Name

    ARB_draw_buffers_blend

Name Strings

    GL_ARB_draw_buffers_blend

Contact

    Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com)
    Mark Young, AMD (mark.young 'at' amd.com)

Contributors

    Jeff Bolz, NVIDIA
    Pierre Boudier, AMD
    Nick Haemel, AMD
    Timothy Lamb, AMD
    Greg Roth, NVIDIA
    Graham Sellers, AMD
    Mike Strauss, NVIDIA
    Mark Young, AMD

Notice

    Copyright (c) 2009-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete. Approved by the ARB on July 3, 2009.

Version

    Last Modified Date:         01/19/2011
    Revision:                   4

Number

    ARB Extension #69

Dependencies

    The extension is written against the OpenGL 2.0 Specification.

    OpenGL 2.0 is required.

    EXT_draw_buffers2 is required.

Overview

    This extension builds upon the ARB_draw_buffers and
    EXT_draw_buffers2 extensions. In ARB_draw_buffers (part of OpenGL
    2.0), separate values could be written to each color buffer. This
    was further enhanced by EXT_draw_buffers2 by adding in the ability
    to enable blending and to set color write masks independently per
    color output.

    This extension provides the ability to set individual blend
    equations and blend functions for each color output.

New Procedures and Functions

    void BlendEquationiARB(uint buf, enum mode);

    void BlendEquationSeparateiARB(uint buf, enum modeRGB,
                                   enum modeAlpha);

    void BlendFunciARB(uint buf, enum src, enum dst);

    void BlendFuncSeparateiARB(uint buf, enum srcRGB, enum dstRGB,
                               enum srcAlpha, enum dstAlpha);

New Tokens

    None.

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

    None.

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

    None.

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

    Modify Section 4.1.8, Blending p. 205

    (modify section Blend Equation starting with the first paragraph)

   "Blending is controlled by the blend equation. This equation can be
    simultaneously set to the same value for all draw buffers using the
    commands:

       void BlendEquation(enum mode);
       void BlendEquationSeparate(enum modeRGB, enum modeAlpha);

    or for an individual draw buffer using the indexed versions of the
    same commands:

        void BlendEquationiARB(uint buf, enum mode);
        void BlendEquationSeparateiARB(uint buf
                                       enum modeRGB,
                                       enum modeAlpha);

    BlendEquationSeparate and BlendEquationSeparateiARB argument
    <modeRGB> determines the RGB blend equation while <modeAlpha>
    determines the alpha blend equation. BlendEquation and
    BlendEquationiARB argument <mode> determines both the RGB and alpha
    blend equations. <mode>, <modeRGB>, and <modeAlpha> must be one of
    FUNC_ADD, FUNC_SUBTRACT, FUNC_REVERSE_SUBTRACT, MIN, MAX.
    BlendEquation and BlendEquationSeparate modify the blend equations
    for all draw buffers. BlendEquationiARB and
    BlendEquationSeparateiARB modify the blend equations associated with
    an individual draw buffer. The <buf> argument is an integer i that
    indicates that the blend equations should be modified for
    DRAW_BUFFERi.

    (modify section Blend Functions starting with the first paragraph)

    The weighting factors used by the blend equation are determined by
    the blend functions. Blend functions are simultaneously specified
    for all draw buffers using the commands:

        void BlendFunc(enum src, enum dst);
        void BlendFuncSeparate(enum srcRGB, enum dstRGB,
                               enum srcAlpha, enum dstAlpha);

    or for an individual draw buffer using the indexed versions of the
    same commands:

        void BlendFunciARB(uint buf, enum src, enum dst);
        void BlendFuncSeparateiARB(uint buf, enum srcRGB,
                                   enum dstRGB, enum srcAlpha,
                                   enum dstAlpha);

    BlendFuncSeparate and BlendFuncSeparateiARB arguments <srcRGB> and
    <dstRGB> determine the source and destination RGB blend functions,
    respectively, while <srcAlpha> and <dstAlpha> determine the source
    and destination alpha blend functions. BlendFunc and BlendFunciARB
    argument <src> determines both RGB and alpha source functions, while
    <dst> determines both RGB and alpha destination functions.
    BlendFuncSeparate and BlendFunc modify the blend functions for all
    draw buffers. BlendFuncSeparateiARB and BlendFunciARB modify the
    blend functions associated with an individual draw buffer. The <buf>
    argument is an integer i that indicates that the blend equations
    should be modified for DRAW_BUFFER<i>."


    (modify section Blending State)

   "The state required for blending, for each draw buffer, is two
    integers for the RGB and alpha blend equations, four integers
    indicating the source and destination RGB and alpha blending
    functions, and a bit indicating whether blending is enabled or
    disabled. Additionally, four floating-point values to store the RGBA
    constant blend color are required.

    For all draw buffers, the initial blend equations for RGB and alpha
    are both FUNC_ADD, and the initial blending functions are ONE for
    the source RGB and alpha functions, and ZERO for the destination RGB
    and alpha functions. Initially, blending is disabled for all draw
    buffers. The initial constant blend color is (R,G,B,A) = (0,0,0,0).

    The value of the blend enable for draw buffer <i> can be queried by
    calling IsEnabledi with <target> BLEND and <index> <i>, and the
    values of the blend equations and functions can be queried by caling
    GetIntegerIndexedvEXT with the corresponding <target> as shown in
    table 6.20 and <index> $i$.
    
    The value of the blend enable, or the blend equations and functions
    for draw buffer zero may also be queried by calling IsEnabled, or
    GetInteger, respectively, with the same symbolic constants but no
    <index> parameter."


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)

    None.

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

    None.

Additions to the AGL/GLX/WGL Specifications

    None.

Errors

    The error INVALID_ENUM is generated by BlendEquationiARB,
    BlendEquationSeparateiARB, BlendFuncSeparateiARB, and
    BlendFunciARB if the <buf> parameter is outside the range
    [0, MAX_DRAW_BUFFERS-1].

    The error INVALID_ENUM is generated by GetIntegerIndexedvEXT
    if <target> is BLEND_EQUATION, BLEND_EQUATION_RGB,
    BLEND_EQUATION_ALPHA, BLEND_SRC, BLEND_SRC_RGB, BLEND_SRC_ALPHA,
    BLEND_DST, BLEND_DST_RGB, BLEND_DST_ALPHA, and <index> is outside
    the range [0, MAX_DRAW_BUFFERS-1].

    The error INVALID_ENUM is generated by BlendFunciARB
    if either <src>, or <dst> is not an accepted value.

    The error INVALID_ENUM is generated by BlendFuncSeparateiARB
    if either <srcRGB>, <dstRGB>, <srcAlpha>, or <dstAlpha> is not
    an accepted value.

    The error INVALID_ENUM is generated if the <mode> parameter of
    BlendEquationiARB is not one of FUNC_ADD, FUNC_SUBTRACT,
    FUNC_REVERSE_SUBTRACT, MAX, or MIN.

    The error INVALID_ENUM is generated if either the <modeRGB> or
    <modeAlpha> parameter of BlendEquationSeparateiARB is not one of
    FUNC_ADD, FUNC_SUBTRACT, FUNC_REVERSE_SUBTRACT, MAX, or MIN.

    The error INVALID_OPERATION is generated if any of these new
    entry-points are executed between the execution of glBegin and the
    corresponding execution of glEnd.


New State

    Modify Table 6.20, p. 281

    (modify the entries for the blend functions and equations and add
    new entries for indexed access to the per draw buffer functions
    and equations)


                                                                         Initial
    Get Target                           Type      Get Command           Value    Description                                          Section Attribute
    -----------------------------------  --------- --------------------- -------- ---------------------------------------------------- ------- -------------
    BLEND_SRC_RGB (BLEND_SRC)            1* x Z_15 GetIntegerIndexedvEXT ONE      Blending source RGB function for draw buffer i       4.2.2   color-buffer
                                                                                  where i is specified as <index>
    BLEND_SRC_ALPHA                      1* x Z_15 GetIntegerIndexedvEXT ONE      Blending source A function for draw buffer i         4.2.2   color-buffer
                                                                                  where i is specified as <index>
    BLEND_DST_RGB (BLEND_DST)            1* x Z_14 GetIntegerIndexedvEXT ZERO     Blending destination RGB function for draw buffer i  4.2.2   color-buffer
                                                                                  where i is specified as <index>
    BLEND_DST_ALPHA                      1* x Z_14 GetIntegerIndexedvEXT ZERO     Blending destination A function for draw buffer i    4.2.2   color-buffer
                                                                                  where i is specified as <index>
    BLEND_EQUATION_RGB (BLEND_EQUATION)  1* x Z    GetIntegerIndexedvEXT FUNC_ADD RGB blending equation for draw buffer i              4.2.2   color-buffer
                                                                                  where i is specified as <index>
    BLEND_EQUATION_ALPHA                 1* x Z    GetIntegerIndexedvEXT FUNC_ADD Alpha blending equation for draw buffer i            4.2.2   color-buffer
                                                                                  where i is specified as <index>

Issues

    1.  What should we call this?

        RESOLVED: ARB_draw_buffers_blend.

        Other suggestions were ARB_drawbuffers_blend and ARB_mrt_blend.

    2. What effect does using the non-indexed versions of the blend
       function or equation entry-points have upon the new indexed per
       draw buffer versions?

        RESOLVED: Similar to the way EXT_draw_buffers2 handles the
        indexed and non-indexed ColorMask interaction in issue 3. The
        non-indexed versions of the functions will set all buffers to
        the passed in parameters.

        For example, BlendFunc will set all color buffer blend functions
        to the values passed in.

    3. Should the new functions be ---Indexed (as in EXT_draw_buffers2)
       or ---i (as in OpenGL 3.0 core)?

        RESOLVED: Because this is to be promoted to core, the i index is
        used

Revision History

    Rev.    Date    Author     Changes
    ----  -------- ---------  -----------------------------------------
      4   1/19/11  Jon Leech  Fix GetIntegeri_v -> GetIntegerIndexedi_v
                              (Bug 7229).
      3   2/11/10  Jon Leech  Clarify in required state section that
                              the constant blend color is global,
                              not per-draw-buffer. Remove duplicate
                              state for draw buffer zero from state
                              tables and describe how non-indexed
                              queries access it from required state
                              section (Bug 5920).
      2   8/02/09  Jon Leech  Reformat to 80 columns and assign
                              ARB extension number.
      1   5/14/09  jbolz      copy & rename from EXT version rev 5.