Name

    NV_multisample_coverage

Name Strings

    GL_NV_multisample_coverage
    GLX_NV_multisample_coverage
    WGL_NV_multisample_coverage

Contact

    Mike Strauss (mstrauss 'at' nvidia.com)

Status

    Shipping in NVIDIA release 169.xx drivers (May 2008) for GeForce
    8 and up

Version

    Revision:                   4
    Last Modified Date:         March 7, 2013

Number

    393

Dependencies

    ARB_multisample is required.
    Written using the language of the OpenGL 2.1 specification.
    Written using the language of the GLX 1.4 specification.

Overview

    The ARB_multisample extension provides a mechanism for antialiasing
    primitives.  This mechanism allows an application to request an
    additional buffer, the multisample buffer, that is added to the
    framebuffer.  An application can request the number of samples per
    fragment that are stored in the multisample buffer.  Rendering 
    proceeds by writing color, depth, and stencil values for each 
    sample to the multisample buffer.  The results are automatically
    resolved to a single displayable color each time a pixel is 
    updated.

    Coverage Sample Anti-Aliasing (CSAA) is an extension to multisample
    antialiasing.  The technique separates "samples" into two types of
    samples.  "Color samples" are samples with color, depth, and 
    stencil information stored in the multisample buffer.  "Coverage
    samples" include both color samples and additional samples that only
    provide pixel coverage information.

    This extension follows the example of the
    NV_framebuffer_multisample_coverage extension, which adds CSAA
    support for framebuffer objects.  The base description of 
    multisample rendering is written in terms of coverage samples and
    color samples.  The windows system notion of "samples"
    (SAMPLES_ARB) is layered on top of coverage and color samples.

IP Status

    None.

New Procedures and Functions

    None.

New Tokens

    Accepted by the <attribList> parameter of glXChooseVisual, and by
    the <attrib> parameter of glXGetConfig, the <attrib_list> parameter
    of glXChooseFBConfig, and the <attribute> parameter of
    glXGetFBConfigAttrib:

        GLX_COVERAGE_SAMPLES_NV              100001
        GLX_COLOR_SAMPLES_NV                 0x20B3

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

        WGL_COVERAGE_SAMPLES_NV              0x2042
        WGL_COLOR_SAMPLES_NV                 0x20B9

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

        COLOR_SAMPLES_NV                     0x8E20

    (Note:  Earlier versions of this extension included a token
     COVERAGE_SAMPLES_NV that was an alias for SAMPLES/SAMPLES_ARB.  This was
     removed to avoid a name collision with a similar COVERAGE_SAMPLES_NV
     token from the NV_coverage_sample extension to OpenGL ES.)

Additions to Chapter 2 of the OpenGL 2.0 Specification (OpenGL Operation)

    None.

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

    Modify Section 3.2.1, Multisampling (p. 93).

    (replace third and fourth paragraphs, p. 94)

    During multisample rendering the contents of a pixel fragment are
    changed in three 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 coverage samples.  The
    number of coverage samples includes both samples that indicate only
    coverage information, as well as samples that include coverage,
    color, depth, and texture coordinate information.  The location of
    the coverage information only samples may be anywhere within the
    pixel.  These samples may be used with an implementation-dependent
    algorithm as additional weights when resolving the color values to a
    single pixel color.

    Third, each fragment includes COLOR_SAMPLES_NV depth values,
    color values, and sets of texture coordinates instead of the
    single depth value, color value, and set of texture coordinates
    that are maintained in single-sample rendering mode.  These samples
    are referred to as color samples, although they contain more than
    color information.  COLOR_SAMPLES_NV is an implementation-dependent
    constant, and is queried by calling GetIntegerv with <pname> set to
    COLOR_SAMPLES_NV.  An implementation may choose to assign the same 
    color value and the same set of texture coordinates to more than 
    one sample.  The location for evaluating the color value and the 
    set of texture coordinates can be anywhere within the pixel 
    including the fragment center or any of the color sample locations.
    The color value and the set of texture coordinates need not be 
    evaluated at the same location.  Each pixel fragment thus 
    consists of integer x and y grid coordinates, SAMPLES_ARB
    samples, COLOR_SAMPLES_NV of which contain color and depth values,
    COLOR_SAMPLES_NV sets of texture coordinates, and a coverage value
    with a maximum of SAMPLES_ARB bits.

    (modify seventh paragraph, p. 94)

    If MULTISAMPLE 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.  Of these locations, COLOR_SAMPLES_NV
    have associated color, depth, and texture coordinates.  The
    remaining locations only supply additional pixel coverage
    information.  These locations are exact positions, rather than
    regions or areas, and each is referred to as a sample point.  The
    sample points...

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

    Modify Section 4.1.11, Additional Multisample Fragment 
    Operations (p. 213).

    (modify second paragraph, p. 213)

    If MULTISAMPLE is enabled, and the value of SAMPLE_BUFFERS is one,
    the alpha test, stencil test, depth test, blending, and dithering
    operations are performed for each pixel color sample, rather than
    just once for each fragment...

    (modify third paragraph, p 215)

    After all operations have been completed on the multisample...
    The method of combination is not specified, though a simple average
    computed independently for each color component is recommended when
    the number of coverage samples and color samples are equal.


Additions to Chapter 5 of the OpenGL 2.0 Specification (Special Functions)

    None.

Additions to Chapter 6 of the OpenGL 2.0 Specification (State and
State Requests)

 
Additions to Appendix A of the OpenGL 2.0 Specification (Invariance)

    None.

Additions to the GLX Specification 

    Modify Section 3.3.3 Configuration Management (p. 12)

    (modify fourth paragraph, p. 12)

    The attribute GLX_SAMPLE_BUFFERS indicates the number of multisample
    buffers, which must be zero or one.  The attribute
    GLX_COVERAGE_SAMPLES_NV gives the number of coverage samples per
    pixel, and the attribute GLX_COLOR_SAMPLES_NV gives the number of
    color samples per pixel;  if GLX_SAMPLE_BUFFERS is zero, then
    GLX_COVERAGE_SAMPLES_NV and GLX_COLOR_SAMPLES_NV will also be zero.
    If GLX_SAMPLE_BUFFERS is one, then the number of color, depth, and
    stencil bits for each sample in the multisample buffer are
    specified by the GLX_*_SIZE attributes.  GLX_SAMPLES is an alias for
    GLX_COVERAGE_SAMPLES_NV.

    (modify GLX_SAMPLES entry in Table 3.1: GLXFBConfig attributes,
     p. 12)

    Attribute               Type    Notes
    ----------------------- ------- ------------------------------
    GLX_COVERAGE_SAMPLES_NV integer no. coverage samples per pixel

    (add a new entry to Table 3.1: GLXFBConfig attributes, p.12,
     following the GLX_COVERAGE_SAMPLES_NV entry)

    Attribute               Type    Notes
    ----------------------- ------- ------------------------------
    GLX_COLOR_SAMPLES_NV    integer no. color samples per pixel


    (add a new following the discussion of GLX_TRASPARENT_TYPE,
     p 18.)

    If GLX_COVERAGE_SAMPLES_NV is in <attrib_list>, but
    GLX_COLOR_SAMPLES_NV is not in <attrib_list> the selected
    GLXFBConfigs must have the same number of color samples as
    coverage samples.

    (modify the GLX_SAMPLES entry in Table 3.4: Default values and
    match criteria for GLXFBConfig attributes, p. 19)

                                    Selection
                                    and Sorting Sort 
    Attribute               Default Criteria    Priority
    ----------------------- ------- ----------- --------
    GLX_COVERAGE_SAMPLES_NV 0       SMALLER     7

    
    (add a new entry to table 3.4:  Default values and match criteria
    for GLXFBConfig attributes, p. 19, following the 
    GLX_COVERAGE_SAMPLES_NV entry.
    
                                    Selection
                                    and Sorting Sort 
    Attribute               Default Criteria    Priority
    ----------------------- ------- ----------- --------
    GLX_COLOR_SAMPLES_NV    0       SMALLER     8


    (modify the GLX_DEPTH_SIZE, GLX_STENCIL_SIZE, GLX_ACCUM_*_SIZE,
    and GLX_X_VISUAL_TYPE entries in table 3.4: Default values and
    match criteria for GLXFBConfig attributes, p. 19, by incrementing
    the value of Sort Priority for each)

    (modify sixth paragraph, p. 20)

    7. Smaller GLX_COVERAGE_SAMPLES_NV.

    (add a new paragraph following the sixth paragraph, p. 20)

    8. Smaller GLX_COLOR_SAMPLES_NV.

    (modify paragraphs seven through ten, p. 20, by incrementing
    the list number for each)

    Modify Section 3.4.1 Using Visuals for Configuration Management

    (modify the GLX_SAMPLES entry in Table 3.7: GLX attributes for
    Visuals, p. 37)

    Attribute                 Type     Notes   
    ------------------------- -------- --------------------------
    GLX_COVERAGE_SAMPLES_NV   integer  number of coverage samples
                                       per pixel.

    (add a new entry to table 3.7: GLX attributes for Visuals, p.37,
    following the GLX_COVERAGE_SAMPLES_NV entry)

    Attribute                 Type     Notes   
    ------------------------- -------- --------------------------
    GLX_COLOR_SAMPLES_NV      integer  number of color samples
                                       per pixel.


    (modify the GLX_SAMPLES entry in table 3.8: Defaults and selection
    criteria used by glXChooseVisual, p.38)

    Attribute                 Default Selection Criteria   
    ------------------------- ------- ------------------
    GLX_COVERAGE_SAMPLES_NV      0    Smaller

    (add a new entry to table 3.8: Defaults and selection criteria used
    by glXChooseVisual, p.38, following the GLX_COVERAGE_SAMPLES_NV
    entry)

    Attribute                 Default Selection Criteria   
    ------------------------- ------- ------------------
    GLX_COLOR_SAMPLES_NV         0    Smaller


GLX Protocol

    None.

Additions to the WGL Specification

    Modify additions from ARB_multisample

    (modify first and second paragraphs)

    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 coverage
    samples per pixel is queried by calling wglGetPixelFormatAttrib*v 
    with <piAttributes> set to WGL_COVERAGE_SAMPLES_NV.  The number of
    color samples per pixel is queried by calling 
    wglGetPixelFormatAttrib*v with <piAttributes> set to 
    WGL_COLOR_SAMPLES_NV. 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 pixel format -- the only depth and stencil buffers are those 
    in the multisample buffer.  WGL_COVERAGE_SAMPLES_NV and 
    WGL_COLOR_SAMPLES_NV are zero if WGL_SAMPLES_BUFFERS_ARB is zero.
    WGL_SAMPLES_ARB is an alias for WGL_COVERAGE_SAMPLES_NV.

    wglChoosePixelFormatARB 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 operations with more than one multisample
    buffer are undefined, so the returned value will be either zero or
    one.

    (add a new paragraph, missing from ARB_multisample, following the
     second paragraph)

    wglChoosePixelFormatARB accepts WGL_COVERAGE_SAMPLES_NV in
    <piAttribIList> and  <pfAttibIList> with the corresponding value
    set to the requested number of coverage samples for the multisample 
    buffer.  wglChoosePixelFormatARB accepts WGL_COLOR_SAMPLES_NV in 
    <piAttribIList> and <pfAttribIList> with the corresponding value 
    set to the requested number of color samples for the multisample
    buffer.  A pixel format with differing numbers of coverage and
    color samples may only be chosen when WGL_COLOR_SAMPLES_NV is
    specified.


Errors

    None.

New State


New Implementation Dependent State

                                          Minimum
    Get Value            Type Get Command Value   Description                Sec.
    -------------------- ---- ----------- ------- -------------------------- -----
    SAMPLES_ARB          Z+   GetIntegerv 0       Number of coverage samples 3.2.1
                                                  (state already in ARB_multisample,
                                                   just defined to be coverage samples)
    COLOR_SAMPLES_NV     Z+   GetIntegerv 0       Number of color samples    3.2.1

Issues

    1)  How do we provide backward compatibility for glXChooseFBConfig
        and wglChoosePixelFormatARB if COVERAGE_SAMPLES_NV aliases to
        SAMPLES_ARB?

        RESOLVED:  Traditional multisample antialiasing can be
        be described as CSAA where the number of coverage and color
        samples are the same (all samples provide coverage, color, 
        depth, and stencil information).  Existing applications and
        applications that do not wish to use this extension will use
        SAMPLES_ARB when choosing a multisample GLXFBConfig or pixel 
        format.  In this case, glXChooseFBConfig and 
        wglChoosePixelFormatARB should never return a CSAA visual or 
        pixel format.  This is enforced in the spec by requiring the 
        presence of COLOR_SAMPLES_NV in the attribute list in order to
        return a CSAA GLXFBConfig or pixel format.

        (NOTE:  The token COVERAGE_SAMPLES_NV was removed in version 4
         of the extension, but its value was always indistinguishable
         from SAMPLES_ARB.)

    2)  How do we provide backward compatibility for 
        glXGetFBConfigAttrib and wglGetPixelFormatAttrib*v if 
        COVERAGE_SAMPLES_NV aliases to SAMPLES_ARB?

        RESOLVED: The glXGetFBConfigAttrib and 
        wglGetPixelFormatAttrib*v routines provide a mechanism for 
        applications to query properties of a GLXFBConfig or pixel 
        format.  This is useful for obtaining additional information 
        about a GLXFBConfig or pixel format after choosing one with the
        appropriate routine.  An application can also use these 
        routines to apply custom GLXFBConfig/pixel format selection 
        logic when presented with a set of GLXFBConfigs or pixel 
        formats.

        In the latter case, existing applications and future 
        applications that do not use this extension may accidentally 
        select a CSAA GLXFBConfig or pixel format because they only 
        look at SAMPLES_ARB, which aliases to COVERAGE_SAMPLES_NV.  
        This problem is not unique to CSAA GLXFBConfigs and pixel 
        formats.  An application can run into a problem selecting the 
        wrong GLXFBConfig or pixel format any time a new GLXFBConfig or
        pixel format attribute is added that differs from the old 
        default.  The GLX specification provides GLXFBConfig sorting
        guidelines to aid in working around this problem.  For WGL,
        vendors are encouraged to order their visuals and pixel formats
        in such a way that gives preference to old formats.  With 
        regards to CSAA, it is recommended that vendors list traditional
        multisample pixel formats first, and CSAA pixel formats later.
        The assumption being that applications will tend to choose the
        first pixel format that meets the criteria.

        (NOTE:  The token COVERAGE_SAMPLES_NV was removed in version 4
         of the extension, but its value was always indistinguishable
         from SAMPLES_ARB.)

    3)  The NV_framebuffer_multisample_coverage extension supplies
        specific rules with regards to how the requested number of
        coverage and color samples are rounded to an implementation
        supported value.  Should this extension provide similar rules?

        RESOLVED.  The NV_framebuffer_multisample_coverage extension
        is built on top of EXT_framebuffer_multisample, which provides
        explicit rules for rounding the number of requested samples.  
        Specifically, the spec says to round the number of requested 
        samples up to the next implementation supported value.  
        NV_framebuffer_multisample_coverage says that the number of 
        coverage samples is rounded to the next highest implementation
        supported value.  The number of color samples is rounded up to
        the next highest implementation supported value given the 
        chosen number of coverage samples.

        Choosing WGL pixel formats has always been a flexible
        and fuzzy process.  On the other hand, the GLX specification 
        provides explicit rules for sorting and choosing GLXFBConfigs.
        Language has been added to the GLX spec to sort first on 
        coverage samples, and then on color samples.  This has a
        similar effect as the rules given in 
        NV_framebuffer_multisample_coverage as long as both 
        COVERAGE_SAMPLES_NV and COLOR_SAMPLES_NV are specified.  For
        WGL pixel format selection, the implementation is free to 
        sort in whatever way it wishes.  Although it is recommended
        that sorting be done in a way that is compatible with 
        NV_framebuffer_mulitsample_coverage. 

Revision History

        Revision 1, 6/22/2007, mstrauss:  Initial version
        Revision 2, 6/23/2007, mstrauss:  Removed GLX visual language.
                                          Added GLXFBconfig language.
        Revision 3, 4/20/2009, jajones:   Filled in GLX token values.
                                          Restored GLX 1.2 compatibility
                                          table entries.
        Revision 4, 3/7/2013, pbrown:     Removed the OpenGL API token
                                          COVERAGE_SAMPLES_NV, which was
                                          just an alias for SAMPLES_ARB.