Native bindings to the NV_shading_rate_image extension.

By default, OpenGL runs a fragment shader once for each pixel covered by a primitive being rasterized. When using multisampling, the outputs of that fragment shader are broadcast to each covered sample of the fragment's pixel. When using multisampling, applications can also request that the fragment shader be run once per color sample (when using the "sample" qualifier on one or more active fragment shader inputs), or run a fixed number of times per pixel using SAMPLE_SHADING enable and the MinSampleShading frequency value. In all of these approaches, the number of fragment shader invocations per pixel is fixed, based on API state.

This extension allows applications to bind and enable a shading rate image that can be used to vary the number of fragment shader invocations across the framebuffer. This can be useful for applications like eye tracking for virtual reality, where the portion of the framebuffer that the user is looking at directly can be processed at high frequency, while distant corners of the image can be processed at lower frequency. The shading rate image is an immutable-format two-dimensional or two-dimensional array texture that uses a format of R8UI. Each texel represents a fixed-size rectangle in the framebuffer, covering 16x16 pixels in the initial implementation of this extension. When rasterizing a primitive covering one of these rectangles, the OpenGL implementation reads the texel in the bound shading rate image and looks up the fetched value in a palette of shading rates. The shading rate used can vary from (finest) 16 fragment shader invocations per pixel to (coarsest) one fragment shader invocation for each 4x4 block of pixels.

When this extension is advertised by an OpenGL implementation, the implementation must also support the GLSL extension "GL_NV_shading_rate_image" (documented separately), which provides new built-in variables that allow fragment shaders to determine the effective shading rate used for each fragment. Additionally, the GLSL extension also provides new layout qualifiers allowing the interlock functionality provided by ARB_fragment_shader_interlock to guarantee mutual exclusion across an entire fragment when the shading rate specifies multiple pixels per fragment shader invocation.

Note that this extension requires the use of a framebuffer object; the shading rate image and related state are ignored when rendering to the default framebuffer.

Requires OpenGL 4.5 and NV_framebuffer_mixed_samples.