Name

    ARB_draw_buffers

Name Strings

    GL_ARB_draw_buffers

Contributors

    Benj Lipchak, AMD
    Bill Licea-Kane, AMD

Contact

    Rob Mace, NVIDIA (rmace 'at' nvidia.com)

Notice

    Copyright (c) 2004-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

IP Status

    No known IP issues.

Status

    Complete. Approved by the ARB on July 23, 2004.

Version

    Last Modified Date: February 19, 2008
    Revision: 17

Number

    ARB Extension #37

Dependencies

    The extension is written against the OpenGL 1.5 Specification.

    OpenGL 1.3 is required.

    ARB_fragment_program affects the definition of this extension.

    ARB_fragment_shader affects the definition of this extension.


Overview

    This extension extends ARB_fragment_program and ARB_fragment_shader
    to allow multiple output colors, and provides a mechanism for
    directing those outputs to multiple color buffers.


Issues

    (1) How many GL_DRAW_BUFFER#_ARB enums should be reserved?  

      RESOLVED: We only need 4 currently, but for future expandability 
      it would be nice to keep the enums in sequence.  We'll specify
      16 for now, which will be more than enough for a long time.

    (2) How should multisample work when there are multiple output
        colors being rendered to multiple draw buffers?

      Basic options are:
        (a) Color 0 is written to the multisample buffer and then the
            multisample buffer is resolved to all the color buffers.
            This option would be consistent with GL's idea of a single
            multisample buffer, but would be really useless and defeat
            the purpose of multiple output colors.
        (b) Have a separate multisample color buffer for each output
            color/draw buffer.  This would be useful but would all
            implementations be able to handle it?
        (c) Don't allow multiple output colors and multisampling to
            be combined by restricting MAX_DRAW_BUFFERS_ARB to 1
            for contexts with multisample buffers.  This is simple
            and would allow a future extension to allow (b).

      RESOLUTION: (b) and (c).  Samples will contain separate color
      values for each output color.  Implementations that can not
      support this can restrict MAX_DRAW_BUFFERS_ARB to 1 for contexts
      with multisample buffers.

    (3) Should gl_FragColor be aliased to gl_FragData[0]?

      RESOLUTION: No.  A shader should write either gl_FragColor, or
      gl_FragData[n], but not both.

      Writing to gl_FragColor will write to all draw buffers specified
      with DrawBuffersARB.

    (4) Should gl_FragData[n] be clamped?

      RESOLUTION: They will be clamped if fragment color clamping is
      enabled.


New Procedures and Functions

    void DrawBuffersARB(sizei n, const enum *bufs);


New Tokens

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

        MAX_DRAW_BUFFERS_ARB                    0x8824
        DRAW_BUFFER0_ARB                        0x8825
        DRAW_BUFFER1_ARB                        0x8826
        DRAW_BUFFER2_ARB                        0x8827
        DRAW_BUFFER3_ARB                        0x8828
        DRAW_BUFFER4_ARB                        0x8829
        DRAW_BUFFER5_ARB                        0x882A
        DRAW_BUFFER6_ARB                        0x882B
        DRAW_BUFFER7_ARB                        0x882C
        DRAW_BUFFER8_ARB                        0x882D
        DRAW_BUFFER9_ARB                        0x882E
        DRAW_BUFFER10_ARB                       0x882F
        DRAW_BUFFER11_ARB                       0x8830
        DRAW_BUFFER12_ARB                       0x8831
        DRAW_BUFFER13_ARB                       0x8832
        DRAW_BUFFER14_ARB                       0x8833
        DRAW_BUFFER15_ARB                       0x8834


Additions to Chapter 2 of the OpenGL 1.5 Specification (OpenGL
Operation)

    None


Additions to Chapter 3 of the OpenGL 1.5 Specification (Rasterization)

    Modify Section 3.2.1, Multisampling (p. 71)

    (replace the second paragraph with)

    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.  Samples contain separate
    color values for each output color.  When the framebuffer includes a
    multisample buffer, it does not include 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.


    Modify Section 3.11.2, Fragment Program Grammar and Semantic
    Restrictions (ARB_fragment_program)

    (replace <resultBinding> grammar rule with these rules)

    <resultBinding>        ::= "result" "." "color" <optOutputColorNum>
                             | "result" "." "depth"

    <optOutputColorNum>    ::= ""
                             | "[" <outputColorNum> "]"

    <outputColorNum>       ::= <integer> from 0 to MAX_DRAW_BUFFERS_ARB-1


    Modify Section 3.11.3.4, Fragment Program Results

    (modify Table X.3)

        Binding                        Components  Description
        -----------------------------  ----------  ----------------------------
        result.color[n]                (r,g,b,a)   color n 
        result.depth                   (*,*,*,d)   depth coordinate

        Table X.3:  Fragment Result Variable Bindings.  Components labeled
        "*" are unused.  "[n]" is optional -- color <n> is used if 
        specified; color 0 is used otherwise.

    (modify third paragraph)  If a result variable binding matches 
    "result.color[n]", updates to the "x", "y", "z", and "w" components 
    of the result variable modify the "r", "g", "b", and "a" components, 
    respectively, of the fragment's corresponding output color.  If 
    "result.color[n]" is not both bound by the fragment program and 
    written by some instruction of the program, the output color <n> of 
    the fragment program is undefined.


    Add a new Section 3.11.4.5.3 (ARB_fragment_program)

    3.11.4.5.3  Draw Buffers Program Option

    If a fragment program specifies the "ARB_draw_buffers" option,
    it will generate multiple output colors, and the result binding
    "result.color[n]" is allowed, as described in section 3.11.3.4,
    and with modified grammar rules as set forth in section 3.11.2.
    If this option is not specified, a fragment program that attempts
    to bind "result.color[n]" will fail to load, and only "result.color"
    will be allowed.


    Add a new section 3.11.6 (ARB_fragment_shader)

    Section 3.11.6 Fragment Shader Output

    The OpenGL Shading Language specification describes the values that
    may be output by a fragment shader. These are gl_FragColor,
    gl_FragData[n], and gl_FragDepth.  If fragment color clamping is
    enabled, the final fragment color values or the final fragment data
    values written by a fragment shader are clamped to the range [0,1]
    and then converted to fixed-point as described in section 2.13.9,
    Final Color Processing.

    The final fragment depth written by a fragment shader is first
    clamped to [0,1] then  converted to fixed-point as if it were a
    window z value. See Section 2.10.1, Controlling the Viewport.  Note
    that the depth range computation is NOT applied here, only the
    conversion to fixed-point.

    The OpenGL Shading Language specification defines what happens when
    color and/or depth are not written. Those rules are repeated here.

    Writing to gl_FragColor specifies the fragment color that will be
    used by the subsequent fixed functionality pipeline. If subsequent
    fixed functionality consumes fragment color and an execution of a
    fragment shader does not write a value to gl_FragColor then the
    fragment color consumed is undefined.

    Writing to gl_FragData[n] specifies the fragment data that will be
    used by the subsequent fixed functionality pipeline.  If subsequent
    fixed functionality consumes fragment data and an execution of a
    fragment shader does not write a value to gl_FragData[n] then the
    fragment data consumed is undefined.

    If a shader statically assigns a value to gl_FragColor, it may not
    assign a value to gl_FragData[n].  If a shader statically writes a
    value to gl_FragData[n], it may not assign a value to gl_FragColor.
    That is, a shader may assign values to either gl_FragColor or
    gl_FragData[n], but not both.

    Writing to gl_FragDepth will establish the depth value for the
    fragment being processed. If depth buffering is enabled, and a
    shader does not write gl_FragDepth, then the fixed function value
    for depth will be used as the fragment's depth value. If a shader
    statically assigns a value to gl_FragDepth, and there is an
    execution path through the shader that does not set gl_FragDepth,
    then the value of the fragment's depth may be undefined for some
    executions of the shader. That is, if a shader statically writes
    gl_FragDepth, then it is responsible for always writing it.

    Note, statically assigning a value to gl_FragColor, gl_FragData[n]
    or gl_FragDepth means that there is a line of code in the fragment
    shader source that writes a value to gl_FragColor, gl_FragData[n]
    or gl_FragDepth, respectively, even if that line of code is never
    executed.


Additions to Chapter 4 of the OpenGL 1.5 Specification (Per-Fragment
Operations and the Frame Buffer)


    Replace Section 4.2.1, Selecting a Buffer for Writing (p. 183)

    4.2.1 Selecting Color Buffers for Writing

    The first such operation is controlling the color buffers into
    which each of the output colors are written.  This is accomplished
    with either DrawBuffer or DrawBuffersARB.

    The command

      void DrawBuffer(enum buf);

    defines the set of color buffers to which output color 0 is written.
    <buf> is a symbolic constant specifying zero, one, two, or four
    buffers for writing.  The constants are NONE, FRONT_LEFT,
    FRONT_RIGHT, BACK_LEFT, BACK_RIGHT, FRONT, BACK, LEFT, RIGHT,
    FRONT_AND_BACK, and AUX0 through AUXn, where n + 1 is the number
    of available auxiliary buffers.

    The constants refer to the four potentially visible buffers front
    left, front right, back left, and back right, and to the auxiliary
    buffers.  Arguments other than AUXi that omit reference to LEFT or
    RIGHT refer to both left and right buffers.  Arguments other than
    AUXi that omit reference to FRONT or BACK refer to both front and
    back buffers.  AUXi enables drawing only to auxiliary buffer i.
    Each AUXi adheres to AUXi = AUX0 + i.  The constants and the buffers
    they indicate are summarized in Table 4.3.  If DrawBuffer is
    supplied with a constant (other than NONE) that does not indicate
    any of the color buffers allocated to the GL context, the error
    INVALID_OPERATION results.

      symbolic         front  front  back  back   aux
      constant         left   right  left  right   i
      --------         -----  -----  ----  -----  ---
      NONE
      FRONT_LEFT         *
      FRONT_RIGHT               *
      BACK_LEFT                       *
      BACK_RIGHT                             *
      FRONT              *      *
      BACK                            *      *
      LEFT               *            *
      RIGHT                     *            *
      FRONT_AND_BACK     *      *     *      *
      AUXi                                          *

      Table 4.3: Arguments to DrawBuffer and the buffers that they
      indicate.


    DrawBuffer will set the draw buffer for output colors other than 0
    to NONE.

    The command

      void DrawBuffersARB(sizei n, const enum *bufs);

    defines the draw buffers to which all output colors are written.
    <n> specifies the number of buffers in <bufs>.  <bufs> is a pointer
    to an array of symbolic constants specifying the buffer to which
    each output color is written.  The constants may be NONE,
    FRONT_LEFT, FRONT_RIGHT, BACK_LEFT, BACK_RIGHT, and AUX0 through
    AUXn, where n + 1 is the number of available auxiliary buffers.  The
    draw buffers being defined correspond in order to the respective
    output colors.  The draw buffer for output colors beyond <n> is set
    to NONE.

    Except for NONE, a buffer should not appear more then once in the
    array pointed to by <bufs>.  Specifying a buffer more then once
    will result in the error INVALID_OPERATION.

    If a fragment program is not using the "ARB_draw_buffers" option,
    DrawBuffersARB specifies a set of draw buffers into which output
    color 0 is written.

    If a fragment shader writes to "gl_FragColor", DrawBuffersARB
    specifies a set of draw buffers into which the color written to
    "gl_FragColor" is written.

    The maximum number of draw buffers is implementation dependent and
    must be at least 1.  The number of draw buffers supported can
    be queried with the state MAX_DRAW_BUFFERS_ARB.

    The constants FRONT, BACK, LEFT, RIGHT, and FRONT_AND_BACK that
    refer to multiple buffers are not valid for use in DrawBuffersARB
    and will result in the error INVALID_OPERATION.

    If DrawBuffersARB is supplied with a constant (other than NONE)
    that does not indicate any of the color buffers allocated to
    the GL context, the error INVALID_OPERATION will be generated.  If
    <n> is greater than MAX_DRAW_BUFFERS_ARB, the error
    INVALID_OPERATION will be generated.

    Indicating a buffer or buffers using DrawBuffer or DrawBuffersARB
    causes subsequent pixel color value writes to affect the indicated
    buffers.  If more than one color buffer is selected for drawing,
    blending and logical operations are computed and applied
    independently for each buffer.  If there are multiple output colors
    being written to multiple buffers, the alpha used in alpha to
    coverage and alpha test is the alpha of output color 0.

    Specifying NONE as the draw buffer for an output color will inhibit
    that output color from being written to any buffer.

    Monoscopic contexts include only left buffers, while stereoscopic
    contexts include both left and right buffers.  Likewise, single
    buffered contexts include only front buffers, while double buffered
    contexts include both front and back buffers.  The type of context
    is selected at GL initialization.

    The state required to handle color buffer selection is an integer
    for each supported output color.  In the initial state, draw buffer
    for output color 0 is FRONT if there are no back buffers; otherwise
    it is BACK.  The initial state of draw buffers for output colors
    other then 0 is NONE.


Additions to Chapter 5 of the OpenGL 1.5 Specification (Special 
Functions)

    None


Additions to Chapter 6 of the OpenGL 1.5 Specification (State and
State Requests)

    None


Additions to Chapter 3 of the OpenGL Shading Language 1.10 Specification
(Basics)

    Add a new Section 3.3.1, GL_ARB_draw_buffers Extension (p. 13)

    3.3.1 GL_ARB_draw_buffers Extension

    To use the GL_ARB_draw_buffers extension in a shader it must be
    enabled using the #extension directive.

    The shading language preprocessor #define GL_ARB_draw_buffers will
    be defined to 1, if the GL_ARB_draw_buffers extension is supported.


Dependencies on ARB_fragment_program

    If ARB_fragment_program is not supported then all changes to
    section 3.11 of ARB_fragment_program and the fragment program
    specific part of section 4.2.1 are removed.


Dependencies on ARB_fragment_shader

    If ARB_fragment_shader is not supported then all changes to
    section 3.11 of ARB_fragment_shader, section 3.3.1 of the Shading
    Language Specification, and the fragment shader specific part of
    section 4.2.1 are removed.


Interactions with possible future extensions

    If there is some other future extension that defines multiple
    color outputs then this extension and glDrawBuffersARB could be
    used to define the destinations for those outputs.  This extension
    need not be used only with ARB_fragment_program.

GLX Protocol

    The following rendering command is potentially large, and hence can
    be sent in a glxRender or glxRenderLarge request.
    
        DrawBuffersARB
            2           8+(4*n)         rendering command length
            2           233             rendering command opcode
            4           CARD32          n
            n*4         LISTofCARD32    list of draw buffers
            
    If the command is encoded in a glxRenderLarge request, the command
    opcode and command length fields above are expanded to 4 bytes each:

            4           12+(4*n)        rendering command length
            4           233             rendering command opcode

Errors
  
    The error INVALID_OPERATION is generated by DrawBuffersARB if a
    color buffer not currently allocated to the GL context is specified.

    The error INVALID_OPERATION is generated by DrawBuffersARB if <n>
    is greater than the state MAX_DRAW_BUFFERS_ARB.
    
    The error INVALID_OPERATION is generated by DrawBuffersARB if value
    in <bufs> does not correspond to one of the allowed buffers.

    The error INVALID_OPERATION is generated by DrawBuffersARB if a draw
    buffer other then NONE is specified more then once in <bufs>.


New State

    (table 6.19, p227) add the following entry:

    Get Value                        Type    Get Command    Initial Value Description           Section       Attribute
    -------------------------------  ------  -------------  ------------- --------------------  ------------  ------------
    DRAW_BUFFERi_ARB                 Z10*    GetIntegerv    see 4.2.1     Draw buffer selected  4.2.1         color-buffer
                                                                          for output color i
    

New Implementation Dependent State

    Get Value                        Type  Get Command     Minimum Value    Description             Sec.     Attribute
    ---------                        ----  -----------     -------------    -------------------     -----    ---------
    MAX_DRAW_BUFFERS_ARB             Z+    GetIntegerv     1                Maximum number of       4.2.1    -
                                                                            active draw buffers


Revision History

   Date: 2/19/2008
   Revision: 17 (Mark Kilgard, NVIDIA)
      - Updated contact

   Date: 11/4/2006
   Revision: 16 (Benj Lipchak, AMD)
      - Updated contact info after ATI/AMD merger.

   Date: 12/13/2004
   Revision: 15 (Ian Romanick, IBM)
      - Added GLX protocol.

   Date: 7/26/2004
   Revision: 14
      - Clarified interaction of gl_FragColor and multiple draw buffers.
      - Updated dependencies section.
      - Added real ARB extension #.

   Date: 7/22/2004
   Revision: 13
      - Converted from ATI_draw_buffers to ARB_draw_buffers.

   Date: 7/21/2004
   Revision: 12
      - Updated intro to mention ARB_fragment_shader.
      - Marked which sections modify ARB_fragment_program and
        ARB_fragment_shader.
      - Added "Dependencies on ARB_fragment_shader".
      - Added extension section 3.3.1 to Shading Language spec.
      - Resolved interaction with multisample (issue 2).
      - Fixed typos.

   Date: 6/9/2004
   Revision: 11
      - Added GLSL integration.

   Date: 4/27/2004
   Revision: 10
      - Replaced modification to section 4.2.1 with a complete
        replacement for the section, the individual modifications were
        getting too cumbersome.
      - Added issue (2) on multisampling.

   Date: 4/15/2004
   Revision: 9
      - Specified that it is the alpha of color 0 that is used for alpha
        test.

   Date: 12/30/2002
   Revision: 8
      - Clarified that DrawBuffersATI will set the set of draw buffers
        to write color output 0 to when the "ATI_draw_buffer" fragments
        program option is not in use.

   Date: 9/27/2002
   Revision: 7
      - Fixed confusion between meaning of color buffer and draw buffer
        in last revision.
      - Fixed mistake in when an error is generated based on the <n>
        argument of DrawBuffersATI.

   Date: 9/26/2002
   Revision: 6
      - Cleaned up and put in sync with latest ARB_fragment_program
        revision (#22).  Some meaningless changes made just in the name
        of consistency.

   Date: 9/11/2002
   Revision: 5
      - Added section 3.11.4.5.3.
      - Added enum numbers to New Tokens.

   Date: 9/9/2002
   Revision: 4
      - Changed error from MAX_OUTPUT_COLORS to MAX_DRAW_BUFFERS_ATI.
      - Changed 3.10 section numbers to 3.11 to match change to
        ARB_fragment_program spec.
      - Changed ARB_fragment_program from required to affects, and
        added section on interactions with it and future extensions
        that define multiple color outputs.

   Date: 9/6/2002
   Revision: 3
      - Changed error to INVALID OPERATION.
      - Cleaned up typos.

   Date: 8/19/2002
   Revision: 2
      - Added a paragraph that specifically points out that the 
        constants that refer to multiple buffers are not allowed with
        DrawBuffersATI.
      - Changed bufs to <bufs> in a couple of places.

   Date: 8/16/2002
   Revision: 1
      - First draft for circulation.