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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.