Name
NV_texture_compression_s3tc
Name Strings
GL_NV_texture_compression_s3tc
Contributors
Contributors to the OpenGL EXT_texture_compression_s3tc extension
Greg Roth, NVIDIA
Contact
Ian Stewart, NVIDIA Corporation (istewart 'at' nvidia.com)
Status
Complete.
Version
Last Modifed Date: Sep 27, 2012
NVIDIA Revision: 1
Number
OpenGL ES Extension #128
Dependencies
This extension is written against the OpenGL ES 2.0.25
Specification.
This extension interacts with NV_texture_array
Overview
This extension adds support for S3's S3TC texture compression
formats, including DXT1, DXT3, and DXT5. For the DXT1 format,
this specification supports an RGB-only mode and a special RGBA
mode with single-bit alpha.
IP Status
Contact S3 Incorporated (http://www.s3.com) regarding any
intellectual property issues associated with implementing this
extension.
WARNING: Vendors able to support S3TC texture compression in
Direct3D drivers do not necessarily have the right to use the same
functionality in OpenGL ES.
New Tokens
Accepted by the parameter of CompressedTexImage2D
and CompressedTexSubImage2D:
COMPRESSED_RGB_S3TC_DXT1_NV 0x83F0
COMPRESSED_RGBA_S3TC_DXT1_NV 0x83F1
COMPRESSED_RGBA_S3TC_DXT3_NV 0x83F2
COMPRESSED_RGBA_S3TC_DXT5_NV 0x83F3
Additions to Chapter 3 of the OpenGL ES 2.0.25 Specification
Modify Section 3.7.3, "Compressed Texture Images"
(Replace first two sentences with)
Texture images may also be specified or modified using image data
already stored in a known compressed image format. The GL defines
some specific compressed formats, and others may be defined by GL
extensions.
(Insert after section describing CompressedTexImage2D)
The specific compressed texture formats supported by
CompressedTexImage2D, and the corresponding base internal format
for each specific format, are defined in table 3.X.
Table 3.X: "Specific compressed texture formats"
Compressed Internal Formats Base Internal Format
=========================== ====================
COMPRESSED_RGB_S3TC_DXT1_NV RGB
COMPRESSED_RGBA_S3TC_DXT1_NV RGBA
COMPRESSED_RGBA_S3TC_DXT3_NV RGBA
COMPRESSED_RGBA_S3TC_DXT5_NV RGBA
(Replace last paragraph with)
If the internal format is one of COMPRESSED_RGB_S3TC_DXT1_NV,
COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, or
COMPRESSED_RGBA_S3TC_DXT5_NV, the compressed texture is stored
using one of several S3TC compressed texture image formats and is
easily edited along 4x4 texel boundaries. In this case,
CompressedTexImage2D will result in an INVALID_OPERATION error
if one of the following conditions occurs:
* is not a multiple of four, and plus
is not equal to TEXTURE_WIDTH;
* is not a multiple of four, and plus
is not equal to TEXTURE_HEIGHT; or
* or is not a multiple of four.
For any other formats, calling CompressedTexSubImage2D will result
in an INVALID_OPERATION error if or is not
equal to zero, or if and do not match the width
and height of the texture, respectively. The contents of any texel
outside the region modified by the call are undefined. These
restrictions may be relaxed for other specific compressed internal
formats whose images are easily modified.
Interactions with NV_texture_array
If NV_texture_array is supported, the S3TC compressed formats may
also be used as the internal formats given to
CompressedTexImage3DNV and CompressedTexSubImage3DNV. The
restrictions for the , , , and
parameters of the CompressedTexSubImage2D function when used with
S3TC compressed texture formats, described in this extension, also
apply to the identically named parameters of
CompressedTexSubImage3DNV.
Errors
INVALID_OPERATION is generated by CopyTexSubImage2D if the texture
image bound to has internal format
COMPRESSED_RGB_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT1_NV,
COMPRESSED_RGBA_S3TC_DXT3_NV, or COMPRESSED_RGBA_S3TC_DXT5_NV.
INVALID_OPERATION is generated by CompressedTexSubImage2D if
is COMPRESSED_RGB_S3TC_DXT1_NV,
COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, or
COMPRESSED_RGBA_S3TC_DXT5_NV and any of the following apply:
* is not a multiple of four, and plus
is not equal to the texture width;
* is not a multiple of four, and plus
is not equal to the texture height; or
* or is not a multiple of four.
New State
The queries for NUM_COMPRESSED_TEXTURE_FORMATS and
COMPRESSED_TEXTURE_FORMATS include COMPRESSED_RGB_S3TC_DXT1_NV,
COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, and
COMPRESSED_RGBA_S3TC_DXT5_NV.
Appendix
S3TC Compressed Texture Image Formats
Compressed texture images stored using the S3TC compressed image
formats are represented as a collection of 4x4 texel blocks, where
each block contains 64 or 128 bits of texel data. The image is
encoded as a normal 2D raster image in which each 4x4 block is
treated as a single pixel. If an S3TC image has a width or height
that is not a multiple of four, the data corresponding to texels
outside the image are irrelevant and undefined.
When an S3TC image with a width of , height of , and block
size of (8 or 16 bytes) is decoded, the corresponding
image size (in bytes) is:
ceil(/4) * ceil(/4) * blocksize.
When decoding an S3TC image, the block containing the texel at
offset (, ) begins at an offset (in bytes) relative to the
base of the image of:
blocksize * (ceil(/4) * floor(/4) + floor(/4)).
The data corresponding to a specific texel (, ) are
extracted from a 4x4 texel block using a relative (x,y) value of
( modulo 4, modulo 4).
There are four distinct S3TC image formats:
COMPRESSED_RGB_S3TC_DXT1_NV: Each 4x4 block of texels consists of
64 bits of RGB image data.
Each RGB image data block is encoded as a sequence of 8 bytes,
called (in order of increasing address):
c0_lo, c0_hi, c1_lo, c1_hi, bits_0, bits_1, bits_2, bits_3
The 8 bytes of the block are decoded into three quantities:
color0 = c0_lo + c0_hi * 256
color1 = c1_lo + c1_hi * 256
bits = bits_0 + 256 * (bits_1 + 256 * (bits_2 + 256 * bits_3))
color0 and color1 are 16-bit unsigned integers that are
unpacked to RGB colors RGB0 and RGB1 as though they were
16-bit packed pixels with a of RGB and a type of
UNSIGNED_SHORT_5_6_5.
bits is a 32-bit unsigned integer, from which a two-bit
control code is extracted for a texel at location (x,y) in the
block using:
code(x,y) = bits[2*(4*y+x)+1 .. 2*(4*y+x)+0]
where bit 31 is the most significant and bit 0 is the least
significant bit.
The RGB color for a texel at location (x,y) in the block is
given by:
RGB0, if color0 > color1 and code(x,y) == 0
RGB1, if color0 > color1 and code(x,y) == 1
(2*RGB0+RGB1)/3, if color0 > color1 and code(x,y) == 2
(RGB0+2*RGB1)/3, if color0 > color1 and code(x,y) == 3
RGB0, if color0 <= color1 and code(x,y) == 0
RGB1, if color0 <= color1 and code(x,y) == 1
(RGB0+RGB1)/2, if color0 <= color1 and code(x,y) == 2
BLACK, if color0 <= color1 and code(x,y) == 3
Arithmetic operations are done per component, and BLACK refers
to an RGB color where red, green, and blue are all zero.
Since this image has an RGB format, there is no alpha
component and the image is considered fully opaque.
COMPRESSED_RGBA_S3TC_DXT1_NV: Each 4x4 block of texels consists
of 64 bits of RGB image data and minimal alpha information. The
RGB components of a texel are extracted in the same way as
COMPRESSED_RGB_S3TC_DXT1_NV.
The alpha component for a texel at location (x,y) in the block
is given by:
0.0, if color0 <= color1 and code(x,y) == 3
1.0, otherwise
COMPRESSED_RGBA_S3TC_DXT3_NV: Each 4x4 block of texels consists
of 64 bits of uncompressed alpha image data followed by 64 bits of
RGB image data.
Each RGB image data block is encoded according to the
COMPRESSED_RGB_S3TC_DXT1_NV format, with the exception that
the two code bits always use the non-transparent encodings.
In other words, they are treated as though color0 > color1,
regardless of the actual values of color0 and color1.
Each alpha image data block is encoded as a sequence of 8
bytes, called (in order of increasing address):
a0, a1, a2, a3, a4, a5, a6, a7
The 8 bytes of the block are decoded into one 64-bit integer:
alpha = a0 + 256 * (a1 + 256 * (a2 + 256 * (a3 + 256 * (a4 +
256 * (a5 + 256 * (a6 + 256 * a7))))))
alpha is a 64-bit unsigned integer, from which a four-bit
alpha value is extracted for a texel at location (x,y) in the
block using:
alpha(x,y) = bits[4*(4*y+x)+3 .. 4*(4*y+x)+0]
where bit 63 is the most significant and bit 0 is the least
significant bit.
The alpha component for a texel at location (x,y) in the block
is given by alpha(x,y) / 15.
COMPRESSED_RGBA_S3TC_DXT5_NV: Each 4x4 block of texels consists
of 64 bits of compressed alpha image data followed by 64 bits of
RGB image data.
Each RGB image data block is encoded according to the
COMPRESSED_RGB_S3TC_DXT1_NV format, with the exception that
the two code bits always use the non-transparent encodings.
In other words, they are treated as though color0 > color1,
regardless of the actual values of color0 and color1.
Each alpha image data block is encoded as a sequence of 8
bytes, called (in order of increasing address):
alpha0, alpha1, bits_0, bits_1, bits_2, bits_3, bits_4, bits_5
The alpha0 and alpha1 are 8-bit unsigned bytes converted to
alpha components by multiplying by 1/255.
The 6 "bits" bytes of the block are decoded into one 48-bit
integer:
bits = bits_0 + 256 * (bits_1 + 256 * (bits_2 + 256 * (bits_3 +
256 * (bits_4 + 256 * bits_5))))
bits is a 48-bit unsigned integer, from which a three-bit
control code is extracted for a texel at location (x,y) in the
block using:
code(x,y) = bits[3*(4*y+x)+2 .. 3*(4*y+x)+0]
where bit 47 is the most significant and bit 0 is the least
significant bit.
The alpha component for a texel at location (x,y) in the block
is given by:
alpha0, code(x,y) == 0
alpha1, code(x,y) == 1
(6*alpha0 + 1*alpha1)/7, alpha0 > alpha1 and code(x,y) == 2
(5*alpha0 + 2*alpha1)/7, alpha0 > alpha1 and code(x,y) == 3
(4*alpha0 + 3*alpha1)/7, alpha0 > alpha1 and code(x,y) == 4
(3*alpha0 + 4*alpha1)/7, alpha0 > alpha1 and code(x,y) == 5
(2*alpha0 + 5*alpha1)/7, alpha0 > alpha1 and code(x,y) == 6
(1*alpha0 + 6*alpha1)/7, alpha0 > alpha1 and code(x,y) == 7
(4*alpha0 + 1*alpha1)/5, alpha0 <= alpha1 and code(x,y) == 2
(3*alpha0 + 2*alpha1)/5, alpha0 <= alpha1 and code(x,y) == 3
(2*alpha0 + 3*alpha1)/5, alpha0 <= alpha1 and code(x,y) == 4
(1*alpha0 + 4*alpha1)/5, alpha0 <= alpha1 and code(x,y) == 5
0.0, alpha0 <= alpha1 and code(x,y) == 6
1.0, alpha0 <= alpha1 and code(x,y) == 7
Issues
None
Revision History
Rev. Date Author Changes
---- -------- --------- -------------------------------------
1 09/27/2012 istewart First revision, based of the
EXT_texture_compression_s3tc
specification.
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