1 Name 2 3 EXT_shader_samples_identical 4 5 Name Strings 6 7 GL_EXT_shader_samples_identical 8 9 Contact 10 11 Ian Romanick, Intel (ian.d.romanick 'at' intel.com) 12 13 Contributors 14 15 Chris Forbes, Mesa 16 Magnus Wendt, Intel 17 Neil S. Roberts, Intel 18 Graham Sellers, AMD 19 20 Status 21 22 XXX - Not complete yet. 23 24 Version 25 26 Last Modified Date: November 19, 2015 27 Revision: 6 28 29 Number 30 31 TBD 32 33 Dependencies 34 35 OpenGL 3.2, or OpenGL ES 3.1, or ARB_texture_multisample is required. 36 37 This extension is written against the OpenGL 4.5 (Core Profile) 38 Specification 39 40 Overview 41 42 Multisampled antialiasing has become a common method for improving the 43 quality of rendered images. Multisampling differs from supersampling in 44 that the color of a primitive that covers all or part of a pixel is 45 resolved once, regardless of the number of samples covered. If a large 46 polygon is rendered, the colors of all samples in each interior pixel will 47 be the same. This suggests a simple compression scheme that can reduce 48 the necessary memory bandwidth requirements. In one such scheme, each 49 sample is stored in a separate slice of the multisample surface. An 50 additional multisample control surface (MCS) contains a mapping from pixel 51 samples to slices. 52 53 If all the values stored in the MCS for a particular pixel are the same, 54 then all the samples have the same value. Applications can take advantage 55 of this information to reduce the bandwidth of reading multisample 56 textures. A custom multisample resolve filter could optimize resolving 57 pixels where every sample is identical by reading the color once. 58 59 color = texelFetch(sampler, coordinate, 0); 60 if (!textureSamplesIdenticalEXT(sampler, coordinate)) { 61 for (int i = 1; i < MAX_SAMPLES; i++) { 62 vec4 c = texelFetch(sampler, coordinate, i); 63 64 //... accumulate c into color 65 66 } 67 } 68 69 New Procedures and Functions 70 71 None. 72 73 New Tokens 74 75 None. 76 77 Additions to the OpenGL 4.5 (Core Profile) Specification 78 79 None. 80 81 Modifications to The OpenGL Shading Language Specification, Version 4.50.5 82 83 Including the following line in a shader can be used to control the 84 language features described in this extension: 85 86 #extension GL_EXT_shader_samples_identical 87 88 A new preprocessor #define is added to the OpenGL Shading Language: 89 90 #define GL_EXT_shader_samples_identical 91 92 Add to the table in section 8.7 "Texture Lookup Functions" 93 94 Syntax: 95 96 bool textureSamplesIdenticalEXT(gsampler2DMS sampler, ivec2 coord) 97 98 bool textureSamplesIdenticalEXT(gsampler2DMSArray sampler, 99 ivec3 coord) 100 101 Description: 102 103 Returns true if it can be determined that all samples within the texel 104 of the multisample texture bound to <sampler> at <coord> contain the 105 same values or false if this cannot be determined." 106 107 Additions to the AGL/EGL/GLX/WGL Specifications 108 109 None 110 111 Errors 112 113 None 114 115 New State 116 117 None 118 119 New Implementation Dependent State 120 121 None 122 123 Issues 124 125 1) What should the new functions be called? 126 127 RESOLVED: textureSamplesIdenticalEXT. Initially 128 textureAllSamplesIdenticalEXT was considered, but 129 textureSamplesIdenticalEXT is more similar to the existing textureSamples 130 function. 131 132 2) It seems like applications could implement additional optimization if 133 they were provided with raw MCS data. Should this extension also 134 provide that data? 135 136 There are a number of challenges in providing raw MCS data. The biggest 137 problem being that the amount of MCS data depends on the number of 138 samples, and that is not known at compile time. Additionally, without new 139 texelFetch functions, applications would have difficulty utilizing the 140 information. 141 142 Another option is to have a function that returns an array of tuples of 143 sample number and count. This also has difficulties with the maximum 144 array size not being known at compile time. 145 146 RESOLVED: Do not expose raw MCS data in this extension. 147 148 3) Should this extension also extend SPIR-V? 149 150 RESOLVED: Yes, but this has not yet been written. 151 152 4) Is it possible for textureSamplesIdenticalEXT to report false negatives? 153 154 RESOLVED: Yes. It is possible that the underlying hardware may not detect 155 that separate writes of the same color to different samples of a pixel are 156 the same. The shader function is at the whim of the underlying hardware 157 implementation. It is also possible that a compressed multisample surface 158 is not used. In that case the function will likely always return false. 159 160 Revision History 161 162 Rev Date Author Changes 163 --- ---------- -------- --------------------------------------------- 164 1 2014/08/20 cforbes Initial version 165 2 2015/10/23 idr Change from MESA to EXT. Rebase on OpenGL 4.5, 166 and add dependency on OpenGL ES 3.1. Initial 167 draft of overview section and issues 1 through 168 3. 169 3 2015/10/27 idr Typo fixes. 170 4 2015/11/10 idr Rename extension from EXT_shader_multisample_compression 171 to EXT_shader_samples_identical. 172 Add issue #4. 173 5 2015/11/18 idr Fix some typos spotted by gsellers. Change the 174 name of the name of the function to 175 textureSamplesIdenticalEXT. 176 6 2015/11/19 idr Fix more typos spotted by Nicolai Hhnle. 177
