Name EXT_shader_samples_identical Name Strings GL_EXT_shader_samples_identical Contact Ian Romanick, Intel (ian.d.romanick 'at' intel.com) Contributors Chris Forbes, Mesa Magnus Wendt, Intel Neil S. Roberts, Intel Graham Sellers, AMD Nicolai Hähnle, AMD Status Complete. Version Last Modified Date: November 19, 2015 Revision: 6 Number OpenGL Extension #557 OpenGL ES Extension #339 Dependencies OpenGL 3.2, or OpenGL ES 3.1, or ARB_texture_multisample is required. This extension is written against the OpenGL 4.5 (Core Profile) Specification Overview Multisampled antialiasing has become a common method for improving the quality of rendered images. Multisampling differs from supersampling in that the color of a primitive that covers all or part of a pixel is resolved once, regardless of the number of samples covered. If a large polygon is rendered, the colors of all samples in each interior pixel will be the same. This suggests a simple compression scheme that can reduce the necessary memory bandwidth requirements. In one such scheme, each sample is stored in a separate slice of the multisample surface. An additional multisample control surface (MCS) contains a mapping from pixel samples to slices. If all the values stored in the MCS for a particular pixel are the same, then all the samples have the same value. Applications can take advantage of this information to reduce the bandwidth of reading multisample textures. A custom multisample resolve filter could optimize resolving pixels where every sample is identical by reading the color once. color = texelFetch(sampler, coordinate, 0); if (!textureSamplesIdenticalEXT(sampler, coordinate)) { for (int i = 1; i < MAX_SAMPLES; i++) { vec4 c = texelFetch(sampler, coordinate, i); //... accumulate c into color } } New Procedures and Functions None. New Tokens None. Additions to the OpenGL 4.5 (Core Profile) Specification None. Modifications to The OpenGL Shading Language Specification, Version 4.50.5 Including the following line in a shader can be used to control the language features described in this extension: #extension GL_EXT_shader_samples_identical A new preprocessor #define is added to the OpenGL Shading Language: #define GL_EXT_shader_samples_identical Add to the table in section 8.7 "Texture Lookup Functions" Syntax: bool textureSamplesIdenticalEXT(gsampler2DMS sampler, ivec2 coord) bool textureSamplesIdenticalEXT(gsampler2DMSArray sampler, ivec3 coord) Description: Returns true if it can be determined that all samples within the texel of the multisample texture bound to at contain the same values or false if this cannot be determined." Additions to the AGL/EGL/GLX/WGL Specifications None Errors None New State None New Implementation Dependent State None Issues 1) What should the new functions be called? RESOLVED: textureSamplesIdenticalEXT. Initially textureAllSamplesIdenticalEXT was considered, but textureSamplesIdenticalEXT is more similar to the existing textureSamples function. 2) It seems like applications could implement additional optimization if they were provided with raw MCS data. Should this extension also provide that data? There are a number of challenges in providing raw MCS data. The biggest problem being that the amount of MCS data depends on the number of samples, and that is not known at compile time. Additionally, without new texelFetch functions, applications would have difficulty utilizing the information. Another option is to have a function that returns an array of tuples of sample number and count. This also has difficulties with the maximum array size not being known at compile time. RESOLVED: Do not expose raw MCS data in this extension. 3) Should this extension also extend SPIR-V? RESOLVED: Yes, but this has not yet been written. 4) Is it possible for textureSamplesIdenticalEXT to report false negatives? RESOLVED: Yes. It is possible that the underlying hardware may not detect that separate writes of the same color to different samples of a pixel are the same. The shader function is at the whim of the underlying hardware implementation. It is also possible that a compressed multisample surface is not used. In that case the function will likely always return false. Revision History Rev Date Author Changes --- ---------- -------- --------------------------------------------- 1 2014/08/20 cforbes Initial version 2 2015/10/23 idr Change from MESA to EXT. Rebase on OpenGL 4.5, and add dependency on OpenGL ES 3.1. Initial draft of overview section and issues 1 through 3. 3 2015/10/27 idr Typo fixes. 4 2015/11/10 idr Rename extension from EXT_shader_multisample_compression to EXT_shader_samples_identical. Add issue #4. 5 2015/11/18 idr Fix some typos spotted by gsellers. Change the name of the name of the function to textureSamplesIdenticalEXT. 6 2015/11/19 idr Fix more typos spotted by Nicolai Hähnle.