OES_texture_env_crossbar

GL_OES_texture_env_crossbar

Copyright (c) 2005-2013 The Khronos Group Inc. Copyright terms at
    http://www.khronos.org/registry/speccopyright.html

Khronos-approved extension specifications are updated in response to
issues and bugs prioritized by the Khronos OpenGL ES 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

Last modified date: May 18, 2005

OpenGL ES Extension #21    

OpenGL ES 1.1 is required.

This extension is based on the ARB_texture_env_crossbar
extension specification.

This extension adds the capability to use the texture color from
other texture units as sources to the COMBINE enviornment
function. OpenGL ES 1.1 defined texture combine functions which
could use the color from the current texture unit as a source. 
This extension adds the ability to use the color from any texture 
unit as a source.

Please refer to the ARB_texture_env_crossbar extension specification.

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Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
and TexEnviv when the <pname> parameter value is SOURCE0_RGB,
SOURCE1_RGB, SOURCE2_RGB, SOURCE0_ALPHA,
SOURCE1_ALPHA, or SOURCE2_ALPHA

    TEXTURE<n>                        0x84C0+<n>

where <n> is in the range 0 to MAX_TEXTURE_UNITS.

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The arguments Arg0, Arg1 and Arg2 are determined by the values of
SOURCE<n>_RGB, SOURCE<n>_ALPHA, OPERAND<n>_RGB and
OPERAND<n>_ALPHA. In the following two tables, Ct and At are
the filtered texture RGB and alpha values; Ct<n> and At<n> are the
filtered texture RGB and alpha values from the texture bound to
texture unit <n>; 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 table 3.5 and 3.6 of the OpenGL ES 1.2
specification.

Added to table 3.5 of the OpenGL ES 1.2 specification:

    SOURCE<n>_RGB       OPERAND<n>_RGB         Argument
    -------------       --------------         --------

    TEXTURE<n>          SRC_COLOR               Ct<n>
                        ONE_MINUS_SRC_COLOR     (1-Ct<n>)
                        SRC_ALPHA               At<n>
                        ONE_MINUS_SRC_ALPHA     (1-At<n>)

    Table 3.6: COMBINE_RGB texture functions

Added to table 3.6 of the OpenGL ES 1.2 specification:


    SOURCE<n>_ALPHA     OPERAND<n>_ALPHA       Argument
    ---------------     ----------------       --------

    TEXTURE<n>          SRC_ALPHA               At<n>
                        ONE_MINUS_SRC_ALPHA     (1-At<n>)

    Table 3.6: COMBINE_ALPHA texture functions

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Inserted after the second paragraph of F.2.12:

If the value of TEXTURE_ENV_MODE is COMBINE, the texture
function associated with a given texture unit is computed using
the values specified by SOURCE<n>_RGB, SOURCE<n>_ALPHA,
OPERAND<n>_RGB and OPERAND<n>_ALPHA. If TEXTURE<n> is
specified as SOURCE<n>_RGB or SOURCE<n>_ALPHA, the texture
value from texture unit <n> will be used in computing the texture
function for this texture unit.

Inserted after the third paragraph of F.2.12:

If a texture environment for a given texture unit references a
texture unit that is disabled or does not have a valid texture
object bound to it, then it is as if texture blending is disabled 
for the given texture unit. Every texture unit implicitly 
references the texture object that is bound to it, regardless 
of the texture function specified by COMBINE_RGB or COMBINE_ALPHA.

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INVALID_ENUM is generated if <params> value for SOURCE0_RGB,
SOURCE1_RGB, SOURCE2_RGB, SOURCE0_ALPHA,
SOURCE1_ALPHA or SOURCE2_ALPHA is not one of TEXTURE,
CONSTANT, PRIMARY_COLOR, PREVIOUS, or TEXTURE<n>,
where <n> is in the range 0 to MAX_TEXTURE_UNITS.

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