docs/specs/EXT_shader_samples_identical.txt

   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 Hähnle.