracting the red, green and blue components of myRGBarray[9] given its base address is, in a sense, orthogonal to finding the base address of myRGBarray[9].

Note also that sizeof(MyRGB) will often exceed the total size of red, green and blue due to padding; the difference in address between one MyRGB and the next can be described as the pixel stride in bytes.

Figure 2. (Trivial) 1D address offsets for 1-byte elements, start of buffer

images/1DByteOffset.svg

Figure 3. 1D address offsets for 2-byte elements, start of buffer

images/1DWordOffset.svg

Figure 4. 1D address offsets for R,G,B elements (padding in gray), start of buffer

images/1DRGBOffset.svg

An alternative representation is a “structure of arrays”, distinct from the “array of structures” myRGBarray:

typedef struct _MyRGBSoA {
  unsigned char *red;
  unsigned char *green;
  unsigned char *blue;
} MyRGBSoA;

MyRGBSoA myRGBSoA;
myRGBSoA.red = (unsigned char *) malloc(100);
myRGBSoA.green = (unsigned char *) malloc(100);
myRGBSoA.blue = (unsigned char *) malloc(100);

In this case, accessing a value requires the sizeof each channel element. The best approach depends on the operations performed: calculations on one whole MyRGB a time likely favor MyRGB, those processing multiple values from a single channel may prefer MyRGBSoA. A “pixel” need not fill an entire byte — nor need pixel stride be a whole number of bytes. For example, a C++ std::vector<bool> can be considered to be a 1-D bulk data structure of individual bits.

2.2. Simple 2-D texel addressing

The simplest way to represent two-dimensional data is in consecutive rows, each representing a one-dimensional array — as with a 2-D array in C. There may be padding after each row to achieve the required alignment: in some cases each row should begin at the start of a cache line, or rows may be deliberately offset to different cache lines to ensure that vertically-adjacent values can be cached concurrently. The offset from the start of one horizontal row to the next is a line stride or row stride (or just stride for short), and is necessarily at least the width of the row. If each row holds an whole number of pixels, row stride can be described either in bytes or pixels; it is rare not to start each row on a byte boundary. In a simple 2-D representation, the row stride and the offset from the start of the storage can be described as follows:

\begin{align*} \textit{row stride}_\textit{pixels} &= \textit{width}_\textit{pixels} + \textit{padding}_\textit{pixels} \\ \textit{row stride}_\textit{bytes} &= \textit{width}_\textit{pixels} \times \textit{pixel stride}_\textit{bytes} + \textit{padding}_\textit{bytes} \\ \textit{offset}_\textit{pixels} &= x + (y \times \textit{rowstride}_\textit{pixels}) \\ \textit{address}_\textit{bytes} &= \textit{base} + (x \times \textit{pixel stride}_\textit{bytes}) + (y \times \textit{row stride}_\textit{bytes}) \end{align*}

Figure 5 shows example coordinate byte offsets for a 13×4 buffer, padding row stride to a multiple of four elements.

Figure 5. 2D linear texel offsets from coordinates

images/2DLinearOffsets.svg

Table 1. Order of byte storage in memory for coordinates in a linear 5×3 buffer, padded (italics) to 8×3

Byte

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

Coords

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

0,1

1,1

2,1

3,1

4,1

5,1

6,1

7,1

0,2

1,2

2,2

3,2

4,2

5,2

6,2

7,2


Figure 6. 2D R,G,B byte offsets (padding in gray) from coordinates for a 4×4 image

images/2DRGBOffsets.svg

By convention, this “linear” layout is in y-major order (with the x axis changing most quickly). This does not necessarily imply that an API which accesses the 2D data will do so in this orientation, and there are image layouts which have x and y swapped. The direction of the axes (particularly y) in relation to the image orientation also varies between representations.

Each contiguous region described in this way can be considered as a plane of data. In the same way that 1-D data can be described in structure-of-arrays form, two-dimensional data can also be implemented as multiple planes.

Figure 7. 2D R,G,B plane offsets from coordinates for 8×4 texels

images/2DRGBplanes.svg

In early graphics, planes often contributed individual bits to the pixel. This allowed pixels of less than a byte, reducing bandwidth and storage; combining 1-bit planes allowed, for example, 64-color, 6-bit pixels. Storing six bits per pixel consecutively would make the number of memory bytes holding a pixel vary by alignment; a planar layout needs simpler hardware. This hardware could often process a subset of the planes — scrolling text may require blitting just one plane, and changes to the display controller start position for each plane independently provide cheap parallax effects.

In modern architectures, planes are typically byte-aligned and hold separate channels. One motivation is avoiding padding. Some common hardware can only access powers of two bytes for each pixel: for example, texels of three 1-byte channels may need padding to four bytes for alignment restrictions; separate planes of one byte per texel need no padding. Other benefits are supporting different downsampling of each channel (described below), and more efficient independent processing of each channel: many common operations (such as filtering and image compression) treat channels separately.

Note that, once all the data from any contributing channels have been combined, the interpretation of the resulting values is again orthogonal to addressing the data.

2.3. More complex 2-D texel addressing

In many applications, the simple “linear” layout of memory has disadvantages. The primary concern is that data which is vertically adjacent in two-dimensional space may be widely separated in memory address. Many computer graphics rendering operations involve accessing the frame buffer in an order not aligned with its x-axis; for example, traversing horizontally across the width of a textured triangle while writing to the frame buffer will result in a texture access pattern which depends on the orientation of the texture relative to the triangle and the triangle relative to the frame buffer. If each texel access processed a different cache line, the resulting performance would be heavily compromised. Modern GPUs process multiple texels in parallel, meaning that many nearby texels in different orientations may need to be accessed quickly. Additionally, texture filtering operations typically read a 2×2 quad of texels, inherently requiring access to texels which would be distant in memory in the linear representation.

One solution to this is to divide the image into smaller rectangular tiles of texels (of tw × th) rather than horizontal rows. The texel ordering within the tiles may be treated as though each tile were an independent, smaller image, and the order of tiles in memory may be as in the linear layout:

\begin{align*} \textit{If}\ a\ \%\ b &= a - \left(b\times\left\lfloor{a\over b}\right\rfloor\right), & \textit{texel offset} &= \left(x\ \%\ \textit{tw}\right) + \textit{tw}\times\left(y\ \%\ \textit{th}\right) + \left\lfloor{x\over\textit{tw}}\right\rfloor\times\textit{th}\times\textit{tw} + \left\lfloor{y\over\textit{th}}\right\rfloor\times\textit{th}\times\textit{line stride} \end{align*}

This approach can be implemented efficiently in hardware, so long as the tile dimensions are powers of two, by interleaving some bits from the y-coordinate with the bits contributed by the x-coordinate. If twb = log2(tw) and thb = log2(th):

pixelOffset = (x & ((1 << twb) - 1)) | ((y & ((1 << thb) - 1)) << twb)
            | ((x & ~((1 << twb) - 1)) << thb) | ((y & ~((1 << thb)-1)) * lineStride);

For example, if a linear 16×12 image calculates a pixel offset relative to the start of the buffer as:

pixelOffset = x | (y * 16);

a 16×12 image comprised of 4×4 tiles may calculate the pixel offset relative to the start of the buffer as:

pixelOffset = (x & 3) | ((y & 3) << 2) | ((x & ~3) << 2) | ((y & ~3) * 16);

Figure 8. 4×4 tiled order texel offsets from coordinates (consecutive values linked in red to show the pattern)

images/2DTile4x4.svg

Textures which have dimensions that are not a multiple of the tile size require padding.

For so long as the size of a tile fits into an on-chip cache and can be filled efficiently by the memory subsystem, this approach has the benefit that only one in $1\over n$ transitions between vertically-adjacent lines requires accessing outside a tile of height n. The larger the tile, the greater the probability that vertically-adjacent accesses fall inside the same scan line. However, horizontally-adjacent texels that cross a tile boundary are made more distant in memory the larger the tile size, and tile sizes which do not conveniently fit the cache and bus sizes of the memory subsystem have inefficiencies; thus the tile size is an architectural trade-off.

While any non-linear representation is typically referred to as “tiling”, some hardware implementations actually use a more complex layout in order to provide further locality of reference. One such scheme is Morton order:

Figure 9. Morton order texel offsets from coordinates (consecutive values linked in red)

images/2DMortonOffsets.svg

uint32_t mortonOffset(uint32_t x, uint32_t y,
                      uint32_t width, uint32_t height) {

    const uint32_t minDim = (width <= height) ? width : height;
    uint32_t offset = 0, shift = 0, mask;

    // Tests xy bounds AND that width and height != 0
    assert(x < width && y < height);

    // Smaller size must be a power of 2
    assert((minDim & (minDim - 1)) == 0);

    // Larger size must be a multiple of the smaller
    assert(width % minDim == 0 && height % minDim == 0);

    for (mask = 1; mask < minDim; mask <<= 1) {
        offset |= (((y & mask) << 1) | (x & mask)) << shift;
        shift++;
    }

    // At least one of width and height will have run out of most-significant bits
    offset |= ((x | y) >> shift) << (shift * 2);
    return offset;
}

Note that this particular implementation assumes that the smaller of width and height is a power of two, and the larger is a multiple of the smaller. A practical hardware implementation of the calculation is likely much more efficient than this C code would imply.

Another approach, with even more correlation between locality in x,y space and memory offset, is Hilbert curve order:

Figure 10. Hilbert curve order texel offsets from coordinates (consecutive values linked in red)

images/2DHilbertOffsets.svg

uint32_t h(uint32_t size, uint32_t x, uint32_t y) {
    const uint32_t z = x ^ y;
    uint32_t offset = 0;

    while (size >>= 1) { // Iterate in decreasing block size order
        // Accumulate preceding blocks of size * size
        offset += size * (((size & x) << 1) + (size & z));
        y = z ^ x; // Transpose x and y
        if (!(size & y)) x = (size & x) ? ~y : y; // Conditionally swap/mirror
    }
    return offset;
}

uint32_t hilbertOffset(uint32_t x, uint32_t y, uint32_t width, uint32_t height) {
    const uint32_t minDim = (width <= height) ? width : height;

    // Tests xy bounds AND that width and height != 0
    assert(x < width && y < height);

    // Smaller size must be a power of 2
    assert((minDim & (minDim - 1)) == 0);

    // Larger size must be a multiple of the smaller
    assert(width % minDim == 0 && height % minDim == 0);

    if (width < height) return (width * width) * (y / width) + h(width, y % width, x);
    else return (height * height) * (x / height) + h(height, x % height, y);
}

This code assumes that the smaller of width and height is a power of two, with the larger a multiple of the smaller.

Some implementations will mix these approaches — for example, having linear tiles arranged in Morton order, or Morton order texels within tiles which are themselves in linear order. Indeed, for non-square areas, the tiling, Morton and Hilbert orderings shown here can be considered as a linear row of square areas with edges of the shorter dimension.

In all these cases, the relationship between coordinates and the memory storage location of the buffer depends on the image size and, other than needing to know the amount of memory occupied by a single texel, is orthogonal to the “format”. Tiling schemes tend to be complemented by proprietary hardware that performs the coordinate-to-address mapping and depends on cache details, so many APIs expose only a linear layout to end-users, keeping the tiled representation opaque. The variety of possible mappings between coordinates and addresses mandates leaving the calculation to the application.

2.4. 3-dimensional texel addressing

These storage approaches can be extended to additional dimensions, for example treating a 3-D image as being made up of 2-D “planes”, which are distinct from the concept of storing channels or bits of data in separate planes. In a linear layout, 3-dimensional textures can be accessed as:

\begin{align*} \textit{address}_\textit{bytes} &= \textit{base} + (x \times \textit{pixel stride}_\textit{bytes}) + (y \times \textit{row stride}_\textit{bytes}) + (z \times \textit{plane stride}_\textit{bytes}) \end{align*}

Here, the plane stride is the offset between the start of one contiguous 2-D plane of data and the next. The plane stride is therefore height × row stride plus any additional padding.

Again, non-linear approaches can be used to increase the correlation between storage address and coordinates.

uint32_t tileOffset3D(uint32_t x, uint32_t y, uint32_t z,
                      uint32_t twb, uint32_t thb, uint32_t tdb,
                      uint32_t lineStride, uint32_t planeStride) {
  // twb = tile width bits (log2 of tile width)
  // thb = tile height bits (log2 of tile height)
  // tdb = tile depth bits (log2 of tile depth)
  return (x & ((1 << twb) - 1)) |
         ((y & ((1 << thb) - 1)) << twb) |
         ((z & ((1 << tdb) - 1)) << (twb + thb)) |
         ((x & ~((1 << twb) - 1)) << (thb + tdb)) |
         ((y & ~((1 << thb) - 1)) << tdb) * lineStride |
         ((z & ~((1 << tdb) - 1)) * planeStride);
}
uint32_t mortonOffset3D(uint32_t x, uint32_t y, uint32_t z,
                        uint32_t width, uint32_t height, uint32_t depth) {
    const uint32_t max = width | height | depth;
    uint32_t offset = 0, shift = 0, mask;
    for (mask = 1; max > mask; mask <<= 1) {
        if (width > mask)  offset |= (x & mask) << shift++;
        if (height > mask) offset |= (y & mask) << shift++;
        if (depth > mask)  offset |= (z & mask) << shift++;
        --shift;
    }
    return offset;
}

There are multiple 3D variations on the Hilbert curve; one such is this:

uint32_t hilbertOffset3D(uint32_t x, uint32_t y, uint32_t z, uint32_t size) {
    // "Harmonious" 3D Hilbert curve for cube of power-of-two edge "size":
    // http://herman.haverkort.net/recursive_tilings_and_space-filling_curves
    uint32_t offset = 0;
    while (size >>= 1) {
        uint32_t tx = (size & x) > 0, ty = (size & y) > 0, tz = (size & z) > 0;
        switch (tx + 2 * ty + 4 * tz) {
        case 0: tx =  z; ty =  y; tz =  x; break;
        case 1: tx =  x; ty =  z; tz =  y; offset += size * size * size; break;
        case 2: tx = ~y; ty = ~x; tz =  z; offset += size * size * size * 3; break;
        case 3: tx =  z; ty =  x; tz =  y; offset += size * size * size * 2; break;
        case 4: tx = ~z; ty =  y; tz = ~x; offset += size * size * size * 7; break;
        case 5: tx =  x; ty = ~z; tz = ~y; offset += size * size * size * 6; break;
        case 6: tx = ~y; ty = ~x; tz =  z; offset += size * size * size * 4; break;
        case 7: tx = ~z; ty =  x; tz = ~y; offset += size * size * size * 5; break;
        }
        x = tx; y = ty; z = tz;
    }
    return offset;
}

2.5. Downsampled channels

The examples provided so far have assumed that a unique value from each color channel is present at each access coordinate. However, some common representations break this assumption.

One reason for this variation comes from representing the image in the Y′CBCR color model, described in Section 15.1; in this description, the Y′ channel represents the intensity of light, with CB and CR channels describing how the color differs from a gray value of the same intensity in the blue and red axes. Since the human eye is more sensitive to high spatial frequencies in brightness than in the hue of a color, the CB and CR channels can be recorded at lower spatial resolution than the Y′ channel with little loss in visual quality, saving storage space and bandwidth.

In one common representation known colloquially as YUV 4:2:2, each horizontal pair of Y′ values has a single CB and CR value shared for the pair. For example, Figure 11 shows a 6-element 1-D buffer with one Y′ value for each element, but as shown in Figure 12 the CB and CR values are shared across pairs of elements.

Figure 11. 1-D Y′CBCR 4:2:2 buffer, texels associated with Y′ values

images/1DYCbCr_Y.svg

Figure 12. 1-D Y′CBCR 4:2:2 buffer, texels associated with CB and CR values

images/1DYCbCr_CbCr.svg

In this case, we can say that the 2×1 coordinate region forms a texel block which contains two Y′ values, one CB value and one CR value; our bulk-data buffer or image is composed of a repeating pattern of these texel blocks.

Note that this representation raises a question: while we have assumed so far that accessing a value at texel coordinates will provide the value contained in the texel, how should the shared CB and CR values relate to the coordinates of the Y′ channel? Each texel coordinate represents a value in the coordinate space of the image or buffer, which can be considered as a sample of the value of a continuous surface at that location. The preferred means of reconstructing this surface is left to the application: since this specification only defines the values in the image and not how they are used, it is concerned with the sample values rather than the reconstruction algorithm or means of access. For example, in graphics APIs the coordinates used to access a 2-D texture may offset the sample locations of a texture by half a coordinate relative to the origin of sample space, and filtering between samples is typically used to implement antialiasing. However, to interpret the data correctly, any application still needs to know the theoretical location associated with the samples, so that information is within the remit of this specification.

Our Y′ samples should fall naturally on our native coordinates. However, the CB and CR sample locations (which are typically at the same location as each other) could be considered as located coincident with one or other of the Y′ values as shown in Figure 13, or could be defined as falling at the mid-point between them as in Figure 14. Different representations have chosen either of these alternatives — in some cases, choosing a different option for each coordinate axis. The application can choose how to treat these sample locations: in some cases it may suffice to duplicate CB and CR across the pair of Y′ values, in others bilinear or bicubic filtering may be more appropriate.

Figure 13. 1-D Y′CBCR 4:2:2 buffer, CB and CR cosited with even Y′ values

images/1DYCbCr_cosited.svg

Figure 14. 1-D Y′CBCR 4:2:2 buffer, CB and CR midpoint between Y′ values

images/1DYCbCr_midpoint.svg

Traditional APIs have described the CB and CR as having 2×1 downsampling in this format (there are half as many samples available in the horizontal axis for these channels).

This concept can be extended to more dimensions. Commonly, a two-dimensional image stored in Y′CBCR format may store the CB and CR channels downsampled by a factor of two in each dimension (“2×2 downsampling”, also known for historical reasons as “4:2:0”). This approach is used in, for example, many JPEG images and MPEG video streams.

Because there are twice as many rows of Y′ data as there are CB and CR data, it is convenient to record the channels as separate planes as shown in Figure 15 (with 2×2 texel blocks outlined in red); in addition, image compression schemes often work with the channels independently, which is amenable to planar storage.

Figure 15. 2-D Y′CBCR 4:2:0 planes, downsampled CB and CR at midpoint between Y′ values

images/2DYCbCr420_traditional.svg

In this case, the CB and CR planes are half the width and half the height of the Y′ plane, and also have half the line stride. Therefore if we store one byte per channel, the offsets for each plane in a linear representation can be calculated as:

\begin{align*} \textit{Y}'_\textit{address} &= \textit{Y}'_\textit{base} + x + (y \times \textit{row stride}_\textit{bytes}) \\ \textit{C}_\textit{B address} &= \textit{C}_\textit{B base} + \left\lfloor{x\over 2}\right\rfloor + \left(\left\lfloor{y\over 2}\right\rfloor\times {{\textit{row stride}_\textit{bytes}}\over 2}\right) \\ \textit{C}_\textit{R address} &= \textit{C}_\textit{R base} + \left\lfloor{x\over 2}\right\rfloor + \left(\left\lfloor{y\over 2}\right\rfloor\times {{\textit{row stride}_\textit{bytes}}\over 2}\right) \end{align*}

A description based on downsampling factors is sufficient in the case of common Y′CBCR layouts, but does not extend conveniently to all representations. For example, one common representation used in camera sensors is a Bayer pattern, in which there is only one of the red, green and blue channels at each sample location: one red and blue value per 2×2 texel block, and two green values offset diagonally, as in Figure 16.

Figure 16. An 18×12 RG/GB Bayer filter pattern (repeating pattern outlined in yellow)

images/Bayer18x12.svg

A Bayer pattern can then be used to sample an image, as shown in Figure 17, and this sampled version can later be used to reconstruct the original image by relying on heuristic correlations between the channels. Technology for image processing continues to develop, so in many cases it is valuable to record the “raw” sensor data for later processing, and to pass the raw data unmodified to a range of algorithms; the choice of algorithm for reconstructing an image from samples is beyond the remit of this specification.

Figure 17. A Bayer-sampled image with a repeating 2×2 RG/GB texel block (outlined in yellow)

images/Bayer.svg

In the Bayer representation, the red and blue channels can be considered to be downsampled by a factor of two in each dimension. The two green channels per 2×2 repeating block mean that the “downsampling factor” for the green channel is effectively $\sqrt{2}$ in each direction.

More complex layouts are not uncommon. For example, the X-Trans sample arrangement developed by Fujifilm for their digital camera range, shown in Figure 18, consists of 6×6 texel blocks, with each sample, as in Bayer, corresponding to a single channel. In X-Trans, each block contains eight red, eight blue and twenty green samples; red and blue are “downsampled” by $\sqrt{3\over 2}$ and green is “downsampled” by $3\over\sqrt{5}$ .

Figure 18. An 18×12 X-Trans image (repeating 6×6 pattern outlined in yellow)

images/X-Trans18x12.svg

Allowing for possible alternative orientations of the samples (such as whether the Bayer pattern starts with a row containing red or blue samples, and whether the first sample is green or red/blue), trying to encode these sample patterns implicitly is difficult.

3. The Khronos Data Format Descriptor overview

The data structure defined in this specification is termed a data format descriptor. This is an extensible block of contiguous memory, with a defined layout. The size of the data format descriptor depends on its content, but is also stored in a field at the start of the descriptor, making it possible to copy the data structure without needing to interpret all possible contents.

The data format descriptor is divided into one or more descriptor blocks, each also consisting of contiguous data, as shown in Table 2. These descriptor blocks may, themselves, be of different sizes, depending on the data contained within. The size of a descriptor block is stored as part of its data structure, allowing an application to process a data format descriptor while skipping contained descriptor blocks that it does not need to understand. The data format descriptor mechanism is extensible by the addition of new descriptor blocks.

Table 2. Data format descriptor and descriptor blocks

Data format descriptor

 Descriptor block 1

 Descriptor block 2

 :


The diversity of possible data makes a concise description that can support every possible format impractical. This document describes one type of descriptor block, a basic descriptor block, that is expected to be the first descriptor block inside the data format descriptor where present, and which is sufficient for a large number of common formats, particularly for pixels. Formats which cannot be described within this scheme can use additional descriptor blocks of other types as necessary.

Later sections of this specification describe several common color spaces, and provide a description of the in-memory representation of a number of common compressed texture formats.

3.1. Texel blocks in the Khronos Data Format Descriptor

Instead of an implicit representation, the descriptor specifies a texel block, which is described in terms of its finite extent in each of four dimensions, and which identifies the location of each sample within that region. The texel block is therefore a self-contained, repeating, axis-aligned pattern in the coordinate space of the image. This description conveniently corresponds to the concept of compressed texel blocks used in common block-based texture compression schemes, which similarly divide the image into (typically) small rectangular regions which are encoded collectively. The bounds of the texel block are chosen to be aligned to integer coordinates.

Although it is most common to consider one- and two-dimensional textures and texel blocks, it can be convenient to record additional dimensions; for example, the ASTC compressed texture format described in Section 23 can support compressed texel blocks with three dimensions. For convenience of encoding, this specification uses a four-dimensional space to define sample locations within a texel block — there is no meaning imposed on how those dimensions should be interpreted.

In many formats, all color channels refer to the same location, and the texel block defines a 1×1×1×1 region — that is, a single texel.

[Note]

Tiling schemes for texel addressing can also be seen to break the image into rectangular (or higher-dimensional) sub-regions, and in many schemes these sub-regions are repeating and axis-aligned. Within reason, it is possible to define some coordinate-to-texel mapping in terms of a texel block; for example, instead of a simple tiled layout of 4×4 texels, it would be possible to describe the image in terms of a linear pattern of texel blocks, each of which contain 4×4 samples. In general, this is not a useful approach: it is very verbose to list each sample individually, it does not extend well to larger block sizes (or to infinite ranges in approaches such as Morton order), it does not handle special cases well (such as the “tail” of a mip chain), and encodings such as Hilbert order do not have a repeating mapping. In most contexts where these concepts exist, tiling is not considered to be part of a “format”.

3.2. Planes in the Khronos Data Format Specification

The description above has shown that the data contributing to a texel may be stored in separate locations in memory; for example, R, G and B stored in separate planes may need to be combined to produce a single texel. For the purposes of this specification, a plane is defined to be a contiguous region of bytes in memory that contributes to a single texel block.

This interpretation contradicts the traditional interpretation of downsampled channels: if two rows of Y′ data correspond to a single row of CB and CR (in a linear, non-tiled memory layout), the Y′ channel contribution to the texel block is not “a contiguous region of bytes”. Instead, each row of Y′ contributing to a texel block can be treated as a separate “plane”.

In linear layout, this can be represented by offsetting rows of Y′ data with odd y coordinates by the row stride of the original Y′ plane; each new Y′ plane’s stride is then double that of the original Y′ plane, as in Figure 19 (c.f. Figure 15). If the planes of a 6×4 Y′CBCR 4:2:0 texture are stored consecutively in memory with no padding, which might be described in a traditional API as Table 3, the representation used by this specification would be that shown in Table 4.

Table 3. Plane descriptors of a 6×4 Y′CBCR-format buffer in a conventional API

Plane Byte offset Byte stride Downsample factor

Y′

0

6

1×1

CB

24

3

2×2

CR

30

3

2×2


Table 4. Plane descriptors of a 6×4 Y′CBCR-format buffer using this standard

Plane Byte offset Byte stride Bytes per plane

Y′ plane 1

0

12

2

Y′ plane 2

6

12

2

CB

24

3

1

CR

30

3

1


[Note]

There is no expectation that an API must actually use this representation in accessing the data: it is simple for an API with explicit support for the special case of integer chroma downsampling to detect interleaved planes and to deduce that they should be treated as a single plane of double vertical resolution. Many APIs will not support the full flexibility of formats supported by this specification, and will map to a more restrictive internal representation.

The Khronos Basic Descriptor Block indicates a number of bytes contributing to the texel block from each of up to eight planes — if more than eight planes are required, an extension is needed. Eight planes are enough to encode, for example:

  • 8-bit data stored as individual bit planes
  • Stereo planar R, G, B, A data
  • 4× vertically-downsampled Y′CBCR data, as might be produced by rotating a (relatively common) 4× horizontally-downsampled Y′CBCR (4:1:1) video frame
  • A 2×2×2×2 4-D texel block in a linear layout
  • Interlaced Y′CBCR 4:2:0 data with each field stored separately

If a plane contributes less than a byte to a texel (or a non-integer number of bytes), the texel block size should be expanded to cover more texels until a whole number of bytes are covered — q.v. Table 92.

[Note]

Interlaced Y′CBCR data may associate chroma channels with Y′ samples only from the same field, not the frame as a whole. This distinction is not made explicit, in part because a number of interlacing schemes exist. One suggested convention for interlaced data is that the field of a sample be encoded in the fourth sample coordinate (the first field as samplePosition3 = 0, the second field as samplePosition3 = 1, etc.) This interpretation is not mandated to allow other reasons for encoding four-dimensional texels, although it is consistent with the fourth dimension representing “time”.

Figure 19. 2-D Y′CBCR 4:2:0 planes, with separate even and odd Y′ planes

images/2DYCbCr420_KDFS.svg

3.3. Bit pattern interpretation and samples

For the purposes of description, the bytes contributed by each plane are considered to be concatenated into a single contiguous logical bit stream. This “concatenation” of bits is purely conceptual for the purposes of determining the interpretation of the bits that contribute to the texel block, and is not expected to be the way that actual decoders would process the data.

That is, if planes zero and one contribute two bytes of data each and planes two and three contribute one byte of data each, this bit stream would consist of the two bytes from plane zero followed by the two bytes from plane one, then the byte from plane two, then the byte from plane three.

The data format descriptor then describes a series of samples that are contained within the texel block. Each sample represents a series of contiguous bits from the bit stream, interpreted in little-endian order, and is associated with a single channel and four-dimensional coordinate offset within the texel block; in formats for which only a single texel is being described, this coordinate offset will always be 0,0,0,0.

The descriptor block for a Y′CBCR 4:2:0 layout is shown in Table 96. Figure 20 shows this representation graphically: the three disjoint regions for each channels and the texel block covering them are shown on the left, represented in this specification as four planes of contiguous bytes. These are concatenated into a 48-bit logical bit stream (shown in blue, in top-to-bottom order); these samples then describe the contributions from these logical bits, with geometric location of the sample at the right.

Figure 20. Y′CBCR 4:2:0 described in six samples

images/2DYCbCrKDFSEncoding.svg

Consecutive samples with the same channel, location and flags may describe additional bits that contribute to the same final value, and appear in increasing order. For example, a 16-bit big-endian word in memory can be described by one sample describing bits 0..7, coming from the higher byte address, followed by one sample describing bits 8..15, coming from the lower address. These samples comprise a single virtual sample, of 16 bits.

Figure 21 shows how multiple contributions to single value can be used to represent a channel which, due to a big-endian native word type (high bits stored in lower byte addresses), is not contiguous in a little-endian representation (low bits stored in lower byte addresses). Here, channels are comprised of consecutive bits from a 16-bit big-endian word; the bits of each channel cease to be contiguous when the memory bytes are interpreted in little-endian order for the logical bit stream. The bit contributions to each channel from each bit location are shown in superscript, as later in this specification; the channel contributions start at bit 0 with the first sample contributing to the value, and are deduced implicitly from the number of bits each sample contributes. This example assumes that we are describing a single texel, with the same 0,0,0,0 coordinates (not shown) for each sample. The descriptor block for this format is shown in Table 97.

Figure 21. RGB 565 big-endian encoding

images/RGB565BEKDFSEncoding.svg

In Figure 21, bit 0 of the logical bit stream corresponds to bit 3 of the virtual sample that describes the green channel; therefore the first channel to be encoded is green. Bits 0 to 2 of the green channel virtual sample correspond to bits 13..15 of the logical bit stream, so these are described by the first sample. The second sample continues describing bits 3..5 of the green channel virtual sample, in bits 0..2 of the logical bit stream. The next bit from the logical bit stream that has not yet been described is bit 3, which corresponds to bit 0 of the red channel. The third sample is therefore used to describe bits 3..7 of the logical bit stream as a 5-bit red channel. The last sample encodes the remaining 5-bit blue channel that forms bits 8..12 of the logical bit stream. Note that in the case where some bits of the data are ignored, they do not need to be covered by samples; bits may also appear repeatedly if they contribute to multiple samples.

The precision to which sample coordinates are specified depends on the size of the texel block: coordinates in a 256×256 texel block can be specified only to integer-coordinate granularity, but offsets within a texel block that is only a single coordinate wide are specified to the precision of $1\over 256$ of a coordinate; this approach allows large texel blocks, half-texel offsets for representations such as Y′CBCR 4:2:0, and precise fractional offsets for recording multisampled pattern locations.

The sequence of bits in the (virtual) sample defines a numerical range which may be interpreted as a fixed-point or floating-point value and signed or unsigned. Many common representations specify this range. For example, 8-bit RGB data may be interpreted in “unsigned normalized” form as meaning “0.0” (zero color contribution) when the channel bits are 0 and “1.0” (maximum color contribution) when the channel is 255. In Y′CBCR “narrow-range” encoding, there is head-room and foot-room to the encoding: “black” is encoded in the Y′ channel as the value 16, and “white” is encoded as 235, as shown in Section 16.1 encoding”. To allow the bit pattern of simply-encoded numerical encodings to be mapped to the desired values, each sample has an upper and lower value associated with it, usually representing 1.0 and either 0.0 or -1.0 depending on whether the sample is signed.

Note that it is typically part of the “format” to indicate the numbers which are being encoded; how the application chooses to process these numbers is not part of the “format”. For example, some APIs may have two separate “formats”, in which the 8-bit pattern 0x01 may be interpreted as either the float value 1.0 or the integer value 1; for the purposes of this specification, these formats are identical — “1” means “1”.

A sample has an associated color channel in the color model of the descriptor block — for example, if the descriptor block indicates an RGB color model, the sample’s channel field could specify the R channel. The descriptor block enumerates a range of common color models, color primary values and transfer functions, which apply to the samples.

3.4. Canonical representation

There is some redundancy in the data format descriptor when it comes to the ordering of planes and samples. In the interests of simplifying comparison between formats, it is helpful to define a canonical ordering.

  • Planes should be described first in order of the first channel with which they are associated, then in increasing raster order of coordinates, then in increasing bit order (in a little-endian interpretation). For example:

    • In the Y′CBCR 4:2:0 format described above, the Y′ planes should come before the CB and CR planes in that order, because of the channel order.
    • The Y′ plane corresponding to even y addresses should come before the Y′ plane corresponding to odd y coordinates, because row 0 is even.
    • Planes should be ordered such that sample values that are split across multiple planes should be ordered in increasing order —  e.g. in an 8-bit format with one bit per plane, planes 0 through 7 should encode bits 0 through 7 in that order (thereby minimizing the number of samples required to describe the value).

The order of samples should be defined by the following rules:

  • Samples sharing a channel and location appear consecutively in increasing order of bits contributed to the virtual sample.

    • For example, a big-endian 16-bit red channel at location 0,0 may be composed of two samples: one referencing the eight consecutive virtual bit stream bits from 0..7 (bitOffset = 0, bitLength = 8) and the other referencing the eight consecutive virtual bit stream bits from 8..15 (bitOffset = 8, bitLength = 8). Since the least-significant 8 bits of the virtual red value come from the sample that references bits 8..15 of the virtual bit stream, this sample should appear first, immediately followed by the sample that references bits 0..8, which define the most-significant 8 bits of the virtual value. Table 102 shows a similar virtual sample, split across three samples of contiguous bits.
  • A minimum number of samples describes each sequence of contiguous virtual bit stream bits in a virtual sample value.

    • If an additional sample is required to represent the sample’s sampleLower and sampleUpper values because more than 32 bits are encoded and the existing extension rules for sampleLower and sampleUpper do not result in the desired behavior, the earlier sample(s) should be limited to 32 bits, ensuring that the subsequent sample holds at least one bit (bitLength is greater than or equal to 0).
    • Otherwise, if the contiguous sequence of bits from the virtual bit stream is longer than 256 bits, samples should be concatenated such that only the last sample describes fewer than 256 bits.
  • Samples that qualify an existing virtual sample immediately follow it.

    • Specifically, in an explicitly-described floating-point format, any sample describing a sign bit immediately follows the unsigned mantissa, and any exponent follows thereafter, as seen in Table 103.
  • Virtual samples are described in increasing order of the virtual bit stream bits to which they apply.

    • This means that if bit 0 of the virtual bit stream is part of a virtual sample, that virtual sample should be described first; this does not require that the first sample directly describes bit 0, as in the green channel of Table 97.
  • If the same bit in the virtual bit stream in increasing bit order is the first to be associated with more than one virtual sample, virtual samples are listed in channel number order.

    • For example, an alpha channel may be encoded in bits 0..7 of the virtual bits stream, followed by red, green and blue channels sharing bits 8..15. The sample describing alpha should be listed first, since it uniquely describes bit 0 of the virtual bits stream. Bit 8 of the virtual bit stream is the first bit of the red, green and blue, and since red, green and blue are channel numbers 0, 1 and 2, the corresponding samples are described in this order. See, for example, Table 99 and Table 101.
  • Virtual samples that share bits and channel number but not location would be extremely unusual, but would appear in increasing raster order of the location (that is, sorted first by coordinate three, then two, then one, then zero).

Finally:

  • Fields which are irrelevant (for example, the alpha behavior for a format with no alpha channel) should be set to zero.
  • Samples describing a channel of a single bit should have a linear transfer function — either by selecting the transfer function KHR_DF_TRANSFER_LINEAR for the descriptor as a whole if all samples are linear, or by setting the sample’s KHR_DF_SAMPLE_DATATYPE_LINEAR bit if other samples have multiple bits and a non-linear transfer function is needed.

3.5. Related concepts outside the “format”

The data format descriptor describes only concepts which are conventionally part of the “format”. Additional information is required to access the image data:

  • A formula for mapping accessed coordinates to byte data for each channel.
  • This may be expected to require, for each channel, a start address and a stride in each dimension.
  • This transformation depends on the image size, and may be parameterized by the texel block dimensions and the number of bytes contributed by each plane.
  • (Optionally) for each dimension, a maximum (and arguably minimum) range.
  • Note that padding is independent of the “format”.

For example, if texels are laid out in memory in linear order:

int numPlanes;
char *planeBaseAddresses[8];
unsigned int strides[8][4];

// Note: The strides here are assumed to be in units of the
// corresponding dimension of the texel block
char *planeAddress(uint32_t descriptor, int plane, float coords[4]) {
  return planeBaseAddresses[plane]
    // Block stride
    + ((int) (coords[0] / descriptorSize(descriptor, 0))) * strides[plane][0]
    // Row stride
    + ((int) (coords[1] / descriptorSize(descriptor, 1))) * strides[plane][1]
    // Plane stride
    + ((int) (coords[2] / descriptorSize(descriptor, 2))) * strides[plane][2]
    // Volume(?) stride
    + ((int) (coords[3] / descriptorSize(descriptor, 3))) * strides[plane][3];
}

decodeTexelGivenCoords(uint32_t *descriptor, float coords[4]) {
  char *addresses[8];
  for (int i = 0; i < numPlanes; ++i) {
    addresses[i] = planeAddress(i, coords);
  }

  // processTexel reads bytesPlane[] bytes from each address and
  // decodes the concatenated data according to the descriptor
  processTexel(descriptor, addresses, coords);
}

The processTexel function would typically operate on the coordinates having taken the remainder of dividing them by the texel block size. For example, if the texel block size is 2×2, the block and row stride provide the offset in bytes between adjacent blocks in the first two dimensions. processTexel would then work with the data corresponding to the 2×2 region with coords[0] and coords[1] in the range [0..2). For formats that describe a single texel, coords can be considered to be an integer. Note that for more complex formats such as Bayer layouts, reconstructing an R,G,B value at a location may require information from more than one texel block, depending on the algorithm used, in a manner analogous to sampling using bilinear filtering.

The stride values may be stored explicitly, or derived implicitly from the bytesPlane values and image size information, with application-specific knowledge about alignment constraints.

3.6. Translation to API-specific representations

Despite being designed to balance size against flexibility, the data format container described here is too unwieldy to be expected to be used directly in most APIs, which will generally support only a subset of possible formats. The expectation is that APIs and users will define data descriptors in memory, but have API-specific names for the formats that the API supports. If these names are enumeration values, a mapping can be provided by having an array of pointers to the data descriptors, indexed by the enumeration. It may commonly be necessary to provide API-specific supplementary information in the same array structure, particularly where the API natively associates concepts with the data which is not uniquely associated with the content.

In this approach, it is likely that an API would predefine a number of common data formats which are natively supported. If there is a desire to support dynamic creation of data formats, this array could be made extensible with a manager assigning unique handles.

Even where an API supports only a fixed set of formats, it is flexible to provide a comparison with user-provided format descriptors in order to establish whether a format is compatible (and differs only in a manner irrelevant to the API).

Some APIs have the concept of a native data type for a format, if each channel is stored separately. Since this specification describes a number of bytes per plane and separately contiguous bit sequences, there is no such explicit concept. However, if a sample’s bitOffset and bitLength are byte-aligned and no further samples contribute to the same value, the bitLength trivially defines a little-endian native data type size. Big-endian data types can be identified by observing that in a big-endian format, a sequence of bits in the top bits of byte n may continue in the low bits of byte n - 1. Finally, “packed” formats consist of consecutive bit sequences per channel in either little- or big-endian order; little-endian sequences are a single stand-alone sample, and a big-endian sequence consists of a number of samples adjacent to byte boundaries in decreasing byte order (see Figure 21); the packed field size can typically be deduced from the bytesPlane0 value. There is no way to distinguish a logically “packed”, byte-aligned samples from distinct but consecutively-stored channels that have the same in-memory representation.

Glossary

Data format: The interpretation of individual elements in bulk data. Examples include the channel ordering and bit positions in pixel data or the configuration of samples in a Bayer image. The format describes the elements, not the bulk data itself: an image’s size, stride, tiling, dimensionality, border control modes, and image reconstruction filter are not part of the format and are the responsibility of the application.

Data format descriptor: A contiguous block of memory containing information about how data is represented, in accordance with this specification. A data format descriptor is a container, within which can be found one or more descriptor blocks. This specification does not define where or how the the data format descriptor should be stored, only its content. For example, the descriptor may be directly prepended to the bulk data, perhaps as part of a file format header, or the descriptor may be stored in a CPU memory while the bulk data that it describes resides within GPU memory; this choice is application-specific.

(Data format) descriptor block: A contiguous block of memory with a defined layout, held within a data format descriptor. Each descriptor block has a common header that allows applications to identify and skip descriptor blocks that it does not understand, while continuing to process any other descriptor blocks that may be held in the data format descriptor.

Basic (data format) descriptor block: The initial form of descriptor block as described in this standard. Where present, it must be the first descriptor block held in the data format descriptor. This descriptor block can describe a large number of common formats and may be the only type of descriptor block that many portable applications will need to support.

Texel block: The units described by the Basic Data Format Descriptor: a repeating element within bulk data. In simple texture formats, a texel block may describe a single pixel. In formats where the bytes of each plane do not correspond uniquely to single pixels, as for example with subsampled channels, the texel block may cover several pixels. In a block-based compressed texture, the texel block typically describes the compression block unit. The basic descriptor block supports texel blocks of up to four dimensions.

Plane: In the Basic Data Format Descriptor, a plane describes a contiguous sequence of bytes that contribute to the texel block. The basic format descriptor block defines a texel block as being made of a number of concatenated bits which may come from different regions of memory, where each region is considered a separate plane. For common formats, it is sufficient to require that the contribution from each plane is an integer number of bytes. This specification places no requirements on the ordering of planes in memory — the plane locations are described outside the format. This allows support for multiplanar formats which have proprietary padding requirements that are hard to accommodate in a more terse representation.

Sample: In this standard, texel blocks are considered to be composed of contiguous bit patterns with a single channel or component type and a single spatial location. A typical ARGB pixel has four samples, one for each channel, held at the same coordinate. A texel block from a Bayer sensor might have a different location for different channels, and may have multiple samples representing the same channel at multiple locations. A Y′CBCR buffer with downsampled chroma may have more luma samples than chroma, each at different locations.

4. Khronos Data Format Descriptor

The data format descriptor consists of a contiguous area of memory, as shown in Table 5, divided into one or more descriptor blocks, which are tagged by the type of descriptor that they contain. The size of the data format descriptor varies according to its content.

Table 5. Data Format Descriptor layout

uint32_t  

  totalSize

Descriptor block  

  First descriptor

Descriptor block  

  Second descriptor (optional) etc.


The totalSize field, measured in bytes, allows the full format descriptor to be copied without need for details of the descriptor to be interpreted. totalSize includes its own uint32_t, not just the following descriptor blocks. For example, we will see below that a four-sample Khronos Basic Data Format Descriptor Block occupies 88 bytes; if there are no other descriptor blocks in the data format descriptor, the totalSize field would then indicate 88 + 4 bytes (for the totalSize field itself) for a final value of 92.

For consistency of decode, each descriptor block should be aligned to a multiple of four bytes relative to the start of the descriptor; totalSize will therefore be a multiple of four.

[Note]

This restriction was not present in versions of the Khronos Data Format Specification prior to version 1.3.

The layout of the data structures described here are comprised solely of 32-bit words, and for canonical communication between devices are assumed to be stored with a little-endian representation. For efficiency, applications may choose to convert the descriptor to the native endianness of the underlying hardware where all software using the data structure is prepared for this conversion. Extensions which are composed of quantities other than 32-bit words (for example if a data structure belonging to another standard is incorporated directly) may define the expected impact of endianness changes on the layout. Since the environment is expected to know its own endianness, there is no explicit means of automatically determining the endianness of a descriptor, although it can be observed that it is highly unlikely that a valid descriptor would be large enough for its size to need to be represented in more than 16 bits — meaning that the endianness of most descriptors can be deduced by which half of the uint32_t totalSize field is non-zero.

[Note]

To avoid expanding the size of the data structure, there is no “magic identifier” for a data format descriptor: applications are expected to know the type of the data structure being accessed, and to provide their own means of identifying a data format descriptor if one is embedded in a multi-purpose byte stream.

4.1. Descriptor block

Each descriptor block has the same prefix, shown in Table 6.

Table 6. Descriptor Block layout

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

descriptorType

vendorId

descriptorBlockSize

versionNumber

Format-specific data


The vendorId is a 17-bit value uniquely assigned to organizations. If the organization has a 16-bit identifier assigned by the PCI SIG, this should occupy bits 0..15, with bit 16 set to 0. Other organizations should apply to Khronos of a unique identifier, which will be assigned consecutively starting with 65536. The identifier 0x1FFFF is reserved for internal use which is guaranteed not to clash with third-party implementations; this identifier should not be shipped in libraries to avoid conflicts with development code.

The descriptorType is a unique 15-bit identifier defined by the vendor to distinguish between potential data representations.

[Note]

Prior to version 1.3 of the Khronos Data Format Specification, the vendorId field was 16-bit, and purely assigned through the auspices of this specification; the descriptorType was consequently also 16-bit. Since no vendor has requested an identifier and Khronos does not have a descriptor block with type 1, this change should not cause any ambiguity. This change is intended to allow consistency with the vendor IDs used by the Vulkan specification.

The versionNumber is vendor-defined, and is intended to allow for backwards-compatible updates to existing descriptor blocks.

The DescriptorBlockSize indicates the size in bytes of this Descriptor Block, remembering that there may be multiple Descriptor Blocks within one container, as shown in Table 7. The descriptorBlockSize therefore gives the offset between the start of the current Descriptor Block and the start of the next — so the size includes the vendorId, descriptorType, versionNumber and descriptorBlockSize fields, which collectively contribute 8 bytes.

Having an explicit descriptorBlockSize allows implementations to skip a descriptor block whose format is unknown, allowing known data to be interpreted and unknown information to be ignored. Some descriptor block types may not be of a uniform size, and may vary according to the content within.

This specification initially describes only one type of stand-alone descriptor block, plus two extension blocks which modify the description in the first. Future revisions may define additional descriptor block types for additional applications — for example, to describe data with a large number of channels or pixels described in an arbitrary color space. Vendors can also implement proprietary descriptor blocks to hold vendor-specific information within the standard descriptor.

Unless otherwise specified, descriptor blocks can appear in any order, to make it easier to add and remove additional informative descriptor blocks to a preexisting data format descriptor as part of processing. Descriptor blocks that provide additional capabilities beyond a basic scheme (such as the descriptor block for supporting additional planes beyond the Khronos Basic Descriptor Block) should not be present unless their additional capabilities are needed; that is, redundancy should be resolved so as to minimize the number of descriptor blocks in the data format descriptor.

Table 7. Data format descriptor header and descriptor block headers for two descriptor blocks

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize

First descriptor block

descriptorType

vendorId

descriptorBlockSize

versionNumber

Descriptor body

Second descriptor block

descriptorType

vendorId

descriptorBlockSize

versionNumber

Descriptor body


5. Khronos Basic Data Format Descriptor Block

A basic descriptor block (Table 8) is designed to encode common metadata associated with bulk data — especially image or texture data. While this descriptor holds more information about the data interpretation than is needed by many applications, a comprehensive encoding reduces the risk of metadata needed by different APIs being lost in translation.

The format is described in terms of a repeating axis-aligned texel block composed of samples. Each sample contains a single channel of information with a single spatial offset within the texel block, and consists of an amount of contiguous data. This descriptor block consists of information about the interpretation of the texel block as a whole, supplemented by a description of a number of samples taken from one or more planes of contiguous memory. For example, a 24-bit red/green/blue format may be described as a 1×1 pixel region, in one plane of three samples, one describing each channel. A Y′CBCR 4:2:0 format may consist of a repeating 2×2 region: four Y′ samples and one sample each of CB and CR.

Table 8. Basic Data Format Descriptor layout

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

descriptorType = 0

vendorId = 0

descriptorBlockSize = 24 + 16 × #samples

versionNumber = 2

flags

transferFunction

colorPrimaries

colorModel

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

bytesPlane3

bytesPlane2

bytesPlane1

bytesPlane0

bytesPlane7

bytesPlane6

bytesPlane5

bytesPlane4

Sample information for the first sample

Sample information for the second sample (optional), etc.


The Basic Data Format Descriptor Block should be the first descriptor block in any data format descriptor of which it is a component.

The fields of the Basic Data Format Descriptor Block are described in the following sections.

To simplify code using the Basic Data Format Descriptor Block, the header khr_df.h provides enums of the following form for accessing descriptor block fields:

Table 9. Field location information for field xxx

Word offset into basic descriptor block

KHR_DF_WORD_xxx

Word offset into descriptor

KHR_DF_WORD_xxx + 1

Start bit within word

KHR_DF_SHIFT_xxx

Bit mask of value

KHR_DF_MASK_xxx


If the basic descriptor block is treated as a uint32_t array bdb[], field xxx can be accessed as follows:

xxx = KHR_DF_MASK_xxx & (bdb[KHR_DF_WORD_xxx] >> KHR_DF_SHIFT_xxx);

The macro KHR_DFDVAL(BDB, X) is provided to perform this calculation. For example, KHR_DFDVAL(bdb, MODEL) returns the value:

KHR_DF_MASK_MODEL & (bdb[KHR_DF_WORD_MODEL] >> KHR_DF_SHIFT_MODEL)

5.1. vendorId

The vendorId for the Basic Data Format Descriptor Block is 0, defined as KHR_DF_VENDORID_KHRONOS in the enum khr_df_vendorid_e.

Table 10. Field location information for vendorId

Word offset into basic descriptor block

KHR_DF_WORD_VENDORID

0

Word offset into descriptor

KHR_DF_WORD_VENDORID + 1

1

Start bit within word

KHR_DF_SHIFT_VENDORID

0

Bit mask of value

KHR_DF_MASK_VENDORID

0x1FFFFU


khr_df_vendorid_e vendorId = KHR_DF_MASK_VENDORID & (bdb[KHR_DF_WORD_VENDORID] >> KHR_DF_SHIFT_VENDORID);

5.2. descriptorType

The descriptorType for the Basic Data Format Descriptor Block is 0, a value reserved in the enum of Khronos-specific descriptor types, khr_df_khr_descriptortype_e, as KHR_DF_KHR_DESCRIPTORTYPE_BASICFORMAT.

Table 11. Field location information for descriptorType

Word offset into basic descriptor block

KHR_DF_WORD_DESCRIPTORTYPE

0

Word offset into descriptor

KHR_DF_WORD_DESCRIPTORTYPE + 1

1

Start bit within word

KHR_DF_SHIFT_DESCRIPTORTYPE

17

Bit mask of value

KHR_DF_MASK_DESCRIPTORTYPE

0x7FFFU


khr_df_descriptortype_e descriptorType = KHR_DF_MASK_DESCRIPTORTYPE & (bdb[KHR_DF_WORD_DESCRIPTORTYPE] >> KHR_DF_SHIFT_DESCRIPTORTYPE);

5.3. versionNumber

The versionNumber relating to the Basic Data Format Descriptor Block as described in this specification is 2.

[Note]

The versionNumber is incremented to indicate an incompatible change in the descriptor. The addition of enumerant values, for example to represent more compressed texel formats, does not constitute an “incompatible change”, and implementations should be resilient against enumerants that have been added in later minor updates.

Table 12. Field location information for versionNumber

Word offset into basic descriptor block

KHR_DF_WORD_VERSIONNUMBER

1

Word offset into descriptor

KHR_DF_WORD_VERSIONNUMBER + 1

2

Start bit within word

KHR_DF_SHIFT_VERSIONNUMBER

0

Bit mask of value

KHR_DF_MASK_VERSIONNUMBER

0xFFFFU


uint32_t versionNumber = KHR_DF_MASK_VERSIONNUMBER & (bdb[KHR_DF_WORD_VERSIONNUMBER] >> KHR_DF_SHIFT_VERSIONNUMBER);

5.4. descriptorBlockSize

The memory size of the Basic Data Format Descriptor Block depends on the number of samples contained within it. The memory requirements for this format are 24 bytes of shared data plus 16 bytes per sample. The descriptorBlockSize is measured in bytes.

Table 13. Field location information for descriptorBlockSize

Word offset into basic descriptor block

KHR_DF_WORD_DESCRIPTORBLOCKSIZE

1

Word offset into descriptor

KHR_DF_WORD_DESCRIPTORBLOCKSIZE + 1

2

Start bit within word

KHR_DF_SHIFT_DESCRIPTORBLOCKSIZE

16

Bit mask of value

KHR_DF_MASK_DESCRIPTORBLOCKSIZE

0xFFFFU


uint32_t descriptorBlockSize = KHR_DF_MASK_DESCRIPTORBLOCKSIZE & (bdb[KHR_DF_WORD_DESCRIPTORBLOCKSIZE] >> KHR_DF_SHIFT_DESCRIPTORBLOCKSIZE);

5.5. colorModel

The colorModel determines the set of color (or other data) channels which may be encoded within the data, though there is no requirement that all of the possible channels from the colorModel be present. Most data fits into a small number of common color models, but compressed texture formats each have their own color model enumeration. Note that the data need not actually represent a color — this is just the most common type of content using this descriptor. Some standards use color container for this concept.

The available color models are described in the khr_df_model_e enumeration, and are represented as an unsigned 8-bit value.

Table 14. Field location information for colorModel

Word offset into basic descriptor block

KHR_DF_WORD_MODEL

2

Word offset into descriptor

KHR_DF_WORD_MODEL + 1

3

Start bit within word

KHR_DF_SHIFT_MODEL

0

Bit mask of value

KHR_DF_MASK_MODEL

0xFF


khr_df_model_e colorModel = KHR_DF_MASK_MODEL & (bdb[KHR_DF_WORD_MODEL] >> KHR_DF_SHIFT_MODEL);

Note that the numbering of the component channels is chosen such that those channel types which are common across multiple color models have the same enumeration value. That is, alpha is always encoded as channel ID 15, depth is always encoded as channel ID 14, and stencil is always encoded as channel ID 13. Luma/Luminance is always in channel ID 0. This numbering convention is intended to simplify code which can process a range of color models. Note that there is no guarantee that models which do not support these channels will not use this channel ID. Particularly, RGB formats do not have luma in channel 0, and a 16-channel undefined format is not obligated to represent alpha in any way in channel number 15.

The value of each enumerant is shown in parentheses following the enumerant name.

5.5.1. KHR_DF_MODEL_UNSPECIFIED (= 0)

When the data format is unknown or does not fall into a predefined category, utilities which perform automatic conversion based on an interpretation of the data cannot operate on it. This format should be used when there is no expectation of portable interpretation of the data using only the basic descriptor block.

For portability reasons, it is recommended that pixel-like formats with up to sixteen channels, but which cannot have those channels described in the basic block, be represented with a basic descriptor block with the appropriate number of samples from UNSPECIFIED channels, and then for the channel description to be stored in an extension block. This allows software which understands only the basic descriptor to be able to perform operations that depend only on channel location, not channel interpretation (such as image cropping). For example, a camera may store a raw format taken with a modified Bayer sensor, with RGBW (red, green, blue and white) sensor sites, or RGBE (red, green, blue and “emerald”). Rather than trying to encode the exact color coordinates of each sample in the basic descriptor, these formats could be represented by a four-channel UNSPECIFIED model, with an extension block describing the interpretation of each channel.

5.5.2. KHR_DF_MODEL_RGBSDA (= 1)

This color model represents additive colors of three channels, nominally red, green and blue, supplemented by channels for alpha, depth and stencil, as shown in Table 15. Note that in many formats, depth and stencil are stored in a completely independent buffer, but there are formats for which integrating depth and stencil with color data makes sense.

Table 15. Basic Data Format RGBSDA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_RGBSDA_RED

  Red

1

  KHR_DF_CHANNEL_RGBSDA_GREEN

  Green

2

  KHR_DF_CHANNEL_RGBSDA_BLUE

  Blue

13

  KHR_DF_CHANNEL_RGBSDA_STENCIL

  Stencil

14

  KHR_DF_CHANNEL_RGBSDA_DEPTH

  Depth

15

  KHR_DF_CHANNEL_RGBSDA_ALPHA

  Alpha (opacity)


Portable representation of additive colors with more than three primaries requires an extension to describe the full color space of the channels present. There is no practical way to do this portably without taking significantly more space.

5.5.3. KHR_DF_MODEL_YUVSDA (= 2)

This color model represents color differences with three channels, nominally luma (Y′) and two color-difference chroma channels, U (CB) and V (CR), supplemented by channels for alpha, depth and stencil, as shown in Table 16. These formats are distinguished by CB and CR being a delta between the Y′ channel and the blue and red channels respectively, rather than requiring a full color matrix. The conversion between Y′CBCR and RGB color spaces is defined in this case by the choice of value in the colorPrimaries field as described in Section 15.1.

[Note]

Most single-channel luma/luminance monochrome data formats should select KHR_DF_MODEL_YUVSDA and use only the Y channel, unless there is a reason to do otherwise.

Table 16. Basic Data Format YUVSDA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_YUVSDA_Y

  Y/Y′ (luma/luminance)

1

  KHR_DF_CHANNEL_YUVSDA_CB

  CB (alias for U)

1

  KHR_DF_CHANNEL_YUVSDA_U

  U (alias for CB)

2

  KHR_DF_CHANNEL_YUVSDA_CR

  CR (alias for V)

2

  KHR_DF_CHANNEL_YUVSDA_V

  V (alias for CR)

13

  KHR_DF_CHANNEL_YUVSDA_STENCIL

  Stencil

14

  KHR_DF_CHANNEL_YUVSDA_DEPTH

  Depth

15

  KHR_DF_CHANNEL_YUVSDA_ALPHA

  Alpha (opacity)


[Note]

Terminology for this color model is often abused. This model is based on the idea of creating a representation of monochrome light intensity as a weighted average of color channels, then calculating color differences by subtracting two of the color channels from this monochrome value. Proper names vary for each variant of the ensuing numbers, but YUV is colloquially used for all of them. In the television standards from which this terminology is derived, Y′CBCR is more formally used to describe the representation of these color differences. See Section 15.1 for more detail.

5.5.4. KHR_DF_MODEL_YIQSDA (= 3)

This color model represents color differences with three channels, nominally luma (Y) and two color-difference chroma channels, I and Q, supplemented by channels for alpha, depth and stencil, as shown in Table 17. This format is distinguished by I and Q each requiring all three additive channels to evaluate. I and Q are derived from CB and CR by a 33-degree rotation.

Table 17. Basic Data Format YIQSDA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_YIQSDA_Y

  Y (luma)

1

  KHR_DF_CHANNEL_YIQSDA_I

  I (in-phase)

2

  KHR_DF_CHANNEL_YIQSDA_Q

  Q (quadrature)

13

  KHR_DF_CHANNEL_YIQSDA_STENCIL

  Stencil

14

  KHR_DF_CHANNEL_YIQSDA_DEPTH

  Depth

15

  KHR_DF_CHANNEL_YIQSDA_ALPHA

  Alpha (opacity)


5.5.5. KHR_DF_MODEL_LABSDA (= 4)

This color model represents the ICC perceptually-uniform L*a*b* color space, combined with the option of an alpha channel, as shown in Table 18.

Table 18. Basic Data Format LABSDA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_LABSDA_L

  L* (luma)

1

  KHR_DF_CHANNEL_LABSDA_A

  a*

2

  KHR_DF_CHANNEL_LABSDA_B

  b*

13

  KHR_DF_CHANNEL_LABSDA_STENCIL

  Stencil

14

  KHR_DF_CHANNEL_LABSDA_DEPTH

  Depth

15

  KHR_DF_CHANNEL_LABSDA_ALPHA

  Alpha (opacity)


5.5.6. KHR_DF_MODEL_CMYKA (= 5)

This color model represents secondary (subtractive) colors and the combined key (black) channel, along with alpha, as shown in Table 19.

Table 19. Basic Data Format CMYKA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_CMYKA_CYAN

  Cyan

1

  KHR_DF_CHANNEL_CMYKA_MAGENTA

  Magenta

2

  KHR_DF_CHANNEL_CMYKA_YELLOW

  Yellow

3

  KHR_DF_CHANNEL_CMYKA_KEY

  Key/Black

15

  KHR_DF_CHANNEL_CMYKA_ALPHA

  Alpha (opacity)


5.5.7. KHR_DF_MODEL_XYZW (= 6)

This “color model” represents channel data used for coordinate values, as shown in Table 20 — for example, as a representation of the surface normal in a bump map. Additional channels for higher-dimensional coordinates can be used by extending the channel number within the 4-bit limit of the channelType field.

Table 20. Basic Data Format XYZW channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_XYZW_X

  X

1

  KHR_DF_CHANNEL_XYZW_Y

  Y

2

  KHR_DF_CHANNEL_XYZW_Z

  Z

3

  KHR_DF_CHANNEL_XYZW_W

  W


5.5.8. KHR_DF_MODEL_HSVA_ANG (= 7)

This color model represents color differences with three channels, value (luminance or luma), saturation (distance from monochrome) and hue (dominant wavelength), supplemented by an alpha channel, as shown in Table 21. In this model, the hue relates to the angular offset on a color wheel.

Table 21. Basic Data Format angular HSVA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_HSVA_ANG_VALUE

  V (value)

1

  KHR_DF_CHANNEL_HSVA_ANG_SATURATION

  S (saturation)

2

  KHR_DF_CHANNEL_HSVA_ANG_HUE

  H (hue)

15

  KHR_DF_CHANNEL_HSVA_ANG_ALPHA

  Alpha (opacity)


5.5.9. KHR_DF_MODEL_HSLA_ANG (= 8)

This color model represents color differences with three channels, lightness (maximum intensity), saturation (distance from monochrome) and hue (dominant wavelength), supplemented by an alpha channel, as shown in Table 22. In this model, the hue relates to the angular offset on a color wheel.

Table 22. Basic Data Format angular HSLA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_HSLA_ANG_LIGHTNESS

  L (lightness)

1

  KHR_DF_CHANNEL_HSLA_ANG_SATURATION

  S (saturation)

2

  KHR_DF_CHANNEL_HSLA_ANG_HUE

  H (hue)

15

  KHR_DF_CHANNEL_HSLA_ANG_ALPHA

  Alpha (opacity)


5.5.10. KHR_DF_MODEL_HSVA_HEX (= 9)

This color model represents color differences with three channels, value (luminance or luma), saturation (distance from monochrome) and hue (dominant wavelength), supplemented by an alpha channel, as shown in Table 23. In this model, the hue is generated by interpolation between extremes on a color hexagon.

Table 23. Basic Data Format hexagonal HSVA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_HSVA_HEX_VALUE

  V (value)

1

  KHR_DF_CHANNEL_HSVA_HEX_SATURATION

  S (saturation)

2

  KHR_DF_CHANNEL_HSVA_HEX_HUE

  H (hue)

15

  KHR_DF_CHANNEL_HSVA_HEX_ALPHA

  Alpha (opacity)


5.5.11. KHR_DF_MODEL_HSLA_HEX (= 10)

This color model represents color differences with three channels, lightness (maximum intensity), saturation (distance from monochrome) and hue (dominant wavelength), supplemented by an alpha channel, as shown in Table 24. In this model, the hue is generated by interpolation between extremes on a color hexagon.

Table 24. Basic Data Format hexagonal HSLA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_HSLA_HEX_LIGHTNESS

  L (lightness)

1

  KHR_DF_CHANNEL_HSLA_HEX_SATURATION

  S (saturation)

2

  KHR_DF_CHANNEL_HSLA_HEX_HUE

  H (hue)

15

  KHR_DF_CHANNEL_HSLA_HEX_ALPHA

  Alpha (opacity)


5.5.12. KHR_DF_MODEL_YCGCOA (= 11)

This color model represents low-cost approximate color differences with three channels, nominally luma (Y) and two color-difference chroma channels, Cg (green/purple color difference) and Co (orange/cyan color difference), supplemented by a channel for alpha, as shown in Table 25.

Table 25. Basic Data Format YCoCgA channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_YCGCOA_Y

  Y

1

  KHR_DF_CHANNEL_YCGCOA_CG

  Cg

2

  KHR_DF_CHANNEL_YCGCOA_CO

  Co

15

  KHR_DF_CHANNEL_YCGCOA_ALPHA

  Alpha (opacity)


5.5.13. KHR_DF_MODEL_YCCBCCRC (= 12)

This color model represents the “Constant luminance” $Y'_CC'_\mathit{BC}C'_\mathit{RC}$ color model defined as an optional representation in ITU-T BT.2020 and described in Section 15.2.

Table 26. Basic Data Format Y′CC′BCC′RC channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_YCCBCCRC_YC

  $Y'_C$ (luminance)

1

  KHR_DF_CHANNEL_YCCBCCRC_CBC

  $C'_\mathit{BC}$

2

  KHR_DF_CHANNEL_YCCBCCRC_CRC

  $C'_\mathit{RC}$

13

  KHR_DF_CHANNEL_YCCBCCRC_STENCIL

  Stencil

14

  KHR_DF_CHANNEL_YCCBCCRC_DEPTH

  Depth

15

  KHR_DF_CHANNEL_YCCBCCRC_ALPHA

  Alpha (opacity)


5.5.14. KHR_DF_MODEL_ICTCP (= 13)

This color model represents the “Constant intensity ICTCP color model” defined as an optional representation in ITU-T BT.2100 and described in Section 15.3.

Table 27. Basic Data Format ICTCP channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_ICTCP_I

  I (intensity)

1

  KHR_DF_CHANNEL_ICTCP_CT

  CT

2

  KHR_DF_CHANNEL_ICTCP_CP

  CP

13

  KHR_DF_CHANNEL_ICTCP_STENCIL

  Stencil

14

  KHR_DF_CHANNEL_ICTCP_DEPTH

  Depth

15

  KHR_DF_CHANNEL_ICTCP_ALPHA

  Alpha (opacity)


5.5.15. KHR_DF_MODEL_CIEXYZ (= 14)

This color model represents channel data used to describe color coordinates in the CIE 1931 XYZ coordinate space, as shown in Table 28.

Table 28. Basic Data Format CIE XYZ channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_CIEXYZ_X

  X

1

  KHR_DF_CHANNEL_CIEXYZ_Y

  Y

2

  KHR_DF_CHANNEL_CIEXYZ_Z

  Z


5.5.16. KHR_DF_MODEL_CIEXYY (= 15)

This color model represents channel data used to describe chromaticity coordinates in the CIE 1931 xyY coordinate space, as shown in Table 29.

Table 29. Basic Data Format CIE xyY channels

Channel number   Name   Description

0

  KHR_DF_CHANNEL_CIEXYZ_X

  x

1

  KHR_DF_CHANNEL_CIEXYZ_YCHROMA

  y

2

  KHR_DF_CHANNEL_CIEXYZ_YLUMA

  Y


5.6. colorModel for compressed formats

A number of compressed formats are supported as part of khr_df_model_e. In general, these formats will have the texel block dimensions of the compression block size. Most contain a single sample of channel type 0 at offset 0,0 — where further samples are required, they should also be sited at 0,0. By convention, models which have multiple channels that are disjoint in memory have these channel locations described independently as separate samples; this can simplify some decoders.

The ASTC family of formats have a number of possible channels, and are distinguished by samples which reference some set of these channels. The texelBlockDimension fields determine the compression ratio for ASTC and PVRTC.

Compressed formats necessarily do not have an equivalent integer representation in which to describe the sampleLower and sampleUpper ranges — in particular, some have different ranges on a block-by-block format. Floating-point compressed formats have lower and upper limits specified in floating point format, since this representation indicates the output of compressed decoding. Integer compressed formats with a lower and upper of 0 and UINT32_MAX (for unsigned formats) or INT32_MIN and INT32_MAX (for signed formats) are assumed to map the full representable range to 0..1 or -1..1 respectively.

If a format has a non-linear transfer function, any samples with channel ID 15 (that is, the format has separate alpha encoding, for example KHR_DF_BC2_ALPHA) should set the KHR_DF_SAMPLE_DATATYPE_LINEAR bit for that sample.

Example descriptors for compressed formats are provided after each model in this section.

5.6.1. KHR_DF_MODEL_DXT1A/KHR_DF_MODEL_BC1A (= 128)

This model represents the DXT1 or BC1 format, described in Section 18. Each compressed texel block consists of 4×4 texels in 8 bytes. A single sample with channel ID 0 indicates that the “special value” should be interpreted as black, as described in Section 18.1 — a descriptor block representing this is shown in Table 31. A single sample with channel ID 1 indicates that the “special value” should represent transparency, as described in Section 18.2 — a descriptor block representing this is shown in Table 32.

Enumerant names for these channel ids are listed in Table 30.

Table 30. BC1A channel names

Enumerant Value

KHR_DF_CHANNEL_DXT1A_COLOR

0

KHR_DF_CHANNEL_BC1A_COLOR

KHR_DF_CHANNEL_DXT1A_ALPHAPRESENT

1

KHR_DF_CHANNEL_DXT1A_ALPHA

KHR_DF_CHANNEL_BC1A_ALPHAPRESENT

KHR_DF_CHANNEL_BC1A_ALPHA


Table 31. Example DXT1A descriptor with no punch-through alpha

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: DXT1A

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 32. Example DXT1A descriptor with punch-through alpha

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: DXT1A

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

ALPHA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.2. KHR_DF_MODEL_DXT2/3/KHR_DF_MODEL_BC2 (= 129)

This model represents the DXT2/3 format, also known as BC2, and described in Section 18.3. Each compressed texel block consists of 4×4 texels in 16 bytes. The alpha premultiplication state (the distinction between DXT2 and DXT3) is recorded separately in the descriptor in the flags field. This model has two channels, recorded as separate samples: Sample 0 with channel ID 15 contains the alpha information. Sample 1 with channel ID 0 contains the color information.

Enumerant names for these channel ids are listed in Table 33.

Table 33. BC2 channel names

Enumerant Value

KHR_DF_CHANNEL_DXT2_COLOR

0

KHR_DF_CHANNEL_DXT3_COLOR

KHR_DF_CHANNEL_BC2_COLOR

KHR_DF_CHANNEL_DXT2_ALPHA

15

KHR_DF_CHANNEL_DXT3_ALPHA

KHR_DF_CHANNEL_BC2_ALPHA


The alpha channel is 64 bits and at offset 0; the color channel is 64 bits and at offset 64. No attempt is made to describe the 16 alpha samples for this position independently, since understanding the other channels for any pixel requires the whole texel block.

Table 34. Example DXT2 descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: DXT2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Alpha sample information

0

0

0

0

ALPHA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Color sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 35. Example DXT3 descriptor (premultiplied alpha)

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: PREMULTIPLIED

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: DXT3

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Alpha sample information

0

0

0

0

ALPHA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Color sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.3. KHR_DF_MODEL_DXT4/5/KHR_DF_MODEL_BC3 (= 130)

This model represents the DXT4/5 format, also known as BC3, and described in Section 18.4. Each compressed texel block consists of 4×4 texels in 16 bytes. The alpha premultiplication state (the distinction between DXT4 and DXT5) is recorded separately in the descriptor in the flags field. This model has two channels, recorded as separate samples: Sample 0 with channel ID 15 contains the alpha information. Sample 1 with channel ID 0 contains the color information.

Enumerant names for these channel ids are listed in Table 36.

Table 36. BC3 channel names

Enumerant Value

KHR_DF_CHANNEL_DXT4_COLOR

0

KHR_DF_CHANNEL_DXT5_COLOR

KHR_DF_CHANNEL_BC3_COLOR

KHR_DF_CHANNEL_DXT4_ALPHA

15

KHR_DF_CHANNEL_DXT5_ALPHA

KHR_DF_CHANNEL_BC3_ALPHA


The alpha channel is 64 bits and at offset 0; the color channel is 64 bits and at offset 64. No attempt is made to describe the 16 alpha samples for this position independently, since understanding the other channels for any pixel requires the whole texel block.

Table 37. Example DXT4 descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: DXT4

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Alpha sample information

0

0

0

0

ALPHA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Color sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 38. Example DXT5 descriptor (premultiplied alpha)

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: PREMULTIPLIED

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: DXT5

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Alpha sample information

0

0

0

0

ALPHA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Color sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.4. KHR_DF_MODEL_BC4 (= 131)

This model represents the Direct3D BC4 format for single-channel interpolated 8-bit data, as described in Section 19.1.

Each compressed texel block consists of 4×4 texels in 8 bytes. The model has a single channel of id 0 with offset 0 and length 64 bits.

The enumerant name for this channel id is listed in Table 39.

Table 39. BC4 channel name

Enumerant Value

KHR_DF_CHANNEL_BC4_DATA

0


Table 40. Example BC4 unsigned descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: BC4

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

DATA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 41. Example BC4 signed descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: BC4

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

1

0

0

DATA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: INT32_MIN

sampleUpper: INT32_MAX


5.6.5. KHR_DF_MODEL_BC5 (= 132)

This model represents the Direct3D BC5 format for dual-channel interpolated 8-bit data, as described in Section 19.3.

Each compressed texel block consists of 4×4 texels in 16 bytes. The model has two channels, 0 (red) and 1 (green), which should have their bit depths and offsets independently described: the red channel has offset 0 and length 64 bits and the green channel has offset 64 and length 64 bits.

Enumerant names for these channel ids are listed in Table 42.

Table 42. BC5 channel names

Enumerant Value

KHR_DF_CHANNEL_BC5_RED

0

KHR_DF_CHANNEL_BC5_R

KHR_DF_CHANNEL_BC5_GREEN

1

KHR_DF_CHANNEL_BC5_G


Table 43. Example BC5 unsigned descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: BC5

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Red sample information

0

0

0

0

RED

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Green sample information

0

0

0

0

GREEN

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 44. Example BC5 signed descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: BC5

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Red sample information

0

1

0

0

RED

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: INT32_MIN

sampleUpper: INT32_MAX

F

S

E

L

channelType

Green sample information

0

1

0

0

GREEN

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: INT32_MIN

sampleUpper: INT32_MAX


A legacy variant of this format known as “ATI2n” or “3Dc” swaps the location of the two channels, and can be encoded as follows:

Table 45. Example ATI2n unsigned descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: BC5

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Red sample information

0

0

0

0

GREEN

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Green sample information

0

0

0

0

RED

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.6. KHR_DF_MODEL_BC6H (= 133)

This model represents the Direct3D BC6H format for RGB floating-point data, as described in Section 20.2.

Each compressed texel block consists of 4×4 texels in 16 bytes. The model has a single channel 0, representing all three channels, and occupying 128 bits.

The enumerant names for this channel id are listed in Table 46.

Table 46. BC6H channel names

Enumerant Value

KHR_DF_CHANNEL_BC6H_COLOR

0

KHR_DF_CHANNEL_BC6H_DATA


Table 47. Example BC6H signed descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: BC6H

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

1

1

0

0

COLOR

bitLength: 127 (= “128”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0xBF800000U — -1.0f

sampleUpper: 0x7F800000U — 1.0f


Table 48. Example BC6H unsigned descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: BC6H

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

1

0

0

0

COLOR

bitLength: 127 (= “128”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0xBF800000U — -1.0f

sampleUpper: 0x7F800000U — 1.0f


5.6.7. KHR_DF_MODEL_BC7 (= 134)

This model represents the Direct3D BC7 format for RGBA data, as described in Section 20.1.

Each compressed texel block consists of 4×4 texels in 16 bytes. The model has a single channel 0, representing all four channels, and occupying 128 bits.

The enumerant names for this channel id are listed in Table 49.

Table 49. BC7 channel names

Enumerant Value

KHR_DF_CHANNEL_BC7_COLOR

0

KHR_DF_CHANNEL_BC7_DATA


Table 50. Example BC7 descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: BC7

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 127 (= “128”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.8. KHR_DF_MODEL_ETC1 (= 160)

This model represents the original Ericsson Texture Compression format, described in Section 21, with a guarantee that the format does not rely on the ETC2 extensions described in Section 22.

Each compressed texel block consists of 4×4 texels in 8 bytes. The model has a single channel 0, representing all three channels, and occupying 64 bits.

The enumerant names for this channel id are listed in Table 51.

Table 51. ETC1 channel names

Enumerant Value

KHR_DF_CHANNEL_ETC1_COLOR

0

KHR_DF_CHANNEL_ETC1_DATA


Table 52. Example ETC1 descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC1

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.9. KHR_DF_MODEL_ETC2 (= 161)

This model represents the updated Ericsson Texture Compression format, ETC2, and also the related R11 EAC and RG11 EAC formats. Each compressed texel block consists of 4×4 texels.

The enumerant names for these channel ids are listed in Table 53.

Table 53. ETC2 channel names

Enumerant Value

KHR_DF_CHANNEL_ETC2_RED

0

KHR_DF_CHANNEL_ETC2_R

KHR_DF_CHANNEL_ETC2_GREEN

1

KHR_DF_CHANNEL_ETC2_G

KHR_DF_CHANNEL_ETC2_COLOR

2

KHR_DF_CHANNEL_ETC2_ALPHA

15

KHR_DF_CHANNEL_ETC2_A


Channel ID 0 represents red, and is used for the R11 EAC format, as described in Section 22.5; the texel block size in this format is 8 bytes, represented as a single 64-bit sample.

Table 54. Example R11 unsigned descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

RED

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 55. Example R11 signed descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

1

0

0

RED

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: INT32_MIN

sampleUpper: INT32_MAX


Channel ID 1 represents green; the presence of samples for both red and green, in that order, indicates the RG11 EAC format as described in Section 22.6, which consists of a total of 16 bytes of data.

Table 56. Example RG11 unsigned descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Red sample information

0

0

0

0

RED

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Green sample information

0

0

0

0

GREEN

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 57. Example RG11 signed descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 2) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Red sample information

0

1

0

0

RED

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: INT32_MIN

sampleUpper: INT32_MAX

F

S

E

L

channelType

Green sample information

0

1

0

0

GREEN

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: INT32_MIN

sampleUpper: INT32_MAX


Channel ID 2 represents RGB combined content, for the ETC2 format as described in Section 22.1. A single sample of ID 2 indicates RGB2 with no alpha, occupying 8 bytes.

Table 58. Example ETC2 descriptor (with no alpha)

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Channel ID 15 indicates the presence of alpha. If the texel block size is 8 bytes and the RGB and alpha channels are co-sited, “punch through” alpha is supported as described in Section 22.9.

Table 59. Example ETC2 descriptor with punchthrough alpha

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Color sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Alpha sample information

0

0

0

0

ALPHA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Finally, if the texel block size is 16 bytes and the alpha channel appears in the first 8 bytes, followed by 8 bytes for the RGB channel, 8-bit separate alpha is supported, as described in Section 22.3.

Table 60. Example ETC2 descriptor with separate alpha

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 60

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 56

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Alpha sample information

0

0

0

0

ALPHA

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX

F

S

E

L

channelType

Color sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 64

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.10. KHR_DF_MODEL_ASTC (= 162)

This model represents Adaptive Scalable Texture Compression as a single channel in a texel block of 16 bytes. ASTC HDR (high dynamic range) and LDR (low dynamic range) modes are distinguished by the channelId containing the flag KHR_DF_SAMPLE_DATATYPE_FLOAT: an ASTC texture that is guaranteed by the user to contain only LDR-encoded blocks should have the channelId KHR_DF_SAMPLE_DATATYPE_FLOAT bit clear, and an ASTC texture that may include HDR-encoded blocks should have the channelId KHR_DF_SAMPLE_DATATYPE_FLOAT bit set to 1. ASTC supports a number of compression ratios defined by different texel block sizes; these are selected by changing the texel block size fields in the data format.

ASTC encoding is described in Section 23.

The single sample, of ID 0, has a size of 128 bits.

The enumerant name for this channel id is listed in Table 61.

Table 61. ASTC channel name

Enumerant Value

KHR_DF_CHANNEL_ASTC_DATA

0


Table 62. Example 4×4 ASTC LDR descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ASTC

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

DATA

bitLength: 127 (= “128”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 63. Example 8×5 ASTC HDR descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ASTC

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

4 (= “5”)

7 (= “8”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 16

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

1

1

0

0

DATA

bitLength: 127 (= “128”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0xBF800000U — -1.0f

sampleUpper: 0x7F800000U — 1.0f


5.6.11. KHR_DF_MODEL_ETC1S (= 163)

This model represents a subset of the original Ericsson Texture Compression format, described in Section 21.1, which is restricted in order to facilitate image compression.

Each compressed texel block consists of 4×4 texels in 8 bytes. The model has a single channel 0, representing all three channels, and occupying 64 bits.

The enumerant names for this channel id are listed in Table 64.

Table 64. ETC1S channel names

Enumerant Value

KHR_DF_CHANNEL_ETC1S_COLOR

0

KHR_DF_CHANNEL_ETC1S_DATA


Table 65. Example ETC1S descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: ETC1S

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.12. KHR_DF_MODEL_PVRTC (= 164)

This model represents the first generation of PowerVR Texture Compression as a single channel in a texel block of 8 bytes. 4-bit-per-pixel mode represents a 4×4 texel block; 2-bit-per-pixel mode represents an 8×4 texel block, and these can be distinguished by changing the texel block size fields in the data format. The single sample has a size of 64 bits.

The enumerant names for this channel id are listed in Table 66.

Table 66. PVRTC channel names

Enumerant Value

KHR_DF_CHANNEL_PVRTC_COLOR

0

KHR_DF_CHANNEL_PVRTC_DATA


Table 67. Example PVRTC 4bpp descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: PVRTC

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 68. Example PVRTC 2bpp descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: PVRTC

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

7 (= “8”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.6.13. KHR_DF_MODEL_PVRTC2 (= 165)

This model represents the second generation of PowerVR Texture Compression as a single channel in a texel block of 8 bytes. 4-bit-per-pixel mode represents a 4×4 texel block; 2-bit-per-pixel mode represents an 8×4 texel block, and these can be distinguished by changing the texel block size fields in the data format. The single sample has a size of 64 bits.

The enumerant names for this channel id are listed in Table 69.

Table 69. PVRTC2 channel names

Enumerant Value

KHR_DF_CHANNEL_PVRTC2_COLOR

0

KHR_DF_CHANNEL_PVRTC2_DATA


Table 70. Example PVRTC2 4bpp descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: PVRTC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

3 (= “4”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


Table 71. Example PVRTC2 2bpp descriptor

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 44

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: PVRTC2

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

3 (= “4”)

7 (= “8”)

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 8

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information

0

0

0

0

COLOR

bitLength: 63 (= “64”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: UINT32_MAX


5.7. colorPrimaries

It is not sufficient to define a buffer as containing, for example, additive primaries. Additional information is required to define what “red” is provided by the “red” channel. A full definition of primaries requires an extension which provides the full color space of the data, but a subset of common primary spaces can be identified by the khr_df_primaries_e enumeration, represented as an unsigned 8-bit integer.

More information about color primaries is provided in Section 14.

Table 72. Field location information for colorPrimaries

Word offset into basic descriptor block

KHR_DF_WORD_PRIMARIES

2

Word offset into descriptor

KHR_DF_WORD_PRIMARIES + 1

3

Start bit within word

KHR_DF_SHIFT_PRIMARIES

8

Bit mask of value

KHR_DF_MASK_PRIMARIES

0xFF


khr_df_primaries_e colorPrimaries = KHR_DF_MASK_PRIMARIES & (bdb[KHR_DF_WORD_PRIMARIES] >> KHR_DF_SHIFT_PRIMARIES);

5.7.1. KHR_DF_PRIMARIES_UNSPECIFIED (= 0)

This “set of primaries” identifies a data representation whose color representation is unknown or which does not fit into this list of common primaries. Having an “unspecified” value here precludes users of this data format from being able to perform automatic color conversion unless the primaries are defined in another way. Formats which require a proprietary color space — for example, raw data from a Bayer sensor that records the direct response of each filtered sample — can still indicate that samples represent “red”, “green” and “blue”, but should mark the primaries here as “unspecified” and provide a detailed description in an extension block.

5.7.2. KHR_DF_PRIMARIES_BT709 (= 1)

This value represents the Color Primaries defined by the ITU-R BT.709 specification and described in Section 14.1, which are also shared by sRGB.

RGB data is distinguished between BT.709 and sRGB by the Transfer Function. Conversion to and from BT.709 Y′CBCR (YUV) representation uses the color conversion matrix defined in the BT.709 specification, and described in Section 15.1.1, except in the case of sYCC (which can be distinguished by the use of the sRGB transfer function), in which case conversion to and from BT.709 Y′CBCR representation uses the color conversion matrix defined in the BT.601 specification, and described in Section 15.1.2. This is the preferred set of color primaries used by HDTV and sRGB, and likely a sensible default set of color primaries for common rendering operations.

KHR_DF_PRIMARIES_SRGB is provided as a synonym for KHR_DF_PRIMARIES_BT709.

5.7.3. KHR_DF_PRIMARIES_BT601_EBU (= 2)

This value represents the Color Primaries defined in the ITU-R BT.601 specification for standard-definition television, particularly for 625-line signals, and described in Section 14.2. Conversion to and from BT.601 Y′CBCR (YUV) typically uses the color conversion matrix defined in the BT.601 specification and described in Section 15.1.2.

5.7.4. KHR_DF_PRIMARIES_BT601_SMPTE (= 3)

This value represents the Color Primaries defined in the ITU-R BT.601 specification for standard-definition television, particularly for 525-line signals, and described in Section 14.3. Conversion to and from BT.601 Y′CBCR (YUV) typically uses the color conversion matrix defined in the BT.601 specification and described in Section 15.1.2.

5.7.5. KHR_DF_PRIMARIES_BT2020 (= 4)

This value represents the Color Primaries defined in the ITU-R BT.2020 specification for ultra-high-definition television and described in Section 14.4. Conversion to and from BT.2020 Y′CBCR (YUV uses the color conversion matrix defined in the BT.2020 specification and described in Section 15.1.3.

5.7.6. KHR_DF_PRIMARIES_CIEXYZ (= 5)

This value represents the theoretical Color Primaries defined by the International Color Consortium for the ICC XYZ linear color space.

5.7.7. KHR_DF_PRIMARIES_ACES (= 6)

This value represents the Color Primaries defined for the Academy Color Encoding System and described in Section 14.7.

5.7.8. KHR_DF_PRIMARIES_ACESCC (= 7)

This value represents the Color Primaries defined for the Academy Color Encoding System compositor and described in Section 14.8.

5.7.9. KHR_DF_PRIMARIES_NTSC1953 (= 8)

This value represents the Color Primaries defined for the NTSC 1953 color television transmission standard and described in Section 14.5.

5.7.10. KHR_DF_PRIMARIES_PAL525 (= 9)

This value represents the Color Primaries defined for 525-line PAL signals, described in Section 14.6.

5.7.11. KHR_DF_PRIMARIES_DISPLAYP3 (= 10)

This value represents the Color Primaries defined for the Display P3 color space, described in Section 14.9.

5.7.12. KHR_DF_PRIMARIES_ADOBERGB (= 11)

This value represents the Color Primaries defined in Adobe RGB (1998), described in Section 14.10.

5.8. transferFunction

Many color representations contain a non-linear transfer function which maps between a linear (intensity-based) representation and a more perceptually-uniform encoding; more information is provided in Section 13. Common transfer functions are represented as an unsigned 8-bit integer and encoded in the enumeration khr_df_transfer_e. A fully-flexible transfer function requires an extension with a full color space definition. Where the transfer function can be described as a simple power curve, applying the function is commonly known as “gamma correction”. The transfer function is applied to a sample only when the sample’s KHR_DF_SAMPLE_DATATYPE_LINEAR bit is 0; if this bit is 1, the sample is represented linearly irrespective of the transferFunction.

When a color model contains more than one channel in a sample and the transfer function should be applied only to a subset of those channels, the convention of that model should be used when applying the transfer function. For example, ASTC stores both alpha and RGB data but is represented by a single sample; in ASTC, any sRGB transfer function is not applied to the alpha channel of the ASTC texture. In this case, the KHR_DF_SAMPLE_DATATYPE_LINEAR bit being zero means that the transfer function is “applied” to the ASTC sample in a way that only affects the RGB channels. This is not a concern for most color models, which explicitly store different channels in each sample.

If all the samples are linear, KHR_DF_TRANSFER_LINEAR should be used. In this case, no sample should have the KHR_DF_SAMPLE_DATATYPE_LINEAR bit set. If the samples encode a single bit, KHR_DF_TRANSFER_LINEAR or KHR_DF_TRANSFER_UNSPECIFIED should be used, since there are no intermediate values to which the transfer function should apply.

Table 73. Field location information for transferFunction

Word offset into basic descriptor block

KHR_DF_WORD_TRANSFER

2

Word offset into descriptor

KHR_DF_WORD_TRANSFER + 1

3

Start bit within word

KHR_DF_SHIFT_TRANSFER

16

Bit mask of value

KHR_DF_MASK_TRANSFER

0xFF


khr_df_transfer_e transferFunction = KHR_DF_MASK_TRANSFER & (bdb[KHR_DF_WORD_TRANSFER] >> KHR_DF_SHIFT_TRANSFER);

The enumerant value for each of the following transfer functions is shown in parentheses alongside the title.

5.8.1. KHR_DF_TRANSFER_UNSPECIFIED (= 0)

This value should be used when the transfer function is unknown, or specified only in an extension block, precluding conversion of color spaces and correct filtering of the data values using only the information in the basic descriptor block.

5.8.2. KHR_DF_TRANSFER_LINEAR (= 1)

This value represents a linear transfer function: for color data, there is a linear relationship between numerical pixel values and the intensity of additive colors. This transfer function allows for blending and filtering operations to be applied directly to the data values.

5.8.3. KHR_DF_TRANSFER_SRGB (= 2)

This value represents the non-linear transfer function defined in the sRGB specification for mapping between numerical pixel values and displayed light intensity, as described in Section 13.3.

Mapping from linear intensity to encoding

EOTF -1

Section 13.3.2

Mapping from encoding to linear intensity

EOTF

Section 13.3.1

Encoded values outside the range 0..1 use the extended formulae for EOTF and EOTF-1 described in Section 13.3.4.

5.8.4. KHR_DF_TRANSFER_ITU (= 3)

This value represents the non-linear transfer function defined by the ITU and used in the BT.601, BT.709 and BT.2020 specifications for mapping between represented scene light intensity and numerical pixel values, as described in Section 13.2.

Mapping from linear intensity to encoding

OETF

Section 13.2.1

Mapping from encoding to linear intensity

OETF -1

Section 13.2.2

5.8.5. KHR_DF_TRANSFER_NTSC (= 4)

This value represents the non-linear transfer function defined by the original NTSC television broadcast specification for mapping between represented scene light intensity or display light intensity and numerical pixel values, as described in Section 13.8.

Mapping from linear intensity to encoding

EOTF -1 / OETF

Mapping from encoding to linear intensity

EOTF / OETF -1

[Note]

More recent formulations of this transfer functions, such as that defined in SMPTE 170M-2004, use the “ITU” formulation described above.

5.8.6. KHR_DF_TRANSFER_SLOG (= 5)

This value represents a nonlinear Transfer Function between linear scene light intensity and nonlinear pixel values, used by some Sony video cameras to represent an increased dynamic range, and is described in Section 13.13.

Mapping from linear intensity to encoding

OETF

Mapping from encoding to linear intensity

OETF -1

5.8.7. KHR_DF_TRANSFER_SLOG2 (= 6)

This value represents a nonlinear Transfer Function between linear scene light intensity and nonlinear pixel values, used by some Sony video cameras to represent a further increased dynamic range, and is described in Section 13.14.

Mapping from linear intensity to encoding

OETF

Mapping from encoding to linear intensity

OETF -1

5.8.8. KHR_DF_TRANSFER_BT1886 (= 7)

This value represents the nonlinear $\gamma = 2.4$ EOTF between encoded pixel values and linear image intensity defined in BT.1886 and described in Section 13.4.

Mapping from linear intensity to encoding

EOTF -1

$\{R',G',B'\} = \{R,G,B\}^{2.4}$

Mapping from encoding to linear intensity

EOTF

$\{R,G,B\} = \{R',G',B'\}^{1\over{2.4}}$

5.8.9. KHR_DF_TRANSFER_HLG_OETF (= 8)

This value represents the Hybrid Log Gamma OETF between linear scene light intensity and nonlinear pixel values, defined by the ITU in BT.2100 for high dynamic range television, and described in Section 13.5.

Mapping from linear intensity to encoding

OETF

Section 13.5.1

Mapping from encoding to linear intensity

OETF -1

Section 13.5.2

5.8.10. KHR_DF_TRANSFER_HLG_EOTF (= 9)

This value represents the Hybrid Log Gamma EOTF between nonlinear pixel values and linear image light intensity, defined by the ITU in BT.2100 for high dynamic range television, and described in Section 13.5.

Mapping from linear intensity to encoding

EOTF -1

Section 13.5.9

Mapping from encoding to linear intensity

EOTF

Section 13.5.8

5.8.11. KHR_DF_TRANSFER_PQ_EOTF (= 10)

This value represents the Perceptual Quantization EOTF between nonlinear pixel values and linear image light intensity, defined by the ITU in BT.2100 for high dynamic range television, and described in Section 13.6.

Mapping from linear intensity to encoding

EOTF -1

Section 13.6.2

Mapping from encoding to linear intensity

EOTF

Section 13.6.1

5.8.12. KHR_DF_TRANSFER_PQ_OETF (= 11)

This value represents the Perceptual Quantization OETF between linear scene light intensity and nonlinear pixel values, defined by the ITU in BT.2100 for high dynamic range television, and described in Section 13.6.

Mapping from linear intensity to encoding

OETF

Section 13.6.4

Mapping from encoding to linear intensity

OETF -1

Section 13.6.6

5.8.13. KHR_DF_TRANSFER_DCIP3 (= 12)

This value represents the transfer function between nonlinear pixel values and linear image light intensity defined in DCI P3 and described in Section 13.7.

Mapping from linear intensity to encoding

EOTF -1

Mapping from encoding to linear intensity

EOTF

5.8.14. KHR_DF_TRANSFER_PAL_OETF (= 13)

This value represents the OETF between linear scene light intensity and nonlinear pixel values for legacy PAL systems described in Section 13.9.

Mapping from linear intensity to encoding

OETF

Mapping from encoding to linear intensity

OETF -1

5.8.15. KHR_DF_TRANSFER_PAL625_EOTF (= 14)

This value represents the EOTF between nonlinear pixel values and linear image light intensity for legacy 625-line PAL systems described in Section 13.10.

Mapping from linear intensity to encoding

EOTF -1

Mapping from encoding to linear intensity

EOTF

5.8.16. KHR_DF_TRANSFER_ST240 (= 15)

This value represents the transfer function between linear scene light intensity and nonlinear pixel values associated with the legacy ST-240 (SMPTE240M) standard, described in Section 13.11.

Mapping from linear intensity to encoding

EOTF -1 / OETF

Mapping from encoding to linear intensity

EOTF / OETF -1

5.8.17. KHR_DF_TRANSFER_ACESCC (= 16)

This value represents the nonlinear transfer function between linear scene light intensity and nonlinear pixel values used in the ACEScc Academy Color Encoding System logarithmic encoding system for use within Color Grading Systems, S-2014-003, defined in ACES. This is described in Section 13.15.

Mapping from linear intensity to encoding

OETF

Mapping from encoding to linear intensity

OETF -1

5.8.18. KHR_DF_TRANSFER_ACESCCT (= 17)

This value represents the nonlinear transfer function between linear scene light intensity and nonlinear pixel values used in the ACEScc Academy Color Encoding System quasi-logarithmic encoding system for use within Color Grading Systems, S-2016-001, defined in ACES. This is described in Section 13.16.

Mapping from linear intensity to encoding

OETF

Mapping from encoding to linear intensity

OETF -1

5.8.19. KHR_DF_TRANSFER_ADOBERGB (= 18)

This value represents the transfer function defined in the Adobe RGB (1998) specification and described in Section 13.12.

5.9. flags

The format supports some configuration options in the form of boolean flags; these are described in the enumeration khr_df_flags_e and represented in an unsigned 8-bit integer value.

Table 74. Field location information for flags

Word offset into basic descriptor block

KHR_DF_WORD_FLAGS

2

Word offset into descriptor

KHR_DF_WORD_FLAGS + 1

3

Start bit within word

KHR_DF_SHIFT_FLAGS

24

Bit mask of value

KHR_DF_MASK_FLAGS

0xFF


khr_df_flags_e flags = KHR_DF_MASK_FLAGS & (bdb[KHR_DF_WORD_FLAGS] >> KHR_DF_SHIFT_FLAGS);

5.9.1. KHR_DF_FLAG_ALPHA_PREMULTIPLIED (= 1)

If the KHR_DF_FLAG_ALPHA_PREMULTIPLIED bit is set, any color information in the data should be interpreted as having been previously scaled/modulated by the alpha channel when performing blending operations.

The value KHR_DF_FLAG_ALPHA_STRAIGHT (= 0) is provided to represent this flag not being set, which indicates that color values in the data should be interpreted as needing to be scaled by the alpha channel when performing blending operations. This flag has no effect if there is no alpha channel in the format.

5.10. texelBlockDimension[0..3]

The texelBlockDimension fields define an integer bound on the range of coordinates covered by the repeating block described by the samples; that is, the texel block covers an integer range in each dimension of coordinate space. Four separate values, represented as unsigned 8-bit integers, are supported, corresponding to successive dimensions: the Basic Data Format Descriptor Block supports up to four dimensions of encoding within a texel block, supporting, for example, a texture with three spatial dimensions and one temporal dimension. Nothing stops the data structure as a whole from having higher dimensionality: for example, a two-dimensional texel block can be used as an element in a six-dimensional look-up table.

The value held in each of these fields is one fewer than the size of the block in that dimension — that is, a value of 0 represents a size of 1, a value of 1 represents a size of 2, etc. A texel block which covers fewer than four dimensions should have a size of 1 in each dimension that it lacks, and therefore the corresponding fields in the representation should be 0.

For example, a Y′CBCR 4:2:0 representation may use a Texel Block of 2×2 pixels in the nominal coordinate space, corresponding to the four Y′ samples, as shown in Table 75. The texel block dimensions in this case would be 2×2×1×1 (in the X, Y, Z and T dimensions, if the fourth dimension is interpreted as T). The texelBlockDimension[0..3] values would therefore be:

Table 75. Example Basic Data Format texelBlockDimension values for Y′CBCR 4:2:0

texelBlockDimension0

1

texelBlockDimension1

1

texelBlockDimension2

0

texelBlockDimension3

0


In the descriptor block examples in this specification, block dimensions larger than 1 (encoded as 0) are shown as the value to be stored in the texelBlockDimension field, but with the represented number in parentheses for clarity.

Table 76. Field location information for texelBlockDimension[0..3]

Word offset into basic descriptor block

KHR_DF_WORD_TEXELBLOCKDIMENSION[0..3]

3

Word offset into descriptor

KHR_DF_WORD_TEXELBLOCKDIMENSION[0..3] + 1

4

Start bit within word

KHR_DF_SHIFT_TEXELBLOCKDIMENSION0

0

KHR_DF_SHIFT_TEXELBLOCKDIMENSION1

8

KHR_DF_SHIFT_TEXELBLOCKDIMENSION2

16

KHR_DF_SHIFT_TEXELBLOCKDIMENSION3

24

Bit mask of value

KHR_DF_MASK_TEXELBLOCKDIMENSION[0..3]

0xFF


uint32_t texelBlockDimension0 = KHR_DF_MASK_TEXELBLOCKDIMENSION0 & (bdb[KHR_DF_WORD_TEXELBLOCKDIMENSION0] >> KHR_DF_SHIFT_TEXELBLOCKDIMENSION0);

uint32_t texelBlockDimension1 = KHR_DF_MASK_TEXELBLOCKDIMENSION1 & (bdb[KHR_DF_WORD_TEXELBLOCKDIMENSION1] >> KHR_DF_SHIFT_TEXELBLOCKDIMENSION1);

uint32_t texelBlockDimension2 = KHR_DF_MASK_TEXELBLOCKDIMENSION2 & (bdb[KHR_DF_WORD_TEXELBLOCKDIMENSION2] >> KHR_DF_SHIFT_TEXELBLOCKDIMENSION2);

uint32_t texelBlockDimension3 = KHR_DF_MASK_TEXELBLOCKDIMENSION3 & (bdb[KHR_DF_WORD_TEXELBLOCKDIMENSION3] >> KHR_DF_SHIFT_TEXELBLOCKDIMENSION3);

5.11. bytesPlane[0..7]

The Basic Data Format Descriptor divides the image into a number of planes, each consisting of an integer number of consecutive bytes. The requirement that planes consist of consecutive data means that formats with distinct subsampled channels — such as Y′CBCR 4:2:0 — may require multiple planes to describe a channel. A typical Y′CBCR 4:2:0 image has two planes for the Y′ channel in this representation, offset by one line vertically.

The use of byte granularity to define planes is a choice to allow large texel blocks. A consequence of this is that formats which are not byte-aligned on each addressable unit, such as 1-bit-per-pixel formats, need to represent a texel block of multiple samples, covering multiple texels — as, for example, in Table 92.

A maximum of eight independent planes is supported in the Basic Data Format Descriptor. Formats which require more than eight planes — which are rare — require an extension.

The bytesPlane[0..7] fields each contain an unsigned 8-bit integer which represents the number of bytes which a plane contributes to the format. If the top bit of a bytesPlane[n] field is set, bits 6..0 of the bytesPlane[n+1] field form bits 13..7 of the number of bytes in the plane (and the next plane is described by field bytesPlane[n+2]). For example, if bytesPlane0 is 0xC0 and bytesPlane1 is 0x02, the first plane holds 0x40 + 128 × 0x02 = 0x140 bytes; bytesPlane2 then describes the number of bytes in the second plane.. Since only sixteen bits are used to encode a bit offset for each sample, 14 bits (two bytes excluding the top bits) are sufficient to encode any useful number of bytes — there is no need to “extend” the higher byte. Few formats are expected to require this “extension bit”, so for most of this specification, the number of bytes in a plane is considered to be synonymous with the bytesPlane value.

The first field which contains the value 0 indicates that only a subset of the 8 possible planes are present; that is, planes which are not present should be given the bytesPlane value of 0, and any bytesPlane values after the first 0 are ignored. If no bytesPlane value is zero, 8 planes are considered to exist.

Table 77. Field location information for bytesPlane[0..7]

Word offset into basic descriptor block

KHR_DF_WORD_BYTESPLANE[0..3]

4

KHR_DF_WORD_BYTESPLANE[4..7]

5

Word offset into descriptor

KHR_DF_WORD_BYTESPLANE[0..3] + 1

5

KHR_DF_WORD_BYTESPLANE[4..7] + 1

6

Start bit within word

KHR_DF_SHIFT_BYTESPLANE0

0

KHR_DF_SHIFT_BYTESPLANE1

8

KHR_DF_SHIFT_BYTESPLANE2

16

KHR_DF_SHIFT_BYTESPLANE3

24

KHR_DF_SHIFT_BYTESPLANE4

0

KHR_DF_SHIFT_BYTESPLANE5

8

KHR_DF_SHIFT_BYTESPLANE6

16

KHR_DF_SHIFT_BYTESPLANE7

24

Bit mask of value

KHR_DF_MASK_BYTESPLANE[0..7]

0xFF


uint32_t bytesPlane[0..7] = KHR_DF_MASK_BYTESPLANE[0..7] & (bdb[KHR_DF_WORD_BYTESPLANE[0..7]] >> KHR_DF_SHIFT_BYTESPLANE[0..7]);

[Note]

In versions of this specification prior to 1.3, there was no facility for the “extension bit”, and a bytesPlane0 value of 0 indicated a paletted format. The scheme for encoding paletted formats as of version 1.3 is described in Section 5.18.

5.12. Sample information

The layout and position of the information within each plane is determined by a number of samples, each consisting of a single channel of data and with a single corresponding position within the texel block, as shown in Table 78.

Table 78. Basic Data Format Descriptor Sample Information

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

F

S

E

L

channelType

bitLength

bitOffset

samplePosition3

samplePosition2

samplePosition1

samplePosition0

sampleLower

sampleUpper


Bits F, S, E and L are abbreviations for the following qualifier flags:

F

KHR_DF_SAMPLE_DATATYPE_FLOAT

0x80

S

KHR_DF_SAMPLE_DATATYPE_SIGNED

0x40

E

KHR_DF_SAMPLE_DATATYPE_EXPONENT

0x20

L

KHR_DF_SAMPLE_DATATYPE_LINEAR

0x10

The sample information begins at word KHR_DF_WORD_SAMPLESTART = 6, offset from the start of the basic descriptor block. Each sample occupies KHR_DF_WORD_SAMPLEWORDS = 4 32-bit words, and is stored consecutively.

The bytes from the plane data contributing to the format are treated as though they have been concatenated into a bit stream, with the first byte of the lowest-numbered plane providing the lowest bits of the result. Each sample consists of a number of consecutive bits from this bit stream.

If the content for a channel cannot be represented in a single sample, for example because the data for a channel is non-consecutive within this bit stream, additional samples with the same coordinate position and channel number should follow from the first, in order increasing from the least significant bits from the channel data; the corresponding bits from the bit stream are concatenated in the increasing order of reference to provide the value representing the channel.

For example, some native big-endian formats may need to be supported with multiple samples in a channel, since the constituent bits may not be consecutive in a little-endian interpretation. There is an example, Table 97, in the list of example format descriptors provided.

See Section 3.4 for more information about the order in which samples should appear in the descriptor block.

The number of samples present in the format is determined by the descriptorBlockSize field:

numSamples = (((KHR_DFDVAL(BDB, DESCRIPTORBLOCKSIZE) >> 2) - KHR_DF_WORD_SAMPLESTART) / KHR_DF_WORD_SAMPLEWORDS);

The macro KHR_DFDSAMPLECOUNT(BDB) is provided to perform this calculation.

There is no limit on the number of samples which may be present, other than the maximum size of the Data Format Descriptor Block. There is no requirement that samples should access unique parts of the bit-stream: formats such as combined intensity and alpha, or shared exponent formats, require that bits be reused. Nor is there a requirement that all the bits in a plane be used (a format may contain padding).

It is unusual but legal for a descriptor block to contain no samples provided the color model is KHR_DF_MODEL_UNSPECIFIED. See Section 5.19 for details.

To simplify code using the Basic Data Format Descriptor Block, the header khr_df.h provides enums of the following form for accessing sample fields:

Table 79. Field location information for sample field xxx

Word offset relative to start of sample

KHR_DF_SAMPLEWORD_xxx + 1

Start bit within word

KHR_DF_SAMPLESHIFT_xxx

Bit mask of value

KHR_DF_SAMPLEMASK_xxx


If the basic descriptor block is treated as a uint32_t array bdb[], sample field xxx can be accessed as follows:

xxx = KHR_DF_SAMPLEMASK_xxx & (bdb[KHR_DF_WORD_SAMPLESTART + sample × KHR_DF_WORD_SAMPLEWORDS + KHR_DF_SAMPLEWORD_xxx] >> KHR_DF_SAMPLESHIFT_xxx);

The macro KHR_DFDSVAL(BDB, S, X) is provided to perform this calculation.

For example, KHR_DFDSVAL(bdb, 2, CHANNELID) returns the value:

KHR_DF_SAMPLEMASK_CHANNELID & (bdb[KHR_DF_WORD_SAMPLESTART + 2 × KHR_DF_WORD_SAMPLEWORDS + KHR_DF_SAMPLEWORD_CHANNELID] >> KHR_DF_SAMPLESHIFT_CHANNELID)

5.13. Sample bitOffset

The bitOffset field describes the offset of the least significant bit of this sample from the least significant bit of the least significant byte of the concatenated bit stream for the format. Typically the bitOffset of the first sample is therefore 0; a sample which begins at an offset of one byte relative to the data format would have a bitOffset of 8. The bitOffset is an unsigned 16-bit integer quantity.

In the special case that the bitOffset field contains the reserved value 0xFFFF, the sample contributes a constant value of the specified bit length, encoded in the sampleLower field. This mechanism notably supports values that are zero-extended.

Table 80. Field location information for sample bitOffset

Word offset relative to start of sample

KHR_DF_SAMPLEWORD_BITOFFSET

0

Start bit within word

KHR_DF_SAMPLESHIFT_BITOFFSET

0

Bit mask of value

KHR_DF_SAMPLEMASK_BITOFFSET

0xFFFFU


uint32_t bitoffset = KHR_DF_SAMPLEMASK_BITOFFSET & ((bdb[KHR_DF_WORD_SAMPLESTART + (sample × KHR_DF_WORD_SAMPLEWORDS) + KHR_DF_SAMPLEWORD_BITOFFSET]) >> KHR_DF_SAMPLESHIFT_BITOFFSET);

5.14. Sample bitLength

The bitLength field describes the number of consecutive bits from the concatenated bit stream that contribute to the sample. This field is an unsigned 8-bit integer quantity, and stores the number of bits contributed minus 1; thus a single-byte channel should have a bitLength field value of 7. If a bitLength of more than 256 is required, further samples should be added; the value for the sample is composed in increasing order from least to most significant bit as subsequent samples are processed.

Note that a large bitLength value means a sample can encode more bits than can be described in the sampleLower and sampleUpper fields. If the rules for expanding the sampleLower and sampleUpper values in this case produce the desired result, bitLength can exceed 32; otherwise the bitLength should be limited and multiple samples used to encode sampleLower and sampleUpper: there is no way to indicate a sample contribution of zero bits for a sample that exists only to expand upon sampleLower and sampleUpper.

Except in the case of a paletted texture (described in Section 5.18) or where the special bitOffset value 0xFFFF is used to indicate constant bits, the bitLength added to bitOffset should not be greater than eight times the total number of bytes contributed to the logical bit stream by the bytesPlane values.

In the descriptor block examples in this specification, bit lengths are shown as the value to be stored in the bitLength field, but with the represented number (without the -1 offset) in parentheses for clarity.

Table 81. Field location information for sample bitLength

Word offset relative to start of sample

KHR_DF_SAMPLEWORD_BITLENGTH

0

Start bit within word

KHR_DF_SAMPLESHIFT_BITLENGTH

16

Bit mask of value

KHR_DF_SAMPLEMASK_BITLENGTH

0xFF


uint32_t bitLength = KHR_DF_SAMPLEMASK_BITLENGTH & ((bdb[KHR_DF_WORD_SAMPLESTART + (sample × KHR_DF_WORD_SAMPLEWORDS) + KHR_DF_SAMPLEWORD_BITLENGTH]) >> KHR_DF_SAMPLESHIFT_BITLENGTH);

5.15. Sample channelType and qualifiers

The channelType field is an unsigned 8-bit quantity.

The bottom four bits of the channelType indicates which channel is being described by this sample. The list of available channels is determined by the colorModel field of the Basic Data Format Descriptor Block, and the channelType field contains the number of the required channel within this list — see the colorModel field for the list of channels for each model.

Table 82. Field location information for sample channelType

Word offset relative to start of sample

KHR_DF_SAMPLEWORD_CHANNELID

0

Start bit within word

KHR_DF_SAMPLESHIFT_CHANNELID

24

Bit mask of value

KHR_DF_SAMPLEMASK_CHANNELID

0xF


khr_df_model_channels_e channelType = KHR_DF_SAMPLEMASK_CHANNELID & ((bdb[KHR_DF_WORD_SAMPLESTART + (sample × KHR_DF_WORD_SAMPLEWORDS) + KHR_DF_SAMPLEWORD_CHANNELID]) >> KHR_DF_SAMPLESHIFT_CHANNELID);

The top four bits of the channelType are described by the khr_df_sample_datatype_qualifiers_e enumeration:

Table 83. Field location information for sample qualifiers

Word offset relative to start of sample

KHR_DF_SAMPLEWORD_QUALIFIERS

0

Start bit within word

KHR_DF_SAMPLESHIFT_QUALIFIERS

24

Bit mask of value

KHR_DF_SAMPLEMASK_QUALIFIERS

0xF0


khr_df_sample_datatype_qualifiers_e qualifiers = KHR_DF_SAMPLEMASK_QUALIFIERS & ((bdb[KHR_DF_WORD_SAMPLESTART + (sample × KHR_DF_WORD_SAMPLEWORDS) + KHR_DF_SAMPLEWORD_QUALIFIERS]) >> KHR_DF_SAMPLESHIFT_QUALIFIERS);

If the KHR_DF_SAMPLE_DATATYPE_EXPONENT bit, shown as E in Table 78, is not set, the sample contributes to a base value; multiple samples with the same color channel (bottom four bits of channelType) and same samplePos values are accumulated into the virtual sample in increasing bit order from lowest to highest.

For samples in which the KHR_DF_SAMPLE_DATATYPE_EXPONENT bit is not set:

  • If the KHR_DF_SAMPLE_DATATYPE_LINEAR bit, shown as L in Table 78, is not set, the final sample value (after any modifier has been applied to the base value) is modified by the transfer function defined in the transferFunction field of the descriptor; if this bit is set, the sample is considered to contain a linearly-encoded value irrespective of the format’s transferFunction. All samples referring to the same base value should have the same value stored in the KHR_DF_SAMPLE_DATATYPE_LINEAR bit.
  • If the KHR_DF_SAMPLE_DATATYPE_SIGNED bit, shown as S in Table 78, is set, the sample holds a signed value in two’s complement form. If this bit is not set, the sample holds an unsigned value. It is possible to represent a sign/magnitude integer value by having a sample of unsigned integer type with the same channel and sample location, as a 1-bit signed sample.
  • If the KHR_DF_SAMPLE_DATATYPE_FLOAT bit, shown as F in Table 78, is set, the sample holds floating point data in a conventional format of 10, 11 or 16 bits, as described in Section 10, or of 32, or 64 bits as described in [IEEE 754]. Unless a genuine unsigned format is intended, KHR_DF_SAMPLE_DATATYPE_SIGNED (bit S) should also be set. Less common floating point representations can be generated with multiple samples and a combination of signed integer, unsigned integer and exponent fields, as described above and in Section 10.4.

If the KHR_DF_SAMPLE_DATATYPE_EXPONENT bit, shown as E in Table 78, is set, the sample applies a modifier to the base value, with the interpretation of the modifier determined according to Table 84. In this case, the virtual sample contains both a base value and a modifier.

All samples contributing to a modifier for the same base value should contain the same L and F bits (it is not legal, for example, to define both a multiplier and a divisor). Samples which apply a modifier should directly follow the samples that describe the base value. If no samples have the E bit set for this channel and position, the base value directly represents the pixel value; it is not legal for a virtual sample to describe a modifier but no base value.

[Note]

The same bits of the format may contribute to modifiers for more than one channel — this is commonly the case for high dynamic range formats with a shared exponent or divisor. The descriptor in this case should contain samples for each color channel in turn, with the description of the shared bits replicated for each channel, as shown in Table 98.

Table 84. Qualifier interpretation when KHR_DF_SAMPLE_DATATYPE_EXPONENT = 1

E

L

F

  Interpretation

Formula

1

0

0

  Exponent

$\mathit{base\ value}\times 2^\mathit{modifier}$

1

0

1

  Multiplier

$\mathit{base\ value}\times\mathit{modifier}$

1

1

0

  Divisor

$\mathit{base\ value}\over\mathit{modifier}$

1

1

1

  Power

$\mathit{base\ value}^\mathit{modifier}$


For samples in which the KHR_DF_SAMPLE_DATATYPE_EXPONENT bit is set:

  • If the KHR_DF_SAMPLE_DATATYPE_LINEAR and KHR_DF_SAMPLE_DATATYPE_FLOAT bits are clear, this modifier holds an exponent (in integer form) describing a floating-point offset for this channel. For example, this would be used to describe the exponent of a custom floating point format, as shown in Table 103, or a shared exponent location in shared exponent formats (with the exponent bits listed separately under each channel as shown in Table 98). If this modifer is used, the base value is considered to contain mantissa information and the samples describing it would not normally have the KHR_DF_SAMPLE_DATATYPE_FLOAT bit set. If the KHR_DF_SAMPLE_DATATYPE_SIGNED bit (S) is also set, the exponent is considered to be two’s complement — otherwise it is treated as unsigned. The bias of the exponent can be determined by the exponent’s sampleLower value. The presence or absence of an implicit leading digit in the mantissa of a format with an exponent can be determined by the sampleUpper value of the mantissa. The use of the exponent is described in more detail in Section 10.4.
  • If the KHR_DF_SAMPLE_DATATYPE_FLOAT bit is set and the KHR_DF_SAMPLE_DATATYPE_LINEAR bit is clear, this sample holds a multiplier (in integer form) for this channel, such that the encoded value is a product of this modifier value and the base value. This approach is useful for encoding a shared multiplier as part of a high dynamic range color image, for example.
  • If the KHR_DF_SAMPLE_DATATYPE_LINEAR bit is set and the KHR_DF_SAMPLE_DATATYPE_FLOAT bit is clear, this sample holds a divisor (in integer form) for this channel, such that the encoded value is the base value divided by this modifier value. This approach is useful for encoding a shared divisor as part of a high dynamic range color image, for example.
  • If both KHR_DF_SAMPLE_DATATYPE_FLOAT and KHR_DF_SAMPLE_DATATYPE_LINEAR are set, this sample holds a power term (in integer form) for this channel, such that the encoded value is the base value raised to the power of the modifier value. This approach is useful for encoding a shared multiplier as part of a high dynamic range color image, for example.

Note that in the multiplier, divisor and power cases, the sampleLower and sampleUpper values allow the modifier value to be represented in fixed-point terms, and the values may be signed depending on whether the S bit is set.

5.16. samplePosition[0..3]

The sample has an associated location within the 4-dimensional space of the texel block. Therefore each sample has an offset relative to the 0,0 position of the texel block, represented as an 8-bit unsigned integer quantity.

Table 85. Field location information for sample samplePosition[0..3]

Word offset relative to start of sample

KHR_DF_SAMPLEWORD_SAMPLEPOSITION[0..3]

1

Start bit within word

KHR_DF_SAMPLESHIFT_SAMPLEPOSITION0

0

KHR_DF_SAMPLESHIFT_SAMPLEPOSITION1

8

KHR_DF_SAMPLESHIFT_SAMPLEPOSITION2

16

KHR_DF_SAMPLESHIFT_SAMPLEPOSITION3

24

Bit mask of value

KHR_DF_SAMPLEMASK_SAMPLEPOSITION[0..3]

0xF


khr_df_model_channels_e samplePosition0 = KHR_DF_SAMPLEMASK_SAMPLEPOSITION0 & ((bdb[KHR_DF_WORD_SAMPLESTART + (sample × KHR_DF_WORD_SAMPLEWORDS) + KHR_DF_SAMPLEWORD_SAMPLEPOSITION0]) >> KHR_DF_SAMPLESHIFT_SAMPLEPOSITION0);

khr_df_model_channels_e samplePosition1 = KHR_DF_SAMPLEMASK_SAMPLEPOSITION1 & ((bdb[KHR_DF_WORD_SAMPLESTART + (sample × KHR_DF_WORD_SAMPLEWORDS) + KHR_DF_SAMPLEWORD_SAMPLEPOSITION1]) >> KHR_DF_SAMPLESHIFT_SAMPLEPOSITION1);

khr_df_model_channels_e samplePosition2 = KHR_DF_SAMPLEMASK_SAMPLEPOSITION2 & ((bdb[KHR_DF_WORD_SAMPLESTART + (sample × KHR_DF_WORD_SAMPLEWORDS) + KHR_DF_SAMPLEWORD_SAMPLEPOSITION2]) >> KHR_DF_SAMPLESHIFT_SAMPLEPOSITION2);

khr_df_model_channels_e samplePosition3 = KHR_DF_SAMPLEMASK_SAMPLEPOSITION3 & ((bdb[KHR_DF_WORD_SAMPLESTART + (sample × KHR_DF_WORD_SAMPLEWORDS) + KHR_DF_SAMPLEWORD_SAMPLEPOSITION3]) >> KHR_DF_SAMPLESHIFT_SAMPLEPOSITION3);

The interpretation of each samplePosition field depends on the corresponding texelBlockDimension value as follows:

\begin{align*} n &= \left\lceil\textrm{log}_2(\textbf{texelBlockDimension} + 1)\right\rceil \\ \textit{coordinateOffset} &= \textbf{samplePosition} \times 2^{n - 8} \end{align*}

For example, if texelBlockDimension0 is 1 (indicating a texel block width of two units) samplePosition0 is described in units of ${2\over 256} = {1\over 128}$ . That is, a samplePosition0 of 128 would encode an offset of “1.0” and a samplePosition0 of 64 would encode an offset of “0.5”. If texelBlockDimension0 is 5 (indicating a texel block width of six units), samplePosition0 is described in units of ${8\over 256} = {1\over 32}$ . That is, a samplePosition0 of 64 would encode an offset of “2.0” and a samplePosition0 of 24 would encode an offset of “0.75”.

The adjusted coordinateOffset must be less than the corresponding texel block dimension. That is, since coordinateOffset can represent fractional offsets, coordinateOffset < (texelBlockDimension + 1) for each dimension.

This approach allows the common situation of downsampled channels to have samples conceptually sited at the midpoint between full resolution samples. The direction of the sample offsets is determined by the coordinate addressing scheme used by the API. There is no limit on the dimensionality of the data, but if more than four dimensions need to be contained within a single texel block, an extension will be required.

It is legal, but unusual, to use the same bits to represent multiple samples at different coordinate locations.

[Note]

Versions of this specification prior to 1.3 always recorded the sample position in a 7.1 fixed-point format (with half-coordinate granularity). This change does not affect the representation of single-coordinate texel blocks; that is, a samplePosition of “0” still represents “0.0”.

5.17. sampleLower and sampleUpper

The sampleLower and sampleUpper fields are used to define the mapping between the numerical value stored in the format and the conceptual numerical interpretation.

For unsigned formats, sampleLower typically represents the value which should be interpreted as zero (the black point). For signed formats, sampleLower typically represents “-1”. For color difference models such as Y′CBCR, sampleLower for chroma channels represents the lower extent of the color difference range (which corresponds to an encoding of -0.5 in numerical terms).

sampleUpper typically represents the value which should be interpreted as “1.0” (the “white point”). For color difference models such as Y′CBCR, sampleUpper for chroma channels represents the upper extent of the color difference range (which corresponds to an encoding of 0.5 in numerical terms).

Equation 1. Sample range conversion rules

\begin{align*} \textit{out}_\textit{unsigned} &= \left({{\textit{value} - \textbf{sampleLower}}\over{\textbf{sampleUpper} - \textbf{sampleLower}}}\right) \\ \textit{out}_\textit{signed} &= \left({{\textit{value} - \textbf{sampleLower}}\over{\textbf{sampleUpper} - \textbf{sampleLower}}} - 0.5\right) \times 2 \\ \textit{out}_\textit{color difference} &= \left({{\textit{value} - \textbf{sampleLower}}\over{\textbf{sampleUpper} - \textbf{sampleLower}}} - 0.5\right) \end{align*}

For example, the BT.709 television broadcast standard dictates that the Y′ value stored in an 8-bit encoding should fall between the range 16 and 235, as described in Section 16.1 encoding”. In this case, sampleLower should contain the value 16 and sampleUpper 235.

Sensor data from a camera typically does not cover the full range of the bit depth used to represent it. sampleUpper can be used to specify an upper limit on sensor brightness — or to specify the value which should map to white on the display, which may be less than the full dynamic range of the captured image.

There is no guarantee or expectation that image data be guaranteed to fall between sampleLower and sampleUpper unless the users of a format agree that convention. For example, high dynamic range video formats may define “1.0” as a nominal brightness level substantially lower than the maximum, and coordinates may encode an arbitrary range. In some formats, the integer value should be interpreted directly as a number, in which case sampleUpper and sampleLower should hold “1” and either “0” or “-1” depending on whether the format is signed, respectively.

If the channel encoding is an integer format, the sampleLower and sampleUpper values are represented as 32-bit integers — signed or unsigned according to whether the channel encoding is signed. Signed negative values should be sign-extended if the channel has fewer than 32 bits, such that the value encoded in sampleLower and sampleUpper values are themselves negative if the encoded values are negative. If the channel encoding is a floating point value, the sampleLower and sampleUpper values are also described in floating point.

If the number of bits in the sample is greater than 32, sampleLower and sampleUpper are converted to a data type of the actual number of bits as follows: A floating point value is converted to the native representation (for example, a float value is converted to double in the normal way). An integer value is expanded by preserving any sign bit and replicating the top non-sign bit (for example, signed 0x80000000 is extended to the 40-bit value 0x8000000000). If these rules do not produce the desired result, it may be necessary to describe the contribution to the channel in multiple samples of no more than 32 bits. In this case, the samples corresponding to lower bitOffset values should occupy 32 bits, with any residual bits encoded in samples corresponding to higher bitOffset values.

If multiple samples contribute to a single value, for example because the bits of a channel are non-contiguous in the logical bit stream, the sampleLower and sampleUpper fields of each sample are concatenated in increasing bit order to produce virtual sampleLower and sampleUpper values; each sample contributes a number of bits equal to the number of data bits that the sample describes. If this contribution is fewer than 32 bits and the value being encoded is signed, the sign bit of the final sampleLower and sampleUpper values should be used to pad the fields of the sample.

For example, if a signed 16-bit value with a minimum value of -32767 (0xFFFE) is described as two 8-bit values, the first sample should have a sampleLower value of 0xFFFFFFFE — of which the bottom 8 bits (0xFE) corresponds to the bottom 8 bits of the final minimum value and the upper 24 bits (0xFFFFFF00) are a result of replicating the sign of bit 15 of the final minimum value. The second sample should have a sampleLower value of 0xFFFFFFFF — of which the bottom 8 bits (0xFF) correspond to the top 8 bits of the final minimum value and the upper 24 bits (0xFFFFFF00) are a result of sign-extending this value. Only the sample corresponding to the top bits of the channel may have a sampleLower or sampleUpper occupying more bits than the input.

The sampleLower value for an exponent should represent the exponent bias — the value that should be subtracted from the encoded exponent to indicate that the mantissa’s sampleUpper value will represent 1.0. The sampleUpper value for an exponent should represent the largest conventional legal exponent value. If the encoded exponent exceeds this value, the encoded floating point value encodes either an infinity or a NaN value, depending on the mantissa. See Section 10.4 for more detail on this.

If the channel encoding is the mantissa of a custom floating point format (that is, the encoding is integer but the same sample location and channel is shared by a sample that encodes an exponent), the presence of an implicit “1” digit can be represented by setting the sampleUpper value to a value one larger than can be encoded in the available bits for the mantissa, as described in Section 10.4.

In OpenGL terminology, a “normalized” channel contains an integer value which is mapped to the range 0..1.0; a channel which is not normalized contains an integer value which is mapped to a floating point equivalent of the integer value. Similarly an “snorm” channel is a signed normalized value mapping from -1.0 to 1.0. Setting sampleLower to the minimum signed integer value representable in the channel (which is often the negative version of the maximum signed integer, for example -127 rather than -128 for an 8-bit value in order to allow the value 0.0 to be represented exactly) is equivalent to defining an “snorm” texture. Setting sampleUpper to the maximum signed integer value representable in the channel for a signed channel type is equivalent to defining an “snorm” texture. Setting sampleUpper to the maximum unsigned value representable in the channel for an unsigned channel type is equivalent to defining a “normalized” texture. Setting sampleUpper to “1” is equivalent to defining an “unnormalized” texture.

In the special case that the sample bitOffset field is 0xFFFF, only the bottom 16 bits of the sampleLower field indicate a contribution to the sample lower limit; the upper 16 bits are taken as a constant contribution to the interpreted value; in this case, the bitLength field of the sample must be no more than 16. For example, a 4-bit value which is interpreted as being zero-extended to eight bits before conversion may have four bits with the value 0 stored in bits 19..16 of sampleLower, indicated by a bitOffset of 0xFFFF.

These “virtual bits” may be needed to encode some numerical representations. For example, if an 8-bit integer encodes the value “-0.5” as 0 and “0.5” as 255 (in the manner of the color difference channel in Equation 1, but if we wish to apply this mapping to a channel other than color difference), Equation 1 suggests that sampleLower should hold -127.5 and sampleUpper should hold 382.5. This is impossible to encode, since sampleLower and sampleUpper are integers. However, if a virtual 0-bit is added below the encoded value (such that the effective range is 0..510), storing sampleLower as -255 and sampleUpper as 765 will have the desired effect. Note that in this case, the original 8-bits of data are unsigned, but the bounds must encode a negative value; this can be achieved by describing the data as a signed value that is guaranteed to be positive by another sample storing a virtual 0 bit in bit 9 (so the data is treated as a signed 10-bit quantity, of which the top and bottom bits are guaranteed to be 0). The cost of this flexibility is that multiple samples are needed per channel, which is why the common case of chroma channels which should map to the -0.5..0.5 range are treated specially.

5.18. Paletted formats

The storage of the palette is considered to be outside the remit of this specification; however, the “format” describes both the encoding of the bits which index into the palette and the format of the entries in the palette itself.

[Note]

The convention for encoding paletted formats was different in revisions of this specification prior to 1.3.

If the bitOffset field of any of the samples equals eight times the total number of bytes indicated by the bytesPlane channels (that is, indicating an offset after the end of the logical bit stream), this sample indicates a palette entry. Samples with offsets within the range of the logical bit stream describe the index of the palette; sampleUpper is used to indicate the number of entries in the palette (which may be lower than could be addressed by the number of bits available). The index can be comprised of multiple samples, supporting non-contiguous bits.

The four bits encoding channelType indicate a choice of palette in the index samples. The palette entries indicate which palette they are associated with by encoding the corresponding palette id in the samplePosition0 field, with the other samplePosition fields set to 0. This approach allows both a simple palette in which each entry represents a complete color and per-channel look-up tables.

The index samples should be described first, as though the format were not paletted; samples for palette entries should then follow, sorted first by palette id, then by channel id.

The descriptor’s colorModel, colorPrimaries, transferFunction and flags apply to the palette entries. The texelBlockDimension values on the other hand refer to the storage of per-texel indices.

Table 94 shows a 240-entry palette with an index encoded in eight bits, and six bits for each of R, G and B per palette entry. Table 95 shows a 24-bit format with 256-entry palettes indexed by each of three 256-bit channels, with a 10-bit R, G and B palette associated with the corresponding channels.

5.19. Unsized formats

The data format descriptor can be a convenient representation for describing some data which does not follow the constraint that texel blocks are of a fixed size. For example, it may be useful to use a descriptor to encode the color space of an image which is compressed with a variable-rate compressor (meaning there is no data-independent mapping between memory locations and corresponding pixels). There are two ways to do this, each with its own uses. Only the color primaries and transfer function may be encoded or these and the presence of color and alpha samples may be encoded.

In the first case KHR_DF_MODEL_UNSPECIFIED is used in a descriptor with no samples. In this case all texelBlockDimension and bytesPlane values must be zero.

In the second case the appropriate color model is used, e.g.KHR_DF_MODEL_RGBSDA. The presence of color and alpha samples is used to indicate the presence of these samples in the image. In this case, all texelBlockDimension and bytesPlane values and the bitOffset, bitLength and samplePosition fields of any samples must be set to 0.

5.20. C99 struct mapping (informative)

The basic descriptor block has been specified in terms of an array of uint32_t value. C99 provides a more direct representation, but this relies on the bit ordering of bitfields (which is implementation-defined) and flexible array members, which are not supported in C++.

In the interests of portability, the following summary (which assumes that bitfields are encoded starting at bit 0) is therefore provided for information, but is not canonical:

typedef struct _DFDSampleType {
  uint32_t bitOffset: 16;
  uint32_t bitLength: 8;
  uint32_t channelType: 8; // Includes qualifiers
  uint32_t samplePosition0: 8;
  uint32_t samplePosition1: 8;
  uint32_t samplePosition2: 8;
  uint32_t samplePosition3: 8;
  uint32_t lower;
  uint32_t upper;
} DFDSampleType;

typedef struct _BasicDataFormatDescriptor {
  uint32_t vendorId: 17;
  uint32_t descriptorType: 15;
  uint32_t model: 8;
  uint32_t primaries: 8;
  uint32_t transfer: 8;
  uint32_t flags: 8;
  uint32_t texelBlockDimension0: 8;
  uint32_t texelBlockDimension1: 8;
  uint32_t texelBlockDimension2: 8;
  uint32_t texelBlockDimension3: 8;
  uint32_t bytesPlane0: 8;
  uint32_t bytesPlane1: 8;
  uint32_t bytesPlane2: 8;
  uint32_t bytesPlane3: 8;
  uint32_t bytesPlane4: 8;
  uint32_t bytesPlane5: 8;
  uint32_t bytesPlane6: 8;
  uint32_t bytesPlane7: 8;
  DFDSampleType samples[];
} BasicDataFormatDescriptor;

6. Extension for more complex formats

Some formats will require more channels than can be described in the Basic Format Descriptor, or may have more specific color requirements. For example, it is expected than an extension will be available which places an ICC color profile block into the descriptor block, allowing more color channels to be specified in more precise ways. This will significantly enlarge the space required for the descriptor, and is not expected to be needed for most common uses. A vendor may also use an extension block to associate metadata with the descriptor — for example, information required as part of hardware rendering. So long as software which uses the data format descriptor always uses the totalSize field to determine the size of the descriptor, this should be transparent to user code.

The extension mechanism is the preferred way to support even simple extensions such as additional color spaces transfer functions that can be supported by an additional enumeration. This approach improves compatibility with code which is unaware of the additional values. Simple extensions of this form that have cross-vendor support have a good chance of being incorporated more directly into future revisions of the specification, allowing application code to distinguish them by the versionId field.

If bit 13, KHR_DF_KHR_DESCRIPTORTYPE_NEEDED_FOR_WRITE_BIT, of the descriptorType field of an extension is set, an application must understand the extension in order to write data coherently. If this bit is clear, copying the bits which correspond to one texel to another can be expected to result in a correct transfer of the texel even if the application does not understand the extension.

If bit 14, KHR_DF_KHR_DESCRIPTORTYPE_NEEDED_FOR_DECODE_BIT, of the descriptorType field of an extension is set, an application must understand the extension in order to decode the contents of the texel coherently. If this bit is clear, the data held in the extension can be considered to be “informative” and that ignoring the extension will still result in correct values to the extent specified by the basic descriptor block. For example, an extension may associate an absolute brightness level with a format, but software which does not have need of this concept can continue processing the texel contents correctly.

As an example of the description of an extension, consider a single-channel 32-bit depth buffer, as shown in Table 86. A tiled renderer may wish to indicate that this buffer is “virtual”: it will be allocated real memory only if needed, and will otherwise exist only a subset at a time in an on-chip representation. Someone developing such a renderer may choose to add a vendor-specific extension (with ID 0x1FFFF to indicate development work and avoid the need for a vendor ID) which uses a boolean to establish whether this depth buffer exists only in virtual form. Note that the mere presence or absence of this extension within the data format descriptor itself forms a boolean, but for this example we will assume that an extension block is always present, and that a boolean is stored within. We will give the enumeration 32 bits, in order to simplify the possible addition of further extensions and pad to the alignment requirements.

In this example (which should not be taken as an implementation suggestion), the data descriptor would first contain a descriptor block describing the depth buffer format as conventionally described, followed by a second descriptor block that contains only the enumeration. The descriptor itself has a totalSize that includes both of these descriptor blocks. Note that KHR_DF_KHR_DESCRIPTORTYPE_NEEDED_FOR_WRITE_BIT is not set, indicating that depth data can be written without knowing about the extension, and KHR_DF_KHR_DESCRIPTORTYPE_NEEDED_FOR_DECODE_BIT is not set, indicating that software can safely ignore the information about the form of allocation while reading texel values.

Table 86. Example of a depth buffer with an extension to indicate a virtual allocation

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 56 — total size of the two blocks plus one 32-bit value

Basic descriptor block

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 1) = 40

versionNumber: 2

flags

transferFunction

colorPrimaries

colorModel

ALPHA_STRAIGHT

UNSPECIFIED

UNSPECIFIED

RGBSDA

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

0

0

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 4

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

Sample information for depth

1

1

0

0

DEPTH

bitLength: 31 (= “32”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0xBF800000U — -1.0f

sampleUpper: 0x7F800000U — 1.0f

Extension descriptor block

descriptorType: 0

vendorId: 0x1FFFF

descriptorBlockSize: 8 + (4 × 1) = 12

versionNumber: 0

Data specific to the extension follows

1 — buffer is “virtual”


It is possible for a vendor to use the extension block to store peripheral information required to access the image — plane base addresses, stride, etc. Since different implementations have different kinds of non-linear ordering and proprietary alignment requirements, this is not described as part of the standard. By many conventional definitions, this information is not part of the “format”, and particularly it ensures that an identical copy of the image will have a different descriptor block (because the addresses will have changed) and so a simple bitwise comparison of two descriptor blocks will disagree even though the “format” matches. Additionally, many APIs will use the format descriptor only for external communication, and have an internal representation that is more concise and less flexible. In this case, it is likely that address information will need to be represented separately from the format anyway. For these reasons, it is an implementation choice whether to store this information in an extension block, and how to do so, rather than being specified in this standard.

7. Additional planes descriptor block

Under some relatively unusual circumstances, either the number of planes described by the basic descriptor block or the number of bytes that can be contributed to a texel may be insufficient to describe the memory layout of bulk data. For example, a format may describe 12-bit colors (4 bits each or red, green and blue), and have the contributing bits stored as separate planes. An extension descriptor block, with vendorId = KHR_DF_VENDORID_KHRONOS and descriptorType = KHR_DF_DESCRIPTORTYPE_ADDITIONAL_PLANES, describes additional planes.

Table 87. Additional planes descriptor block

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

descriptorType: 0x6001

vendorId: 0

descriptorBlockSize: 8 + (4 × number of planes)

versionNumber: 0

bytesPlane0

bytesPlane1 (optional)

(etc.)


If this descriptor block is present, the bytesPlane[0..7] fields of the basic descriptor block are ignored, and the number of bytes for plane n is taken directly from word n + 2 of the descriptor block. The number of planes described can be determined by the descriptor block size.

This extension both allows an arbitrary number of planes and makes it easy to specify planes that contribute a large number of bytes to the virtual bit stream. Note that the sample bitOffset field remains limited to 16 bits, so the total texel block memory footprint is limited.

[Note]

Since knowing the bit stream contribution from all planes is necessary when interpreting data, this descriptorType sets KHR_DF_KHR_DESCRIPTORTYPE_NEEDED_FOR_DECODE_BIT. Since the memory mapping is necessary when writing data, this descriptorType sets KHR_DF_KHR_DESCRIPTORTYPE_NEEDED_FOR_WRITE_BIT.

This descriptor block should be used only if the Khronos Basic Descriptor Block is the first descriptor block in the data format descriptor, and cannot represent the format without extension.

8. Additional dimensions descriptor block

The basic descriptor block allows texel blocks of up to four non-trivial dimensions, and with a texel block size of up to 256 coordinate units, with sample positions described in precision up to $1\over 256$ of a coordinate. Under rare circumstances, this may provide insufficient flexibility. An extension descriptor block, with vendorId = KHR_DF_VENDORID_KHRONOS and descriptorType = KHR_DF_DESCRIPTORTYPE_ADDITIONAL_DIMENSIONS, describes additional dimensions. Note that in some cases where this solution might be useful, a texel block is an inappropriate unit. For example, this extension block allows the direct representation of large texel tiling patterns, but it does so in a manner that is very inefficient, having much more potential flexibility than is needed by the users of the layout being described.

Table 88. Additional dimensions descriptor block

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

descriptorType: 0x6002

vendorId: 0

descriptorBlockSize: 8+(4 × dimensions × (samples+1))

versionNumber: 0

uint32_t texelBlockDimension0

uint32_t texelBlockDimension1 (optional)

(etc.)

uint16_t sample0Pos0

uint16_t sample0Pos0Divisor

uint16_t sample0Pos1

uint16_t sample0Pos1Divisor

(etc.)


The fields texelBlockDimension[0..n] describe the size in coordinate units of the texel block in the corresponding dimension; as with the basic descriptor block, the value stored is the corresponding dimension minus 1 (so a stored value of “0” corresponds to a dimension of 1). The texelBlockDimension[0..7] fields of the basic descriptor block are ignored.

For each sample, the samplePos and samplePosDivisor fields store a numerator and denominator pair for the coordinate $\textit{offset} = {\textit{numerator}\over\textit{denominator}}$ for each dimension, with all dimensions for a sample described before describing the next sample. samplePos and samplePosDivisor should be minimized, leaving an irreducible fraction. The samplePos fields of the basic descriptor block are ignored. These fields are present even for samples corresponding to palette entries.

Since the number of samples in the texel block can be deduced from the size of the basic descriptor block, the number of dimensions that are to be described by the additional dimensions descriptor block can be deduced from its size: numDimensions = (descriptorBlockSize - 8) / (numSamples + 1).

[Note]

Since knowing the mapping from coordinates to samples is necessary when interpreting data, this descriptorType sets KHR_DF_KHR_DESCRIPTORTYPE_NEEDED_FOR_DECODE_BIT. Since the coordinate mapping is necessary for writing data, this descriptorType sets KHR_DF_KHR_DESCRIPTORTYPE_NEEDED_FOR_WRITE_BIT.

This descriptor block should be used only if the Khronos Basic Descriptor Block is the first descriptor block in the data format descriptor, and cannot represent the format without extension.

9. Frequently Asked Questions

9.1. Why have a binary format rather than a human-readable one?

While it is not expected that every new container will have a unique data descriptor or that analysis of the data format descriptor will be on a critical path in an application, it is still expected that comparison between formats may be time-sensitive. The data format descriptor is designed to allow relatively efficient queries for subsets of properties, to allow a large number of format descriptors to be stored, and to be amenable to hardware interpretation or processing in shaders. These goals preclude a text-based representation such as an XML schema.

9.2. Why not use an existing representation such as those on FourCC.org?

Formats in FourCC.org do not describe in detail sufficient information for many APIs, and are sometimes inconsistent.

9.3. Why have a descriptive format?

Enumerations are fast and easy to process, but are limited in that any software can only be aware of the enumeration values in place when it was defined. Software often behaves differently according to properties of a format, and must perform a look-up on the enumeration — if it knows what it is — in order to change behaviors. A descriptive format allows for more flexible software which can support a wide range of formats without needing each to be listed, and simplifies the programming of conditional behavior based on format properties.

9.4. Why describe this standard within Khronos?

Khronos supports multiple standards that have a range of internal data representations. There is no requirement that this standard be used specifically with other Khronos standards, but it is hoped that multiple Khronos standards may use this specification as part of a consistent approach to inter-standard operation.

9.5. Why should I use this descriptor if I don’t need most of the fields?

While a library may not use all the data provided in the data format descriptor that is described within this standard, it is common for users of data — particularly pixel-like data — to have additional requirements. Capturing these requirements portably reduces the need for additional metadata to be associated with a proprietary descriptor. It is also common for additional functionality to be added retrospectively to existing libraries — for example, Y′CBCR support is often an afterthought in rendering APIs. Having a consistent and flexible representation in place from the start can reduce the pain of retrofitting this functionality.

Note that there is no expectation that the format descriptor from this standard be used directly, although it can be. The impact of providing a mapping between internal formats and format descriptors is expected to be low, but offers the opportunity both for simplified access from software outside the proprietary library and for reducing the effort needed to provide a complete, unambiguous and accurate description of a format in human-readable terms.

9.6. Why not expand each field out to be integer for ease of decoding?

There is a trade-off between size and decoding effort. It is assumed that data which occupies the same 32-bit word may need to be tested concurrently, reducing the cost of comparisons. When transferring data formats, the packing reduces the overhead. Within these constraints, it is intended that most data can be extracted with low-cost operations, typically being byte-aligned (other than sample flags) and with the natural alignment applied to multi-byte quantities.

9.7. Can this descriptor be used for text content?

For simple ASCII content, there is no reason that plain text could not be described in some way, and this may be useful for image formats that contain comment sections. However, since many multilingual text representations do not have a fixed character size, this use is not seen as an obvious match for this standard.

10. Floating-point formats

Some common floating-point numeric representations are defined in [IEEE 754]. Additional floating point formats are defined in this section.

10.1. 16-bit floating-point numbers

A 16-bit floating-point number has a 1-bit sign (S), a 5-bit exponent (E), and a 10-bit mantissa (M). The value V of a 16-bit floating-point number is determined by the following:

\[ V = \begin{cases} (-1)^S \times 0.0, & E = 0, M = 0 \\ (-1)^S \times 2^{-14} \times { M \over 2^{10} }, & E = 0, M \neq 0 \\ (-1)^S \times 2^{E-15} \times { \left( 1 + { M \over 2^{10} } \right) }, & 0 < E < 31 \\ (-1)^S \times \mathit{Inf}, & E = 31, M = 0 \\ \mathit{NaN}, & E = 31, M \neq 0 \end{cases} \]

If the floating-point number is interpreted as an unsigned 16-bit integer N, then

$$S = \left\lfloor { { N \bmod 65536 } \over 32768 } \right\rfloor$$ $$E = \left\lfloor { { N \bmod 32768 } \over 1024 } \right\rfloor$$ $$M = N \bmod 1024.$$

10.2. Unsigned 11-bit floating-point numbers

An unsigned 11-bit floating-point number has no sign bit, a 5-bit exponent (E), and a 6-bit mantissa (M). The value V of an unsigned 11-bit floating-point number is determined by the following:

\[ V = \begin{cases} 0.0, & E = 0, M = 0 \\ 2^{-14} \times { M \over 64 }, & E = 0, M \neq 0 \\ 2^{E-15} \times { \left( 1 + { M \over 64 } \right) }, & 0 < E < 31 \\ \mathit{Inf}, & E = 31, M = 0 \\ \mathit{NaN}, & E = 31, M \neq 0 \end{cases} \]

If the floating-point number is interpreted as an unsigned 11-bit integer N, then

$$E = \left\lfloor { N \over 64 } \right\rfloor$$ $$M = N \bmod 64.$$

10.3. Unsigned 10-bit floating-point numbers

An unsigned 10-bit floating-point number has no sign bit, a 5-bit exponent (E), and a 5-bit mantissa (M). The value V of an unsigned 10-bit floating-point number is determined by the following:

\[ V = \begin{cases} 0.0, & E = 0, M = 0 \\ 2^{-14} \times { M \over 32 }, & E = 0, M \neq 0 \\ 2^{E-15} \times { \left( 1 + { M \over 32 } \right) }, & 0 < E < 31 \\ \mathit{Inf}, & E = 31, M = 0 \\ \mathit{NaN}, & E = 31, M \neq 0 \end{cases} \]

If the floating-point number is interpreted as an unsigned 10-bit integer N, then

$$E = \left\lfloor { N \over 32 } \right\rfloor$$ $$M = N \bmod 32.$$

10.4. Non-standard floating point formats

Rather than attempting to enumerate every possible floating-point format variation in this specification, the data format descriptor can be used to describe the components of arbitrary floating-point data, as follows. Note that non-standard floating point formats do not use the KHR_DF_SAMPLE_DATATYPE_FLOAT bit.

An example of use of the 16-bit floating point format described in Section 10.1 but described in terms of a custom floating point format is provided in Table 103. Note that this is provided for example only, and this particular format would be better described using the standard 16-bit floating point format as documented in Table 104.

10.4.1. The mantissa

The mantissa of a custom floating point format should be represented as an integer channelType. If the mantissa represents a signed quantity encoded in two’s complement, the KHR_DF_SAMPLE_DATATYPE_SIGNED bit should be set. To encode a signed mantissa represented in sign-magnitude format, the main part of the mantissa should be represented as an unsigned integer quantity (with KHR_DF_SAMPLE_DATATYPE_SIGNED not set), and an additional one-bit sample with KHR_DF_SAMPLE_DATATYPE_SIGNED set should be used to identify the sign bit. By convention, a sign bit should be encoded in a later sample than the corresponding mantissa.

The sampleUpper and sampleLower values for the mantissa should be set to indicate the representation of 1.0 and 0.0 (for unsigned formats) or -1.0 (for signed formats) respectively when the exponent is in a 0 position after any bias has been corrected. If there is an implicit “1” bit, these values for the mantissa will exceed what can be represented in the number of available mantissa bits.

For example, the shared exponent formats shown in Table 98 does not have an implicit “1” bit, and therefore the sampleUpper values for the 9-bit mantissas are 256 — this being the mantissa value for 1.0 when the exponent is set to 0.

For the 16-bit signed floating point format described in Section 10.1, sampleUpper should be set to 1024, indicating the implicit “1” bit which is above the 10 bits representable in the mantissa. sampleLower should be 0 in this case, since the mantissa uses a sign-magnitude representation.

By convention, the sampleUpper and sampleLower values for a sign bit are 0 and -1 respectively.

10.4.2. The exponent

The KHR_DF_SAMPLE_DATATYPE_EXPONENT bit should be set in a sample which contains the exponent of a custom floating point format.

The sampleLower for the exponent should indicate the exponent bias. That is, the mantissa should be scaled by two raised to the power of the stored exponent minus this sampleLower value.

The sampleUpper for the exponent indicates the maximum legal exponent value. Values above this are used to encode infinities and not-a-number (NaN) values. sampleUpper can therefore be used to indicate whether or not the format supports these encodings.

10.4.3. Special values

Floating point values encoded with an exponent of 0 (before bias) and a mantissa of 0 are used to represent the value 0. An explicit sign bit can distinguish between +0 and -0.

Floating point values encoded with an exponent of 0 (before bias) and a non-zero mantissa are assumed to indicate a denormalized number, if the format has an implicit “1” bit. That is, when the exponent is 0, the “1” bit becomes explicit and the exponent is considered to be the negative sample bias minus one.

Floating point values encoded with an exponent larger than the exponent’s sampleUpper value and with a mantissa of 0 are interpreted as representing +/- infinity, depending on the value of an explicit sign bit. Note that in some formats, no exponent above sampleUpper is possible — for example, Table 98.

Floating point values encoded with an exponent larger than the exponent’s sampleUpper value and with a mantissa of non-0 are interpreted as representing not-a-number (NaN).

Note that these interpretations are compatible with the corresponding numerical representations in [IEEE 754].

10.4.4. Conversion formulae

Given an optional sign bit S, a mantissa value of M and an exponent value of E, a format with an implicit “1” bit can be converted from its representation to a real value as follows:

\[ V = \begin{cases} (-1)^S \times 0.0, & E = 0, M = 0 \\ (-1)^S \times 2^{-(E_\mathit{sampleLower}-1)} \times { M \over M_\mathit{sampleUpper} }, & E = 0, M \neq 0 \\ (-1)^S \times 2^{E-E_\mathit{sampleLower}} \times { \left( 1 + { M \over M_\mathit{sampleUpper} } \right) }, & 0 < E \leq E_\mathit{sampleUpper} \\ (-1)^S \times \mathit{Inf}, & E > E_\mathit{sampleUpper}, M = 0 \\ \mathit{NaN}, & E > E_\mathit{sampleUpper}, M \neq 0. \end{cases} \]

If there is no implicit “1” bit (that is, the sampleUpper value of the mantissa is representable in the number of bits assigned to the mantissa), the value can be converted to a real value as follows:

\[ V = \begin{cases} (-1)^S \times 2^{E-E_{\mathit{sampleUower}}} \times { \left( { M \over M_\mathit{sampleUpper} } \right) }, & 0 < E \leq E_\mathit{sampleUpper} \\ (-1)^S \times \mathit{Inf}, & E > E_\mathit{sampleUpper}, M = 0 \\ \mathit{NaN}, & E > E_\mathit{sampleUpper}, M \neq 0. \end{cases} \]

A descriptor block for a format without an implicit “1” (and with the added complication of having the same exponent bits shared across multiple channels, which is why an implicit “1” bit does not make sense) is shown in Table 98. In the case of this particular example, the above equations simplify to:

$$red = \mathit{red}_\mathrm{shared}\times 2^{(\mathit{exp}_\mathrm{shared}-B-N)}$$ $$green = \mathit{green}_\mathrm{shared}\times 2^{(\mathit{exp}_\mathrm{shared}-B-N)}$$ $$blue = \mathit{blue}_\mathrm{shared}\times 2^{(\mathit{exp}_\mathrm{shared}-B-N)}$$

Where:

$$N = 9 \textrm{ (= number of mantissa bits per component)}$$ $$B = 15 \textrm{ (= exponent bias)}$$

Note that in general conversion from a real number to any representation may require rounding, truncation and special value management rules which are beyond the scope of a data format specification and may be documented in APIs which generate these formats.

11. Example format descriptors

[Note]

Example data format descriptors for compressed formats can be found under the colorModel field in Section 5.6.

Table 89. 565 RGB packed 16-bit format as written to memory by a little-endian architecture

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

totalSize: 76

descriptorType: 0

vendorId: 0

descriptorBlockSize: 24 + (16 × 3) = 72

versionNumber: 2

flags: ALPHA_STRAIGHT

transferFunction: LINEAR

colorPrimaries: BT709

colorModel: RGBSDA

texelBlockDimension3

texelBlockDimension2

texelBlockDimension1

texelBlockDimension0

0

0

0

0

bytesPlane3: 0

bytesPlane2: 0

bytesPlane1: 0

bytesPlane0: 2

bytesPlane7: 0

bytesPlane6: 0

bytesPlane5: 0

bytesPlane4: 0

F

S

E

L

channelType

First sample: low five bits blue

0

0

0

0

BLUE

bitLength: 4 (= “5”)

bitOffset: 0

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: 31

F

S

E

L

channelType

Second sample: middle six bits green

0

0

0

0

GREEN

bitLength: 5 = (“6”)

bitOffset: 6

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: 63

F

S

E

L

channelType

Third sample: top five bits red

0

0

0

0

RED

bitLength: 4 (= “5”)

bitOffset: 11

samplePosition3

samplePosition2

samplePosition1

samplePosition0

0

0

0

0

sampleLower: 0

sampleUpper: 31


Table 90. Four co-sited 8-bit sRGB channels, assuming premultiplied alpha

uint32_t bit

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4