Name

    EXT_texture_env_combine

Name Strings

    GL_EXT_texture_env_combine

Contact

    Michael Gold (gold 'at' nvidia.com)
    Tom Frisinger, AMD (tom.frisinger 'at' amd.com)

Status

    Shipping (NVIDIA, ATI)

Version

    $Date$ $Revision$
    Last Modified Date: 2006/11/04 02:49:00  Revision: 1.8

Number

    158

Dependencies

    SGI_texture_color_table affects the definition of this extension
    SGIX_texture_scale_bias affects the definition of this extension

Overview

    New texture environment function COMBINE_EXT allows programmable
    texture combiner operations, including:

        REPLACE                 Arg0
        MODULATE                Arg0 * Arg1
        ADD                     Arg0 + Arg1
        ADD_SIGNED_EXT          Arg0 + Arg1 - 0.5
        INTERPOLATE_EXT         Arg0 * (Arg2) + Arg1 * (1-Arg2)

    where Arg0, Arg1 and Arg2 are derived from

        PRIMARY_COLOR_EXT       primary color of incoming fragment
        TEXTURE                 texture color of corresponding texture unit
        CONSTANT_EXT            texture environment constant color
        PREVIOUS_EXT            result of previous texture environment; on
                                texture unit 0, this maps to PRIMARY_COLOR_EXT

    and Arg2 is restricted to the alpha component of the corresponding source.

    In addition, the result may be scaled by 1.0, 2.0 or 4.0.

Issues

    Should the explicit bias be removed in favor of an implcit bias as
    part of a ADD_SIGNED_EXT function?

     - Yes.  This pre-scale bias is a special case and will be treated
       as such.

    Should the primary color of the incoming fragment be available to
    all texture environments?  Currently it is only available to the
    texture environment of texture unit 0.

     - Yes, PRIMARY_COLOR_EXT has been added as an input source.

    Should textures from other texture units be allowed as sources?

     - No, not in the base spec.  Too many vendors have expressed
       concerns about the scalability of such functionality.  This can
       be added as a subsequent extension.

    All of the 1.2 modes except BLEND can be expressed in terms of
    this extension.  Should texture color be allowed as a source for
    Arg2, so all of the 1.2 modes can be expressed?  If so, should all
    color sources be allowed, to maintain orthogonality?

     - No, not in the base spec.  This can be added as a subsequent
       extension.

New Procedures and Functions

    None

New Tokens

    Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
    and TexEnviv when the <pname> parameter value is TEXTURE_ENV_MODE

        COMBINE_EXT                                     0x8570

    Accepted by the <pname> parameter of TexEnvf, TexEnvi, TexEnvfv,
    and TexEnviv when the <target> parameter value is TEXTURE_ENV

        COMBINE_RGB_EXT                                 0x8571
        COMBINE_ALPHA_EXT                               0x8572
        SOURCE0_RGB_EXT                                 0x8580
        SOURCE1_RGB_EXT                                 0x8581
        SOURCE2_RGB_EXT                                 0x8582
        SOURCE0_ALPHA_EXT                               0x8588
        SOURCE1_ALPHA_EXT                               0x8589
        SOURCE2_ALPHA_EXT                               0x858A
        OPERAND0_RGB_EXT                                0x8590
        OPERAND1_RGB_EXT                                0x8591
        OPERAND2_RGB_EXT                                0x8592
        OPERAND0_ALPHA_EXT                              0x8598
        OPERAND1_ALPHA_EXT                              0x8599
        OPERAND2_ALPHA_EXT                              0x859A
        RGB_SCALE_EXT                                   0x8573
        ALPHA_SCALE

    Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
    and TexEnviv when the <pname> parameter value is COMBINE_RGB_EXT
    or COMBINE_ALPHA_EXT

        REPLACE
        MODULATE
        ADD
        ADD_SIGNED_EXT                                  0x8574
        INTERPOLATE_EXT                                 0x8575

    Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
    and TexEnviv when the <pname> parameter value is SOURCE0_RGB_EXT,
    SOURCE1_RGB_EXT, SOURCE2_RGB_EXT, SOURCE0_ALPHA_EXT,
    SOURCE1_ALPHA_EXT, or SOURCE2_ALPHA_EXT

        TEXTURE
        CONSTANT_EXT                                    0x8576
        PRIMARY_COLOR_EXT                               0x8577
        PREVIOUS_EXT                                    0x8578

    Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
    and TexEnviv when the <pname> parameter value is
    OPERAND0_RGB_EXT or OPERAND1_RGB_EXT

        SRC_COLOR
        ONE_MINUS_SRC_COLOR
        SRC_ALPHA
        ONE_MINUS_SRC_ALPHA     

    Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
    and TexEnviv when the <pname> parameter value is
    OPERAND0_ALPHA_EXT or OPERAND1_ALPHA_EXT

        SRC_ALPHA
        ONE_MINUS_SRC_ALPHA     

    Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
    and TexEnviv when the <pname> parameter value is
    OPERAND2_RGB_EXT or OPERAND2_ALPHA_EXT

        SRC_ALPHA

    Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
    and TexEnviv when the <pname> parameter value is RGB_SCALE_EXT or
    ALPHA_SCALE

        1.0
        2.0
        4.0     

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

    None

Additions to Chapter 3 of the GL Specification (Rasterization)

    Added to subsection 3.8.9, before the paragraph describing the
    state requirements:

    If the value of TEXTURE_ENV_MODE is COMBINE_EXT, the form of the
    texture function depends on the values of COMBINE_RGB_EXT and
    COMBINE_ALPHA_EXT, according to table 3.20.  The RGB and ALPHA
    results of the texture function are then multiplied by the values
    of RGB_SCALE_EXT and ALPHA_SCALE, respectively.  The results are
    clamped to [0,1].


        COMBINE_RGB_EXT or
        COMBINE_ALPHA_EXT       Texture Function
        ------------------      ----------------
        REPLACE                 Arg0
        MODULATE                Arg0 * Arg1
        ADD                     Arg0 + Arg1
        ADD_SIGNED_EXT          Arg0 + Arg1 - 0.5
        INTERPOLATE_EXT         Arg0 * (Arg2) + Arg1 * (1-Arg2)

        Table 3.20: COMBINE_EXT texture functions

    The arguments Arg0, Arg1 and Arg2 are determined by the values of
    SOURCE<n>_RGB_EXT, SOURCE<n>_ALPHA_EXT, OPERAND<n>_RGB_EXT and
    OPERAND<n>_ALPHA_EXT.  In the following two tables, Ct and At are
    the filtered texture RGB and alpha values; Cc and Ac are the
    texture environment RGB and alpha values; Cf and Af are the RGB
    and alpha of the primary color of the incoming fragment; and Cp
    and Ap are the RGB and alpha values resulting from the previous
    texture environment.  On texture environment 0, Cp and Ap are
    identical to Cf and Af, respectively.  The relationship is
    described in tables 3.21 and 3.22.

        SOURCE<n>_RGB_EXT       OPERAND<n>_RGB_EXT      Argument
        -----------------       --------------          --------
        TEXTURE                 SRC_COLOR               Ct
                                ONE_MINUS_SRC_COLOR     (1-Ct)
                                SRC_ALPHA               At
                                ONE_MINUS_SRC_ALPHA     (1-At)
        CONSTANT_EXT            SRC_COLOR               Cc
                                ONE_MINUS_SRC_COLOR     (1-Cc)
                                SRC_ALPHA               Ac
                                ONE_MINUS_SRC_ALPHA     (1-Ac)
        PRIMARY_COLOR_EXT       SRC_COLOR               Cf
                                ONE_MINUS_SRC_COLOR     (1-Cf)
                                SRC_ALPHA               Af
                                ONE_MINUS_SRC_ALPHA     (1-Af)
        PREVIOUS_EXT            SRC_COLOR               Cp
                                ONE_MINUS_SRC_COLOR     (1-Cp)
                                SRC_ALPHA               Ap
                                ONE_MINUS_SRC_ALPHA     (1-Ap)

        Table 3.21: Arguments for COMBINE_RGB_EXT functions

        SOURCE<n>_ALPHA_EXT     OPERAND<n>_ALPHA_EXT    Argument
        -----------------       --------------          --------
        TEXTURE                 SRC_ALPHA               At
                                ONE_MINUS_SRC_ALPHA     (1-At)
        CONSTANT_EXT            SRC_ALPHA               Ac
                                ONE_MINUS_SRC_ALPHA     (1-Ac)
        PRIMARY_COLOR_EXT       SRC_ALPHA               Af
                                ONE_MINUS_SRC_ALPHA     (1-Af)
        PREVIOUS_EXT            SRC_ALPHA               Ap
                                ONE_MINUS_SRC_ALPHA     (1-Ap)

        Table 3.22: Arguments for COMBINE_ALPHA_EXT functions

    The mapping of texture components to source components is
    summarized in Table 3.23.  In the following table, At, Lt, It, Rt,
    Gt and Bt are the filtered texel values.

        Base Internal Format            RGB Values      Alpha Value
        --------------------            ----------      -----------
        ALPHA                           0,  0,  0       At
        LUMINANCE                       Lt, Lt, Lt      1
        LUMINANCE_ALPHA                 Lt, Lt, Lt      At
        INTENSITY                       It, It, It      It
        RGB                             Rt, Gt, Bt      1
        RGBA                            Rt, Gt, Bt      At
        
        Table 3.23: Correspondence of texture components to source
        components for COMBINE_RGB_EXT and COMBINE_ALPHA_EXT arguments

Additions to Chapter 4 of the GL Specification (Per-Fragment Operations
and the Framebuffer)

    None

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

    None

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

    None

Additions to the GLX Specification

    None

GLX Protocol

    None

Errors

    INVALID_ENUM is generated if <params> value for COMBINE_RGB_EXT or
    COMBINE_ALPHA_EXT is not one of REPLACE, MODULATE, ADD,
    ADD_SIGNED_EXT, or INTERPOLATE_EXT.

    INVALID_ENUM is generated if <params> value for SOURCE0_RGB_EXT,
    SOURCE1_RGB_EXT, SOURCE2_RGB_EXT, SOURCE0_ALPHA_EXT,
    SOURCE1_ALPHA_EXT or SOURCE2_ALPHA_EXT is not one of TEXTURE,
    CONSTANT_EXT, PRIMARY_COLOR_EXT or PREVIOUS_EXT.

    INVALID_ENUM is generated if <params> value for OPERAND0_RGB_EXT
    or OPERAND1_RGB_EXT is not one of SRC_COLOR, ONE_MINUS_SRC_COLOR,
    SRC_ALPHA or ONE_MINUS_SRC_ALPHA.

    INVALID_ENUM is generated if <params> value for OPERAND0_ALPHA_EXT
    or OPERAND1_ALPHA_EXT is not one of SRC_ALPHA or
    ONE_MINUS_SRC_ALPHA.

    INVALID_ENUM is generated if <params> value for OPERAND2_RGB_EXT
    or OPERAND2_ALPHA_EXT is not SRC_ALPHA.

    INVALID_VALUE is generated if <params> value for RGB_SCALE_EXT or
    ALPHA_SCALE is not one of 1.0, 2.0, or 4.0.

Dependencies on SGI_texture_color_table

    If SGI_texture_color_table is implemented, the expanded Rt, Gt,
    Bt, and At values are used directly instead of the expansion
    described by Table 3.23.

Dependencies on SGIX_texture_scale_bias

    If SGIX_texture_scale_bias is implemented, the expanded Rt, Gt,
    Bt, and At values are used directly instead of the expansion
    described by Table 3.23.

New State

    Get Value           Get Command    Type     Initial Value   Attribute
    ---------           -----------    ----     -------------   ---------
    COMBINE_RGB_EXT     GetTexEnviv    n x Z4   MODULATE        texture
    COMBINE_ALPHA_EXT   GetTexEnviv    n x Z4   MODULATE        texture
    SOURCE0_RGB_EXT     GetTexEnviv    n x Z3   TEXTURE         texture
    SOURCE1_RGB_EXT     GetTexEnviv    n x Z3   PREVIOUS_EXT    texture
    SOURCE2_RGB_EXT     GetTexEnviv    n x Z3   CONSTANT_EXT    texture
    SOURCE0_ALPHA_EXT   GetTexEnviv    n x Z3   TEXTURE         texture
    SOURCE1_ALPHA_EXT   GetTexEnviv    n x Z3   PREVIOUS_EXT    texture
    SOURCE2_ALPHA_EXT   GetTexEnviv    n x Z3   CONSTANT_EXT    texture
    OPERAND0_RGB_EXT    GetTexEnviv    n x Z6   SRC_COLOR       texture
    OPERAND1_RGB_EXT    GetTexEnviv    n x Z6   SRC_COLOR       texture
    OPERAND2_RGB_EXT    GetTexEnviv    n x Z1   SRC_ALPHA       texture
    OPERAND0_ALPHA_EXT  GetTexEnviv    n x Z4   SRC_ALPHA       texture
    OPERAND1_ALPHA_EXT  GetTexEnviv    n x Z4   SRC_ALPHA       texture
    OPERAND2_ALPHA_EXT  GetTexEnviv    n x Z1   SRC_ALPHA       texture
    RGB_SCALE_EXT       GetTexEnvfv    n x R3   1.0             texture
    ALPHA_SCALE         GetTexEnvfv    n x R3   1.0             texture

New Implementation Dependent State

    None

NVIDIA Implementation Details

    Because of a hardware limitation, TNT, TNT2, GeForce, and Quadro
    treat "scale by 4.0" with the COMBINE_RGB_EXT or COMBINE_ALPHA_EXT
    mode of ADD_SIGNED_EXT as "scale by 2.0".

    GeForce 6 Series and later GPUs (NV40 and on) support "scale by 4.0"
    properly.

Revision History

    2/22/00 mjk - added NVIDIA Implementation Details