EXT_shader_samples_identical.txt

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.