OpenGL.GL.EXT.shader_image_load_store

OpenGL extension EXT.shader_image_load_store

This module customises the behaviour of the OpenGL.raw.GL.EXT.shader_image_load_store to provide a more Python-friendly API

Overview (from the spec)

This extension provides GLSL built-in functions allowing shaders to load from, store to, and perform atomic read-modify-write operations to a single level of a texture object from any shader stage. These built-in functions are named imageLoad(), imageStore(), and imageAtomic*(), respectively, and accept integer texel coordinates to identify the texel accessed. The extension adds the notion of "image units" to the OpenGL API, to which texture levels are bound for access by the GLSL built-in functions. To allow shaders to specify the image unit to access, GLSL provides a new set of data types ("image*") similar to samplers. Each image variable is assigned an integer value to identify an image unit to access, which is specified using Uniform*() APIs in a manner similar to samplers. For implementations supporting the NV_gpu_program5 extensions, assembly language instructions to perform image loads, stores, and atomics are also provided.

This extension also provides the capability to explicitly enable "early" per-fragment tests, where operations like depth and stencil testing are performed prior to fragment shader execution. In unextended OpenGL, fragment shaders never have any side effects and implementations can sometimes perform per-fragment tests and discard some fragments prior to executing the fragment shader. Since this extension allows fragment shaders to write to texture and buffer object memory using the built-in image functions, such optimizations could lead to non-deterministic results. To avoid this, implementations supporting this extension may not perform such optimizations on shaders having such side effects. However, enabling early per-fragment tests guarantees that such tests will be performed prior to fragment shader execution, and ensures that image stores and atomics will not be performed by fragment shader invocations where these per-fragment tests fail.

Finally, this extension provides both a GLSL built-in function and an OpenGL API function allowing applications some control over the ordering of image loads, stores, and atomics relative to other OpenGL pipeline operations accessing the same memory. Because the extension provides the ability to perform random accesses to texture or buffer object memory, such accesses are not easily tracked by the OpenGL driver. To avoid the need for heavy-handed synchronization at the driver level, this extension requires manual synchronization. The MemoryBarrierEXT() OpenGL API function allows applications to specify a bitfield indicating the set of OpenGL API operations to synchronize relative to shader memory access. The memoryBarrier() GLSL built-in function provides a synchronization point within a given shader invocation to ensure that all memory accesses performed prior to the synchronization point complete prior to any started after the synchronization point.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/EXT/shader_image_load_store.txt