xref: /third_party/openGLES/extensions/ARB/ARB_multisample.txt

Name

    ARB_multisample

Name Strings

    GL_ARB_multisample
    GLX_ARB_multisample
    WGL_ARB_multisample

Contact

    Dale Kirkland, NVIDIA (dkirkland 'at' nvidia.com)
    Bill Armstrong, E&S (armstron 'at' es.com)
    Michael Gold, NVIDIA (gold 'at' nvidia.com)
    Jon Leech, SGI (ljp 'at' sgi.com)
    Paula Womack, 3dfx (paulaw 'at' 3dfx.com)

Notice

    Copyright (c) 1999-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Specification Update Policy

    Khronos-approved extension specifications are updated in response to
    issues and bugs prioritized by the Khronos OpenGL Working Group. For
    extensions which have been promoted to a core Specification, fixes will
    first appear in the latest version of that core Specification, and will
    eventually be backported to the extension document. This policy is
    described in more detail at
        https://www.khronos.org/registry/OpenGL/docs/update_policy.php

Status

    Approved by ARB on 12/8/1999.
    GLX protocol must still be defined.

Version

    Last Modified Date: March 12, 2002
    Author Revision: 0.6

    Based on:  SGIS_Multisample Specification
               Date: 1994/11/22 Revision: 1.14

Number

    ARB Extension #5

Dependencies

    WGL_EXT_extensions_string is required.
    WGL_EXT_pixel_format is required.

Overview

    This extension provides a mechanism to antialias all GL primitives:
    points, lines, polygons, bitmaps, and images.  The technique is to
    sample all primitives multiple times at each pixel.  The color
    sample values are resolved to a single, displayable color each time
    a pixel is updated, so the antialiasing appears to be automatic at
    the application level.  Because each sample includes depth and
    stencil information, the depth and stencil functions perform
    equivalently to the single-sample mode.

    An additional buffer, called the multisample buffer, is added to
    the framebuffer.  Pixel sample values, including color, depth, and
    stencil values, are stored in this buffer.  When the framebuffer
    includes a multisample buffer, it does not also include separate
    depth or stencil buffers, even if the multisample buffer does not
    store depth or stencil values.  Color buffers (left/right, front/
    back, and aux) do coexist with the multisample buffer, however.

    Multisample antialiasing is most valuable for rendering polygons,
    because it requires no sorting for hidden surface elimination, and
    it correctly handles adjacent polygons, object silhouettes, and
    even intersecting polygons.  If only points or lines are being
    rendered, the "smooth" antialiasing mechanism provided by the base
    GL may result in a higher quality image.  This extension is
    designed to allow multisample and smooth antialiasing techniques
    to be alternated during the rendering of a single scene.

IP Status

    TBD

Issues

    1. Multiple passes have been taken out.  Is this acceptable?

       RESOLUTION:  Yes.  This can be added back with an additional
       extension if needed.

    2. Would SampleAlphaARB be a better name for the function
       SampleMaskARB?  If so, the name SAMPLE_MASK_ARB should also be
       changed to SAMPLE_ALPHA_ARB.

       RESOLUTION:  Names containing "mask" were changed to use
       "coverage" instead.

    3. Should the SampleCoverageARB function be changed to allow
       blending between more than two objects?

       RESOLUTION:  Not addressed by this extension.  An additional
       extension has been proposed that allows a coverage range for
       each object.  The coverage range is a min and max value that
       can be used to blend multiple objects at different level-of-
       detail fading.  The SampleCoverageARB function will layer on
       this new extension.

New Procedures and Functions

    void SampleCoverageARB(clampf value,
                           boolean invert);

New Tokens

    Accepted by the <attribList> parameter of glXChooseVisual, and by
    the <attrib> parameter of glXGetConfig:

        GLX_SAMPLE_BUFFERS_ARB               100000
        GLX_SAMPLES_ARB                      100001

    Accepted by the <piAttributes> parameter of
    wglGetPixelFormatAttribivEXT, wglGetPixelFormatAttribfvEXT, and
    the <piAttribIList> and <pfAttribIList> of wglChoosePixelFormatEXT:

        WGL_SAMPLE_BUFFERS_ARB               0x2041
        WGL_SAMPLES_ARB                      0x2042

    Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
    and by the <pname> parameter of GetBooleanv, GetIntegerv,
    GetFloatv, and GetDoublev:

        MULTISAMPLE_ARB                      0x809D
        SAMPLE_ALPHA_TO_COVERAGE_ARB         0x809E
        SAMPLE_ALPHA_TO_ONE_ARB              0x809F
        SAMPLE_COVERAGE_ARB                  0x80A0

    Accepted by the <mask> parameter of PushAttrib:

        MULTISAMPLE_BIT_ARB                  0x20000000

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

        SAMPLE_BUFFERS_ARB                   0x80A8
        SAMPLES_ARB                          0x80A9
        SAMPLE_COVERAGE_VALUE_ARB            0x80AA
        SAMPLE_COVERAGE_INVERT_ARB           0x80AB

Additions to Chapter 2 of the 1.2.1 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.2.1 Specification (Rasterization)

    If SAMPLE_BUFFERS_ARB is a value of one, the rasterization of all
    GL primitives is changed, and is referred to as multisample
    rasterization.  Otherwise, primitive rasterization operates as it is
    described in the GL specification, and is referred to as single-
    sample rasterization.  The value of SAMPLE_BUFFERS_ARB is an
    implementation dependent constant, and is queried by calling
    GetIntegerv with <pname> set to SAMPLE_BUFFERS_ARB.  This value is
    the same as GLX_SAMPLE_BUFFERS_ARB or WGL_SAMPLE_BUFFERS_ARB for
    the visual or pixel format associated with the context.

    During multisample rendering the contents of a pixel fragment are
    changed in two ways.  First, each fragment includes a coverage
    value with SAMPLES_ARB bits.  The value of SAMPLES_ARB is an
    implementation-dependent constant, and is queried by calling
    GetIntegerv with <pname> set to SAMPLES_ARB.  Second, each fragment
    includes SAMPLES_ARB depth values, instead of the single depth
    value that is maintained in single-sample rendering mode.  Each
    pixel fragment thus consists of integer x and y grid coordinates,
    a color, SAMPLES_ARB depth values, texture coordinates, and a
    coverage value with a maximum of SAMPLES_ARB bits.

    The behavior of multisample rasterization is a function of
    MULTISAMPLE_ARB, which is enabled and disabled by calling Enable or
    Disable, with <cap> set to MULTISAMPLE_ARB.  Its value is queried
    using IsEnabled, with <cap> set to MULTISAMPLE_ARB.

    If MULTISAMPLE_ARB is disabled, multisample rasterization of all
    primitives is equivalent to single-sample rasterization, except
    that the fragment coverage value is set to full coverage.  The
    depth values may all be set to the single value that would have
    been assigned by single-sample rasterization, or they may be
    assigned as described below for multisample rasterization.

    If MULTISAMPLE_ARB is enabled, multisample rasterization of all
    primitives differs substantially from single-sample rasterization.
    It is understood that each pixel in the framebuffer has SAMPLES_ARB
    locations associated with it.  These locations are exact positions,
    rather than regions or areas, and each is referred to as a sample
    point. The sample points associated with a pixel may be located
    inside or outside of the unit square that is considered to bound
    the pixel. Furthermore, the relative locations of sample points
    may be identical for each pixel in the framebuffer, or they may
    differ.

    If the sample locations differ per pixel, they should be aligned to
    window, not screen, boundaries.  Otherwise rendering results will
    be window-position specific.  The invariance requirement described
    in section 3.1 is relaxed for all enabled multisample rendering,
    because the sample locations may be a function of pixel location.

    It is not possible to query the actual sample locations of a pixel.

    Point Multisample Rasterization
    [Insert before section 3.3.1]

    If MULTISAMPLE_ARB is enabled, and SAMPLE_BUFFERS_ARB is a value of
    one, then points are rasterized using the following algorithm,
    regardless of whether point antialiasing (POINT_SMOOTH) is enabled
    or disabled.  Point rasterization produces a fragment for each
    framebuffer pixel with one or more sample points that intersect the
    region lying within the circle having diameter equal to the current
    point width and centered at the point's (Xw,Yw).  Coverage bits
    that correspond to sample points that intersect the circular region
    are 1, other coverage bits are 0.  All depth values of the fragment
    are assigned the depth value of the point being rasterized. Other
    data associated with each fragment are the data associated with the
    point being rasterized.

    Point size range and number of gradations are equivalent to those
    supported for antialiased points.

    Line Multisample Rasterization
    [Insert before section 3.4.3]

    If MULTISAMPLE_ARB is enabled, and SAMPLE_BUFFERS_ARB is a value of
    one, then lines are rasterized using the following algorithm,
    regardless of whether line antialiasing (LINE_SMOOTH) is enabled
    or disabled. Line rasterization produces a fragment for each
    framebuffer pixel with one or more sample points that intersect the
    rectangular region that is described in the Antialiasing section of
    3.4.2 (Other Line Segment Features).  If line stippling is enabled,
    the rectangular region is subdivided into adjacent unit-length
    rectangles, with some rectangles eliminated according to the
    procedure given under Line Stipple, where "fragment" is replaced
    by "rectangle".

    Coverage bits that correspond to sample points that intersect a
    retained rectangle are 1, other coverage bits are 0.  Each depth
    value is produced by substituting the corresponding sample location
    into equation 3.1, then using the result to evaluate equation 3.3.
    The data associated with each fragment are otherwise computed by
    evaluating equation 3.1 at the fragment center, then substituting
    into equation 3.2.

    Line width range and number of gradations are equivalent to those
    supported for antialiased lines.

    Polygon Multisample Rasterization
    [Insert before section 3.5.6]

    If MULTISAMPLE_ARB is enabled, and SAMPLE_BUFFERS_ARB is a value of
    one, then polygons are rasterized using the following algorithm,
    regardless of whether polygon antialiasing (POLYGON_SMOOTH) is
    enabled or disabled. Polygon rasterization produces a fragment for
    each framebuffer pixel with one or more sample points that satisfy
    the point sampling criteria described in section 3.5.1, including
    the special treatment for sample points that lie on a polygon
    boundary edge.  If a polygon is culled, based on its orientation
    and the CullFace mode, then no fragments are produced during
    rasterization. Fragments are culled by the polygon stipple just as
    they are for aliased and antialiased polygons.

    Coverage bits that correspond to sample points that satisfy the
    point sampling criteria are 1, other coverage bits are 0.  Each
    depth value is produced by substituting the corresponding sample
    location into the barycentric equations described in section 3.5.1,
    using the approximation to equation 3.4 that omits w components.
    The data associated with each fragment are otherwise computed by
    barycentric evaluation using the fragment's center point.

    The rasterization described above applies only to the FILL state of
    PolygonMode.  For POINT and LINE, the rasterizations described in
    the Point Multisample Rasterization and the Line Multisample
    Rasterization sections apply.

    Pixel Rectangle Multisample Rasterization
    [Insert before section 3.6.5]

    If MULTISAMPLE_ARB is enabled, and SAMPLE_BUFFERS_ARB is a value of
    one, then pixel rectangles are rasterized using the following
    algorithm. Let (Xrp,Yrp) be the current raster position.  (If the
    current raster position is invalid, then DrawPixels is ignored.)
    If a particular group (index or components) is the nth in a row and
    belongs to the mth row, consider the region in window coordinates
    bounded by the rectangle with corners

      (Xrp + Zx*n, Yrp + Zy*m)

    and

      (Xrp + Zx*(n+1), Yrp + Zy*(m+1))

    where Zx and Zy are the pixel zoom factors specified by PixelZoom,
    and may each be either positive or negative.  A fragment
    representing group n,m is produced for each framebuffer pixel with
    one or more sample points that lie inside, or on the bottom or
    left boundary, of this rectangle.  Each fragment so produced takes
    its associated data from the group and from the current raster
    position, in a manner consistent with the discussion in the
    Conversion to Fragments subsection of section 3.6.4 of the GL
    specification.  All depth sample values are assigned the same
    value, taken either from the group (if it is a depth component
    group) or from the current raster position (if it is not).

    A single pixel rectangle will generate multiple, perhaps very many
    fragments for the same framebuffer pixel, depending on the pixel
    zoom factors.

    Bitmap Multisample Rasterization
    [Insert at the end section 3.7]

    If MULTISAMPLE_ARB is enabled, and SAMPLE_BUFFERS_ARB is a value of
    one, then bitmaps are rasterized using the following algorithm.  If
    the current raster position is invalid, the bitmap is ignored.
    Otherwise, a screen-aligned array of pixel-size rectangles is
    constructed, with its lower-left corner at (Xrp,Yrp), and its upper
    right corner at (Xrp+w,Yrp+h), where w and h are the width and
    height of the bitmap. Rectangles in this array are eliminated if
    the corresponding bit in the bitmap is zero, and are retained
    otherwise.  Bitmap rasterization produces a fragment for each
    framebuffer pixel with one or more sample points either inside or
    on the bottom or left edge of a retained rectangle.

    Coverage bits that correspond to sample points either inside or on
    the bottom or left edge of a retained rectangle are 1, other
    coverage bits are 0.  The associated data for each fragment are
    those associated with the current raster position.  Once the
    fragments have been produced, the current raster position is
    updated exactly as it is in the single-sample rasterization case.

Additions to Chapter 4 of the 1.2.1 Specification (Per-Fragment
Operations and the Frame Buffer)

    Multisample Fragment Operations
    [Insert after section 4.1.2]

    This step modifies fragment alpha and coverage values based on the
    values of SAMPLE_ALPHA_TO_COVERAGE_ARB, SAMPLE_ALPHA_TO_ONE_ARB,
    SAMPLE_COVERAGE_ARB, SAMPLE_COVERAGE_VALUE_ARB, and
    SAMPLE_COVERAGE_INVERT_ARB.  No changes to the fragment alpha or
    coverage values are made at this step if MULTISAMPLE_ARB is
    disabled, or if SAMPLE_BUFFERS_ARB is not a value of one.

    SAMPLE_ALPHA_TO_COVERAGE_ARB, SAMPLE_ALPHA_TO_ONE_ARB, and
    SAMPLE_COVERAGE_ARB are enabled and disabled by calling Enable and
    Disable with <cap> specified as one of the three token values. All
    three values are queried by calling IsEnabled, with <cap> set to
    the desired token value. If SAMPLE_ALPHA_TO_COVERAGE_ARB is
    enabled, the fragment alpha value is used to generate a temporary
    coverage value, which is then ANDed with the fragment coverage
    value.  Otherwise the fragment coverage value is unchanged at
    this point.

    This specification does not require a specific algorithm for
    converting an alpha value to a temporary coverage value.  It is
    intended that the number of 1's in the temporary coverage be
    proportional to the alpha value, with all 1's corresponding to the
    maximum alpha value, and all 0's corresponding to an alpha value
    of 0.  It is also intended that the algorithm be pseudo-random in
    nature, to avoid image artifacts due to regular coverage sample
    locations.  The algorithm can and probably should be different
    at different pixel locations.  If it does differ, it should be
    defined relative to window, not screen, coordinates, so that
    rendering results are invariant with respect to window position.

    Next, if SAMPLE_ALPHA_TO_ONE_ARB is enabled, fragment alpha is
    replaced by the maximum representable alpha value.  Otherwise,
    fragment alpha value is not changed.

    Finally, if SAMPLE_COVERAGE_ARB is enabled, the fragment coverage
    is ANDed with another temporary coverage.  This temporary coverage
    is generated in the same manner as the one described above, but as
    a function of the value of SAMPLE_COVERAGE_VALUE_ARB.  The function
    need not be identical, but it must have the same properties of
    proportionality and invariance.  If SAMPLE_COVERAGE_INVERT_ARB is
    TRUE, the temporary coverage is inverted (all bit values are
    inverted) before it is ANDed with the fragment coverage.

    The values of SAMPLE_COVERAGE_VALUE_ARB and
    SAMPLE_COVERAGE_INVERT_ARB are specified simultaneously by calling
    SampleCoverageARB, with <value> set to the desired coverage value,
    and <invert> set to TRUE or FALSE. <value> is clamped to [0,1]
    before being stored as SAMPLE_COVERAGE_VALUE_ARB.
    SAMPLE_COVERAGE_VALUE_ARB is queried by calling GetFloatv with
    <pname> set to SAMPLE_COVERAGE_VALUE_ARB.
    SAMPLE_COVERAGE_INVERT_ARB is queried by calling GetBooleanv with
    <pname> set to SAMPLE_COVERAGE_INVERT_ARB.

    Multisample Fragment Operations
    [Insert after section 4.1.8]

    If the DrawBuffers mode is NONE, no change is made to any
    multisample or color buffer.  Otherwise, fragment processing is as
    described below.

    If MULTISAMPLE_ARB is enabled, and SAMPLE_BUFFERS_ARB is one, the
    stencil test, depth test, blending, and dithering operations
    are performed for each pixel sample, rather than just once for each
    fragment.  Failure of the stencil or depth test results in
    termination of the processing of that sample, rather than
    discarding of the fragment.  All operations are performed on the
    color, depth, and stencil values stored in the multisample buffer
    (to be described in a following section).  The contents of the
    color buffers are not modified at this point.

    Stencil, depth, blending, and dithering operations are performed
    for a pixel sample only if that sample's fragment coverage bit is
    a value of 1.  If the corresponding coverage bit is 0, no
    operations are performed for that sample.  Depth operations use
    the fragment depth value that is specific for each sample.  The
    single fragment color value is used for all sample operations,
    however, as is the current stencil value.

    If MULTISAMPLE_ARB is disabled, and SAMPLE_BUFFERS_ARB is one, the
    fragment may be treated exactly as described above, with
    optimization possible because the fragment coverage must be set
    to full coverage. Further optimization is allowed, however.  An
    implementation may choose to identify a centermost sample, and to
    perform stencil and depth tests on only that sample.  Regardless
    of the outcome of the stencil test, all multisample buffer stencil
    sample values are set to the appropriate new stencil value.  If
    the depth test passes, all multisample buffer depth sample values
    are set to the depth of the fragment's centermost sample's depth
    value, and all multisample buffer color sample values are set to
    the color value of the incoming fragment.  Otherwise, no change is
    made to any multisample buffer color or depth value.

    After all operations have been completed on the multisample buffer,
    the color sample values are combined to produce a single color
    value, and that value is written into each color buffer that is
    currently enabled, based on the DrawBuffers mode.  An
    implementation may defer the writing of the color buffer until a
    later time, but the state of the framebuffer must behave as if the
    color buffer was updated as each fragment was processed.  The
    method of combination is not specified, though a simple average
    computed independently for each color component is recommended.

    Fine Control of Multisample Buffer Updates
    [Insert at the end of section 4.2.2]

    When SAMPLE_BUFFERS_ARB is one, ColorMask, DepthMask, and
    StencilMask control the modification of values in the multisample
    buffer.  The color mask has no effect on modifications to the color
    buffers.  If the color mask is entirely disabled, the color sample
    values must still be combined (as described above) and the result
    used to replace the color values of the buffers enabled by
    DrawBuffers.

    Clearing the Multisample Buffer
    [Insert as a subsection for section 4.2.3]

    The color samples of the multisample buffer are cleared when one or
    more color buffers are cleared, as specified by the Clear mask bit
    COLOR_BUFFER_BIT and the DrawBuffers mode.  If the DrawBuffers mode
    is NONE, the color samples of the multisample buffer cannot be
    cleared.

    Clear mask bits DEPTH_BUFFER_BIT and STENCIL_BUFFER_BIT indicate
    that the depth and stencil samples of the multisample buffer are to
    be cleared.  If Clear mask bit DEPTH_BUFFER_BIT is specified, and
    if the DrawBuffers mode is not NONE, then the multisample depth
    buffer samples are cleared.  Likewise, if Clear mask bit
    STENCIL_BUFFER_BIT is specified, and if the DrawBuffers mode is
    not NONE, then the multisample stencil buffer is cleared.

    Reading Pixels
    [These changes are made to the text in section 4.3.2, following the
    subheading Obtaining Pixels from the Framebuffer.]

    Follow the sentence "If there is no depth buffer, the error
    INVALID_OPERATION occurs." with: If there is a multisample buffer
    (SAMPLE_BUFFERS_ARB is 1) then values are obtained from the depth
    samples in this buffer.  It is recommended that the depth value
    of the centermost sample be used, though implementations may choose
    any function of the depth sample values at each pixel.

    Follow the sentence "if there is no stencil buffer, the error
    INVALID_OPERATION occurs." with: If there is a multisample buffer,
    then values are obtained from the stencil samples in this buffer.
    It is recommended that the stencil value of the centermost sample
    be used, though implementations may choose any function of the
    stencil sample values at each pixel.

    [This extension makes no change to the way that color values are
    obtained from the framebuffer.]

Additions to Chapter 5 of the 1.2.1 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.2 Specification (State and State
Requests)

    An additional group of state variables, MULTISAMPLE_BIT_ARB, is
    defined by this extension.  When PushAttrib is called with bit
    MULTISAMPLE_BIT_ARB set, the multisample group of state variables
    is pushed onto the attribute stack.  When PopAttrib is called,
    these state variables are restored to their previous values if
    they were pushed.  Some multisample state is included in the
    ENABLE_BIT group as well. In order to avoid incompatibility with
    GL implementations that do not support SGIS_multisample,
    ALL_ATTRIB_BITS does not include MULTISAMPLE_BIT_ARB.

Additions to the GLX Specification

    The parameter GLX_SAMPLE_BUFFERS_ARB is added to glXGetConfig.
    When queried, by calling glXGetConfig with <attrib> set to
    GLX_SAMPLE_BUFFERS_ARB, it returns the number of multisample
    buffers included in the visual.  For a normal visual, the return
    value is zero. A return value of one indicates that a single
    multisample buffer is available.  The number of samples per pixel
    is queried by calling glXGetConfig with <attrib> set to
    GLX_SAMPLES_ARB.  It is understood that the number of color, depth,
    and stencil bits per sample in the multisample buffer are as
    specified by the GLX_*_SIZE parameters.  It is also understood that
    there are no single-sample depth or stencil buffers associated with
    this visual -- the only depth and stencil buffers are those in the
    multisample buffer.  GLX_SAMPLES_ARB is zero if
    GLX_SAMPLE_BUFFERS_ARB is zero.

    glXChooseVisual accepts GLX_SAMPLE_BUFFERS_ARB in <attribList>,
    followed by the minimum number of multisample buffers that can be
    accepted.  Visuals with the smallest number of multisample buffers
    that meets or exceeds the specified minimum number are preferred.
    Currently operation with more than one multisample buffer is
    undefined, so the returned value will be either zero or one.

    glXChooseVisual accepts GLX_SAMPLES_ARB in <attribList>, followed
    by the minimum number of samples that can be accepted in the
    multisample buffer.  Visuals with the smallest number of samples
    that meets or exceeds the specified minimum number are preferred.

    If the color samples in the multisample buffer store fewer bits
    than are stored in the color buffers, this fact will not be
    reported accurately.  Presumably a compression scheme is being
    employed, and is expected to maintain an aggregate resolution
    equal to that of the color buffers.

GLX Protocol

    One new GL rendering commands is added. The following command is
    sent to the server as part of a glXRender request:

        SampleCoverageARB
            2           12              rendering command length
            2           229             rendering command opcode
            4           FLOAT32         value
            1           BOOL            invert
            3                           unused

Additions to the WGL Specification

    The parameter WGL_SAMPLE_BUFFERS_ARB is added to
    wglGetPixelFormatAttrib*v. When queried, by calling
    wglGetPixelFormatAttrib*v with <piAttributes> set to
    WGL_SAMPLE_BUFFERS_ARB, it returns the number of multisample
    buffers included in the pixel format.  For a normal pixel format,
    the return value is zero.  A return value of one indicates that a
    single multisample buffer is available.  The number of samples per
    pixel is queried by calling wglGetPixelFormatAttrib*v with
    <piAttributes> set to WGL_SAMPLES_ARB.  It is understood that the
    number of color, depth, and stencil bits per sample in the
    multisample buffer are as specified by the WGL_*_SIZE parameters.
    It is also understood that there are no single-sample depth or
    stencil buffers associated with this visual -- the only depth and
    stencil buffers are those in the multisample buffer.
    WGL_SAMPLES_ARB is zero if WGL_SAMPLE_BUFFERS_ARB is zero.

    wglChoosePixelFormatEXT accepts WGL_SAMPLE_BUFFERS_ARB in
    <piAttribIList> and <pfAttribIList> with the corresponding value
    set to the minimum number of multisample buffers that can be
    accepted.  Pixel formats with the smallest number of multisample
    buffers that meets or exceeds the specified minimum number are
    preferred. Currently operation with more than one multisample
    buffer is undefined, so the returned value will be either zero or
    one.

    If the color samples in the multisample buffer store fewer bits
    than are stored in the color buffers, this fact will not be
    reported accurately.  Presumably a compression scheme is being
    employed, and is expected to maintain an aggregate resolution
    equal to that of the color buffers.

Errors

    INVALID_OPERATION is generated if SampleCoverageARB is called
    between the execution of Begin and the execution of the
    corresponding End.

New State

    Get Value                       Get Command    Type    Initial Value    Attribute
    ---------                       -----------    ----    -------------    ---------
    MULTISAMPLE_ARB                 IsEnabled      B       TRUE             multisample/enable
    SAMPLE_ALPHA_TO_COVERAGE_ARB    IsEnabled      B       FALSE            multisample/enable
    SAMPLE_ALPHA_TO_ONE_ARB         IsEnabled      B       FALSE            multisample/enable
    SAMPLE_COVERAGE_ARB             IsEnabled      B       FALSE            multisample/enable
    SAMPLE_COVERAGE_VALUE_ARB       GetFloatv      R+      1                multisample
    SAMPLE_COVERAGE_INVERT_ARB      GetBooleanv    B       FALSE            multisample

New Implementation Dependent State

    Get Value                Get Command    Type    Minimum Value
    ---------                -----------    ----    -------------
    SAMPLE_BUFFERS_ARB       GetIntegerv    Z+      0
    SAMPLES_ARB              GetIntegerv    Z+      0

Conformance Testing

    TBD

Revision History

    09/20/1999  0.1
        - First ARB draft based on the original SGI draft.

    10/1/1999   0.2
        - Added query for the number of passes.

    11/8/1999   0.3
        - Fixed numerous typos reported by E&S.

    12/7/1999   0.4
        - Removed the multiple pass feature.
        - Resolved the working group issues at the ARB meeting.
        - Added language that stated that SAMPLE_BUFFERS_ARB is the
          same value as either GLX_SAMPLE_BUFFERS_ARB or
          WGL_SAMPLE_BUFFERS_ARB.

    12/15/1999  0.5
        - Added back in the statement about ALL_ATTRIB_BITS not
          including MULTISAMPLE_BIT_ARB.

    03/12/2002  0.6
        - Added GLX protocol for SampleCoverageARB. Updated contact
          information.