Name

    CHROMIUM_framebuffer_mixed_samples

Name Strings

    GL_CHROMIUM_framebuffer_mixed_samples

Version

    Last Modifed Date: 7. December, 2015

Dependencies

    OpenGL ES 2.0 is required.

    This extension interacts with CHROMIUM_framebuffer_multisample.

Overview

    This extension allows multisample rendering with a raster and
    depth/stencil sample count that is larger than the color sample count.
    Rasterization and the results of the depth and stencil tests together
    determine the portion of a pixel that is "covered".  It can be useful to
    evaluate coverage at a higher frequency than color samples are stored.
    This coverage is then "reduced" to a collection of covered color samples,
    each having an opacity value corresponding to the fraction of the color
    sample covered.  The opacity can optionally be blended into individual
    color samples.

  The key features of this extension are:

    - It allows a framebuffer object to be considered complete when its depth
      or stencil samples are a multiple of the number of color samples.

    - It redefines SAMPLES to be the number of depth/stencil samples (if any);
      otherwise, it uses the number of color samples. SAMPLE_BUFFERS is one if
      there are multisample depth/stencil attachments.  Multisample
      rasterization and multisample fragment ops are allowed if SAMPLE_BUFFERS
      is one.

    - It replaces several error checks involving SAMPLE_BUFFERS by error
      checks directly referencing the number of samples in the relevant
      attachments.

    - A coverage reduction step is added to Per-Fragment Operations which
      converts a set of covered raster/depth/stencil samples to a set of
      covered color samples.  The coverage reduction step also includes an
      optional coverage modulation step, multiplying color values by a
      fractional opacity corresponding to the number of associated
      raster/depth/stencil samples covered.

New Procedures and Functions

    void CoverageModulationCHROMIUM(enum components);

New Tokens

    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv and
    GetFloatv:

    COVERAGE_MODULATION_CHROMIUM                          0x9332


Additions to specification text related to framebuffers and framebuffer objects
(Framebuffers and Framebuffer Objects)

    Pending CHROMIUM_framebuffer_multisample specification.
    Apply relevant rules in spirit of the overview to the specification text.


Additions to Chapter 3 of OpenGL ES 2.0 Specification (Rasterization)


    Modify Section 3.2 (Multisampling)

    Pending CHROMIUM_framebuffer_multisample specification.
    Apply relevant rules in spirit of the overview to the specification text.

Additions to Chapter 4 of OpenGL ES 2.0 Specification
(Per-Fragment Operations and the Framebuffer)

     Modify Figure 4.1 (Per-fragment operations)

    Add a new stage called "Coverage Reduction" between "Dept Buffer Test" and
    "Blending".

    Add a new Section 4.1.Y (Coverage Reduction) after 4.1.5.

    If the value of effective raster samples is greater than the value of
    color samples, a fragment's coverage is reduced from
    effective raster samples bits to color samples bits. There is an
    implementation-dependent association of raster samples to color samples.
    The reduced "color coverage" is computed such that the coverage bit for
    each color sample is 1 if any of the associated bits in the fragment's
    coverage is on, and 0 otherwise.  Blending and writes to the framebuffer
    are not performed on samples whose color coverage bit is zero.

    For each color sample, the associated bits of the fragment's coverage are
    counted and divided by the number of associated bits to produce a
    modulation factor R in the range (0,1] (a value of zero would have been
    killed due to a color coverage of 0). Specifically:

        N = value of effective raster samples;
        M = value of color samples;
        R = popcount(associated coverage bits) / (N / M);

    For each draw buffer with a floating point or normalized color format, the
    fragment's color value is replicated to M values which may each be
    modulated (multiplied) by that color sample's associated value of R. This
    modulation is controlled by the function:

        CoverageModulationCHROMIUM(enum components);

    <components> may be RGB, RGBA, ALPHA, or NONE. If <components> is RGB or
    RGBA, the red, green, and blue components are modulated. If components is
    RGBA or ALPHA, the alpha component is modulated. The initial value of
    COVERAGE_MODULATION_CHROMIUM is NONE.

New State

    Get Value                       Get Command    Type    Initial Value    Description                 Sec.    Attribute
    ---------                       -----------    ----    -------------    -----------                 ----    ---------

    COVERAGE_MODULATION_CHROMIUM    GetIntegerv    E       NONE             Which components are        4.1.Y  -
                                                                            multiplied by coverage
                                                                            fraction
Issues

    (1) How is coverage modulation intended to be used?

    RESOLVED: Coverage modulation allows the coverage to be converted to
    "opacity", which can then be blended into the color buffer to accomplish
    antialiasing. This is similar to the intent of POLYGON_SMOOTH. For example,
    if non-premultiplied alpha colors are in use (common OpenGL usage):

        glCoverageModulationCHROMIUM(GL_ALPHA);
        glEnable(GL_BLEND);
        glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA,
                            GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

    or if using pre-multiplied alpha colors (common in 2D rendering):

        glCoverageModulationCHROMIUM(GL_RGBA);
        glEnable(GL_BLEND);
        glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);