View source for Image Load Store

{{stack
|{{infobox feature
| core = 4.2
| core_extension = [http://www.opengl.org/registry/specs/ARB/shader_image_load_store.txt ARB_shader_image_load_store]
| ext_extension = [http://www.opengl.org/registry/specs/EXT/shader_image_load_store.txt EXT_shader_image_load_store]
}}
|{{shader float}}
}}

'''Image load/store''' is the ability of [[Shader]]s to more-or-less arbitrarily read from and write to images.

== Overview ==

The idea with image load/store is that the user can bind one of the images in a [[Texture]] to a number of image binding points (which are separate from texture image units). Shaders can read information from these images and write information to them, in ways that they cannot with textures.

This can allow for a number of powerful features, including relatively cheap order-independent transparency.

If you think that this is a great feature, remember that there is no such thing as a free lunch. The cost of using image load/store is that all of its write operations are [[Incoherent Memory Access|not automatically coherent]]. By using image load/store, you take on the responsibility to manage what OpenGL would normally manage for you using regular texture reads/[[Framebuffer Object|FBO]] writes.

== Image variables ==

Image variables in [[GLSL]] are variables that have one of the following {{code|image}} types. The image types are based on the type of the source [[Texture]] for the image. Not all texture types have a corresponding image type. Image variables must be declared with the [[Uniform (GLSL)|{{code|uniform}} storage qualifier]] (or as [[Function Parameter (GLSL)|function parameter inputs]]).

Like samplers, image variables represent either floating-point, signed integer, or unsigned integer [[Image Format]]s. The prefix used for the image variable name denotes which, using standard GLSL conventions. No prefix means floating-point, a prefix of {{code|i}} means signed integer, and {{code|u}} means unsigned integer.

For the sake of clarity, when you see a ''g'' preceding "image" in an image name, it represents any of the 3 possible prefixes. The image variables are:

{| class="wikitable"
! Image Type
! Corresponding Texture Type
|-
| {{code|''g''image1D}}
| {{enum|GL_TEXTURE_1D}}
single layer from
* {{enum|GL_TEXTURE_1D_ARRAY}}
|-
| {{code|''g''image2D}}
| {{enum|GL_TEXTURE_2D}}
single layer from:
* {{enum|GL_TEXTURE_2D_ARRAY}}
* {{enum|GL_TEXTURE_CUBE_MAP}}
* {{enum|GL_TEXTURE_CUBE_MAP_ARRAY}}
* {{enum|GL_TEXTURE_3D}}
|-
| {{code|''g''image3D}}
| {{enum|GL_TEXTURE_3D}}
|-
| {{code|''g''imageCube}}
| [[Cubemap Texture|{{enum|GL_TEXTURE_CUBE_MAP}}]]
|-
| {{code|''g''image2DRect}}
| [[Rectangle Texture|{{enum|GL_TEXTURE_RECTANGLE}}]]
|-
| {{code|''g''image1DArray}}
| [[Array Texture|{{enum|GL_TEXTURE_1D_ARRAY}}]]
|-
| {{code|''g''image2DArray}}
| [[Array Texture|{{enum|GL_TEXTURE_2D_ARRAY}}]]
|-
| {{code|''g''imageCubeArray}}
| [[Cubemap Array Texture|{{enum|GL_TEXTURE_CUBE_MAP_ARRAY}}]] (requires GL 4.0 or {{extref|texture_cube_map_array}})
|-
| {{code|''g''imageBuffer}}
| [[Buffer Texture|{{enum|GL_TEXTURE_BUFFER}}]]
|-
| {{code|''g''image2DMS}}
| [[Multisample Texture|{{enum|GL_TEXTURE_2D_MULTISAMPLE}}]]
single layer from:
* {{enum|GL_TEXTURE_2D_MULTISAMPLE_ARRAY}}
|-
| {{code|''g''image2DMSArray}}
| {{enum|GL_TEXTURE_2D_MULTISAMPLE_ARRAY}}
|-
|}

There are no "shadow" variants.

You will notice several "single layer from" entries in the above table. It is possible to bind a specific layer from certain texture types to an image. When you do so, you must use a different image variable compared to the source texture's actual type, as shown above.

=== Memory qualifiers ===

Image variables can be declared with a [[Type_Qualifier_(GLSL)#Memory_qualifiers|number of qualifiers]] that have different meanings for how the variable is accessed.

{{memory qualifiers}}

Multiple qualifiers can be used, but they must make sense together (a variable cannot be both {{code|readonly}} and {{code|writeonly}}). You are encouraged to use {{code|restrict}} whenever possible.

=== Format qualifiers ===

Image variables can be declared with a [[Layout Image Format|format qualifier]]; this specifies the format for any read operations done on the image. Therefore, a format qualifier is required if you do not declare the variable with the {{code|writeonly}} memory qualifier. Write-only variables cannot be used as in any reading operations; this includes calling load ''and'' atomic (read/modify/write) functions. So if you want to read from an image, you must declare the format.

The format defines how the shader interprets the bits of data that it reads from the image. It also defines how it converts the data passed for write operations when it writes it into the image. This allows the actual [[Image Format]] of the image to [[#Format conversion|differ between what the shader sees and what is stored in the image]], sometimes substantially.

The format are divided into three categories, representing the three types of image variables:

{{div col}}
<!--No snippet link. Looks awful with columns -->
{{layout image formats}}
{{div col end}}

== Image operations ==

OpenGL provides a number of functions for accessing images through image variables.

Image operations have "image coordinates", which serve the purpose of specifying where in an image that an access should take place. Image coordinates are different from texture coordinates in that image coordinates are ''always'' signed integers in texel space.

Each image variable has a specific dimensionality for their image coordinates, which represents the dimensionality of the underlying image. An {{code|image1D}} variable takes an {{code|int}} as a coordinate. {{code|image2DArray}} takes an {{code|ivec3}}; the third component is the array layer.

Cube maps (and cube map arrays) are accessed very differently from texture accesses. The image coordinate for cube map and cube map arrays are both {{code|ivec3}}. The coordinate is ''not a direction''; it is a texel coordinate within the space of the cube. The third component is the face index, [[Template:cubemap layer face ordering|as this is usually defined]]. Image variables effectively treat cube maps as simply a form of array texture; cube map arrays are just bigger arrays, as their third component is the layer-face index.

When accessing multisample textures, the accessing function has another parameter, an {{code|int}} that defines the sample index to read from or write to.

Let us collectively refer to the image coordinate parameters as "IMAGE_COORD". When you see this in a function definition below, this means that the function takes an image coordinate, which may include a sample index parameter if it is a multisample image.

=== Image size ===
{{infobox feature
| name = Image Size
| core = 4.3
| core_extension = {{extref|shader_image_size}}
}}

The size of the image for an image variable can be queries with this function:

  '''ivec''' imageSize('''gimage''' {{param|image}});

The size of the returned vector will be the size of the image coordinate, ''except in the case of cube maps''. For cube maps, the size will be {{code|ivec2}}; the third dimension would always be 6, so it is not returned. Cube map arrays will return {{code|ivec3}}, with the third component being the number of layer-faces.

Accessing any texels outside of this size results in invalid accesses, as defined below.

=== Image load ===

Image load functions read a specific location from the image into the shader.

{{note|Load operations from any texel that is outside of the boundaries of the bound image will return all zeros.}}

Reading from an image is done with this function:

  '''gvec4''' imageLoad('''gimage''' {{param|image}}, IMAGE_COORD);

The return value is the data from the image, [[#Format conversion|converted]] as per the [[#Format qualifiers|format specified by the format layout qualifier]].

=== Image store ===

Image store operations write a value to a specific location in the image.

{{note|Store operations to any texel that is outside the boundaries of the bound image will do nothing.}}

Writing to an image is done with this function:

  '''void''' imageStore('''gimage''' {{param|image}}, IMAGE_COORD, '''gvec4''' {{param|data}});

The value in {{param|data}} will be written into the image at the given coordinate, using [[#Format conversion|format conversion]] based on the {{param|format}} parameter given to {{apifunc|glBindImageTexture}}.

=== Atomic operations ===

Atomic operations perform read/modify/write operations on a location in an image. These operations are guaranteed to be "atomic": if two shaders issue the same atomic operation on the same location in the same image, one will go first, followed by the other.

Consider a shader that reads from a location, adds 1 to it, and then writes to it. It is theoretically possible for two such shaders to read from and and write to the same location in the same image at the same time. Because of the way [[#Memory coherency|memory accesses are handled]], it is entirely possible that this sequence of events works like this:

* Shader A reads the image value, say 0.
* Shader B reads the image value, also 0.
* Shader B adds 1 to its ''local'' value of 0, becoming 1.
* Shader B writes its local value to the image. The image now has 1.
* Shader A adds 1 to its ''local'' value of 0, becoming 1.
* Shader A writes its local value to the image. The image now has 1.

Atomic operations prevent this possibility entirely. Each shader's independent atomic operation will fully complete before the next one starts.

The return value of all atomic operations is the original value before modifications. The value written will be the modified value.

Atomic operations to any texel that is outside of the boundaries of the bound image will return 0 and do nothing.

==== Atomic limitations ====

There are some severe limitations on image atomic operations. First, atomics can only be used on integer images, either signed or unsigned. Second, they can only be used on images with the {{code|GL_R32I/r32i}} or {{code|GL_R32UI/r32ui}} formats.

It is very possible to use [[#Format conversion|format conversions]] to make image operations work with other formats. But the GLSL code logic will be operating on 32-bit integers.

Below, the term "'''gint'''" means either {{code|int}} or {{code|uint}}, as is appropriate for the '''gimage''' type.

==== Atomic set value ====

The value at the location in an image can be directly set via this function:

  '''gint''' imageAtomicExchange('''gimage''' {{param|image}}, IMAGE_COORDS, '''gint''' {{param|data}});

This function is called "exchange" because it effectively exchanges {{param|data}} with the value in the image. The return value of all atomic functions is the original value from the image, so it is exchanging {{param|data}} with that value.

==== Atomic conditional set ====

A very powerful operation is the conditional modify operation. This operation will write a new value ''only'' if the current value in the image is equal to the given value.

  '''gint''' imageAtomicCompSwap('''gimage''' {{param|image}}, IMAGE_COORDS, '''gint''' {{param|compare}}, '''gint''' {{param|data}});

If the current value of {{param|image}} at the image coordinate is exactly equal to {{param|compare}}, then {{param|data}} will be stored into the image at that location. Otherwise, the location will retain the original value.

While the function doesn't provide a direct way to tell if it actually wrote the value, it always returns the original value. So if you need to know, you can test the return value against {{param|compare}} yourself.

==== Atomic math ====

GLSL only provides a single math operation: additions:

  '''gint''' imageAtomicAdd('''gimage''' {{param|image}}, IMAGE_COORDS, '''gint''' {{param|data}});

This will read the value from the image, add {{param|data}} to it, and then write it to the image.

Obviously if you need subtract, simply negate {{param|data}}. This will only work directly with signed integers. However, because GLSL 4.30 mandates two's complement, you can get the same effect with unsigned integers, since {{code|int(uint)}} and {{code|uint(int)}} conversion constructors will preserve the bit pattern.

So if you need unsigned integer subtraction, you can do this:

  imageAtomicAdd(..., uint(-data));

Obviously if this produces a negative value, you'll get a very positive value back instead, since it thinks you're doing unsigned math.

==== Atomic bitwise ====

GLSL provides 3 atomic bitwise operations: and, or, and xor:

  '''gint''' imageAtomicAnd('''gimage''' {{param|image}}, IMAGE_COORDS, '''gint''' {{param|data}});
  '''gint''' imageAtomicOr('''gimage''' {{param|image}}, IMAGE_COORDS, '''gint''' {{param|data}});
  '''gint''' imageAtomicXor('''gimage''' {{param|image}}, IMAGE_COORDS, '''gint''' {{param|data}});

These read the value from the image, perform the operation given {{param|data}}, and writes the results back to the image.

==== Atomic minmax ====

GLSL provides a pair of functions that ensure that the value in the image is no larger than the given value or no smaller than it.

  '''gint''' imageAtomicMin('''gimage''' {{param|image}}, IMAGE_COORDS, '''gint''' {{param|data}});
  '''gint''' imageAtomicMax('''gimage''' {{param|image}}, IMAGE_COORDS, '''gint''' {{param|data}});

For the "Min" function, it will ensure that the image value is no smaller than {{param|data}}. The "Max" function will ensure that the iamge value is no larger than {{param|data}}.

== Image stores and discard ==

The [[Fragment Shader]] has the ability to issue a {{code|discard}} command. This will prevent writing any fragment values to the framebuffer. However, it will ''also'' have the effect of preventing image store and atomic operations from taking place.

== Images in the context ==

The way to associate an image variable in GLSL works very similar to the way of associating [[Sampler (GLSL) #Binding textures to samplers|samplers with textures]].

For each shader stage, there is some number of available ''image units'' (not to be confused with ''texture'' image units). The number of image units can be [[Shader Resource Limit|queried per-stage, using {{enum|GL_MAX_*_IMAGE_UNIFORMS}}]], where * is filled in with the appropriate shader stage. Note that OpenGL 4.3 only requires [[Fragment Shader]]s and [[Compute Shader]]s to have non-zero numbers of image units; the minimum required in those cases is 8.

The total number of image units available is queried via {{enum|GL_MAX_IMAGE_UNITS}}; this represents the total number of images you can bind at one time.

Just as with samplers, image variables reference image unit indices in the context. These are usually set with a [[Layout Binding|binding layout qualifier]], but they can also be set with {{apifunc|glUniform|1i}} or {{apifunc|glProgramUniform|1i}}.

After associating the image variable with its image unit, you then bind an image to the context. This is done via this function:

  void {{apifunc|glBindImageTexture}}(GLuint {{param|unit}}, GLuint {{param|texture}}, GLint {{param|level}}, GLboolean {{param|layered}}, GLint {{param|layer}}, GLenum {{param|access}}, GLenum {{param|format}})

This binds an image from {{param|texture}} to the given image {{param|unit}}, using the given mipmap {{param|level}} and array {{param|layer}}.

Image bindings can be layered or non-layered, which is determined by {{param|layered}}. If {{param|layered}} is {{enum|GL_TRUE}}, then {{param|texture}} must be an [[Array Texture]] (of some type), a [[Cubemap Texture]], or a [[3D Texture]]. If a layered image is being bound, then the entire mipmap level specified by {{param|level}} is bound.

If the image is not layered, then the user must use the {{param|layer}} to select which array layer will be bound. If the texture does not have array layers, then this parameter must be 0. As with other functions, if this is a cubemap array texture, then {{param|layer}} is the layer-face to select.

If an array or cubemap texture is bound and is not layered, then the bound image is not an array or cubemap image. So if you bind a single array layer from a {{enum|GL_TEXTURE_1D_ARRAY}} texture, it should be used with the {{code|image1D}} image variable type. Simiarly, layers from a 2D array texture, cubemap, 3D texture, or cubemap array should be {{code|image2D}}, and a layer from a 2D multisample array should use {{code|image2DMS}}.

The {{param|access}} specifies how the shader may access the image through this image unit. This can be {{enum|GL_READ_ONLY}}, {{enum|GL_WRITE_ONLY}}, or {{enum|GL_READ_WRITE}}. If the shader violates this restriction, then all manner of bad things can happen, including program termination. It is a good idea to use memory qualifiers in the shader itself to catch this at shader compile-time.

The {{param|format}} parameter is an [[Image Format]] which defines the format that will be used for ''writes'' to the image. If a [[#Format qualifiers|format qualifier]] is specified in the shader, this format must match it. The format must be [[#Format compatibility|compatible with the texture's image format]]. The {{param|format}} parameter may only use formats from the following table:

{{snippet|Template:bind image format table}}

Also, note that these are the ''only'' image formats you can use for images in image load/store operations. You must use exactly these image formats and no others.

=== Multibind and images ===
{{infobox feature
| name = Object Multibind
| core = 4.4
| core_extension = {{extref|multi_bind}}
}}

A range of image objects can be bound to a range of image binding points with a single function call:

{{funcdef|void {{apifunc|glBindImageTextures}}(GLuint {{param|first}}, GLsizei {{param|count}}, const GLuint *{{param|textures}});}}

{{param|first}} is the image binding index to start binding the array to. {{param|count}} is the number of indices to bind. Thus, the image binding indices that will be changed are those on the half-open range, [{{param|first}}, {{param|first}} + {{param|count}}).

If {{param|textures}} is NULL, then it works like binding the 0 image to all of the given binding indices. If not, then it will bind each texture in the array to the indices. {{param|textures}} must have {{param|count}} elements.

For each texture in the array, if the texture is 0, then it will be the equivalent of calling {{apifunc|glBindImageTexture|(first + i, 0, 0, GL_FALSE, 0, GL_READ_ONLY, GL_R8)}}. If the texture is an actual texture object, it will be the equivalent of calling {{apifunc|glBindImageTexture|(first + i, textures[i], 0, GL_TRUE, 0, GL_READ_WRITE, lookupInternalFormat(textures[i]))}}, where {{code|lookupInternalFormat}} gets the [[Image Format]] of the texture.

==== Utility ====

You may notice that {{apifunc|glBindImageTexture}} has a lot more parameter options than the multibind {{apifunc|glBindImageTextures}}. If you use multibind, you have some significant restrictions on what you're binding.

The texture will always be bound as a layered texture; you can't multibind a specific layer of a texture. It will always bind mipmap level 0. It will always bind it for reading and writing. And you can't do format remapping; it will always use the exact format.

There is one tool that can mitigate most of these limitations: [[View texture|texture views]]. You can create a view of an existing texture. This allows you to pick a particular mipmap and array layer/face from a texture up-front. You can then multibind this view, effectively preselecting the mipmap and layer that you want. Furthermore, view textures can already do the same kind of [[#Format conversion|formatting conversion]] that images can do.

The only thing view textures can't save you from is the read/write binding. However, ''that'' can be set in the shader just fine. So you don't really need it.

== Format conversion ==

The [[Image Format]] of the image may be different from the format specified to the image binding function and in the shader. Values read and written are converted in the following way, assuming that the formats are [[#Format compatibility|compatible]].

The term "source format" represents the image format of whatever the source of the operation is. Similarly, the "destination format" is the image format of whatever the destination of the operation is. Therefore:

* Read operation:
** Source format: The image's actual format.
** Dest format: The format specified by the image binding function {{apifunc|glBindImageTexture}} (which must match the [[#Format qualifiers|format declared in the shader qualifier]]).
* Write operation:
** Source format: The format specified by the image binding function {{apifunc|glBindImageTexture}}.
** Dest format: The image's actual format.

All operations, whether read or write, function as though they were copying data from/to images with those formats. The first step of a write operation would be taking the value provided by the shader and writing that into a texture in the source image format. Then the format conversion takes place, copying the value into the destination. Similarly, the last step of read operations is reading from the destination image into the value in the shader.

{{note|What follows is a description of how reading and writing ''appears'' to behave. Obviously, no hardware does it this way; this is just a convenient way of describing the behavior in terms of bits and bytes.}}

The conversion works based on memory copies using existing API functions. The source format values are read into memory as though calling {{apifunc|glGetTexImage}}. The destination format values are written into their image as though they were uploaded via a call to one of the {{code|glTexSubImage}} functions.

Both of these functions take [[Pixel Transfer|pixel transfer formats and types]]. The two effective calls will use {{param|format}}s and {{param|type}}s that exactly match the source/destination image format.

For example, if the source image format was {{enum|GL_RGBA8UI}}, then the {{param|format}} and {{param|type}} passed to {{apifunc|glGetTexImage}} would be {{enum|GL_RGBA_INTEGER}} and {{enum|GL_UNSIGNED_BYTE}}. If the destination image format for a copy is {{enum|GL_RGB10_A2}} (which may or may not be compatible with {{enum|GL_RGBA8UI}}), then the parameters would be {{enum|GL_RGBA}} and {{enum|GL_UNSIGNED_INT_2_10_10_10_REV}}. The full set of pixel transfer parameters can be found in {{glref45|Table 8.27|281}}.

The destination format values are written into "memory", using values pulled from the source, as though written with one of the {{code|glTexSubImage}} calls. These calls again use pixel transfer formats and types that exactly match the ''destination'' format.

Note that (compatibility willing) we are perfectly capable of switching between floating-point and integral formats. However, converting between {{enum|GL_R32F}} and {{enum|GL_RGBA8}} is not well-defined, in terms of the endian conversion. The reason is that {{enum|GL_R32F}} will be read using a type of {{enum|GL_FLOAT}}, but the writing will write 4 bytes in RGBA order. The R byte will be the first byte read from the {{enum|GL_FLOAT}}, but the endian storage of {{enum|GL_FLOAT}} is not defined. So the first byte may be the most significant or the least significant.

For any particular platform, you could assume a specific endian. But OpenGL itself provides no guarantees. If you want some guarantees on being able to play with the bits of a 32-bit floating-point texture, convert it to {{enum|GL_R32UI}} instead and do your bit manipulation in the shader.

=== Format compatibility ===

The various image format compatibility matrix for image load/store operations is very similar to the compatiblity for [[View Texture|texture views]], though there are some differences. The first difference is that the list of image formats that can be used for images in load/store operations is more limited: only the formats mentioned above may be used.

Each of these formats has two properties: a size and a class. The size represents the bit-size of each texel. For example, {{enum|GL_R32F}} has a size of 32; {{enum|GL_RGBA32UI}} has a size of 128. The class represents the number of components and the bit-depth of each component. The class of {{enum|GL_R32F}} is 1x32, while the class of {{enum|GL_RGBA8}} is 4x8.

The class for formats with oddball bitdepths ({{enum|GL_R11F_G11F_B10F}}, for example) is the arrangement of components. So {{enum|GL_R11F_G11F_B10F}}'s class is 11/11/10, while {{enum|GL_RGB10_A2UI}}'s class is 10/10/10/2. This has a class match with {{enum|GL_RGB10_A2}}.

If the texture was allocated by OpenGL (it is possible for OpenCL or other interop layers to allocate textures), then the only thing that matters for compatibility is overall texel size. So it is perfectly valid to map an {{enum|GL_R32F}} image to an {{enum|GL_RGBA8UI}} format and back, though again endian conversions may make this unusable in platform-neutral code.

If a texture was allocated from outside of OpenGL, then how compatibility is determined may not be by texel size; it may be by class. You must use glGetTexParameter with {{enum|GL_IMAGE_FORMAT_COMPATIBILITY_TYPE}} to detect which. It will return either {{enum|GL_IMAGE_FORMAT_COMPATIBILITY_BY_SIZE}} or {{enum|GL_IMAGE_FORMAT_COMPATIBILITY_BY_CLASS}}, specifying how compatibility is determined.

You can also detect this at the image format level with an image format query using the same parameter; this will be true for all (foreign) textures using that image format and texture type.

As an alternative to querying with foreign textures, you could just stick to formats that match on class. If the classes match, the sizes also match.

== Memory coherency ==
{{main|Memory Model#Incoherent memory access}}

Writes and atomic operations via image variables are not automatically coherent. Therefore, you must do things to ensure that writes have occurred before you can read those values.

== Limitations ==

[[Category:OpenGL Shading Language]]
[[Category:Shaders]]