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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
Status
XXX - Not complete yet.
Version
Last Modified Date: November 19, 2015
Revision: 6
Number
TBD
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 <sampler> at <coord> 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.
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