OES_blend_subtract

GL_OES_blend_subtract

Benj Lipchak, Apple (lipchak 'at' apple.com)

Copyright (c) 2009-2013 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

Ratified by the Khronos BOP, July 31, 2009.

Date: 05/19/2009  Version 1.0

OpenGL ES Extension #3

Written based on the wording of the OpenGL ES 1.1 specification.

Blending capability is extended by respecifying the entire blend
equation.  While this document defines only two new equations, the
BlendEquationOES procedure that it defines will be used by subsequent
extensions to define additional blending equations.

In addition to the default blending equation, two new blending equations
are specified.  These equations are similar to the default blending 
equation, but produce the difference of its left and right hand sides, 
rather than the sum.  Image differences are useful in many image 
processing applications.

void BlendEquationOES(enum mode);

Accepted by the <mode> parameter of BlendEquationOES:

    FUNC_ADD_OES                     0x8006
    FUNC_SUBTRACT_OES                0x800A
    FUNC_REVERSE_SUBTRACT_OES        0x800B

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

    BLEND_EQUATION_OES               0x8009

None

None

Replace the 1st paragraph of the "Blend Equation" discussion in section 
4.1.7 (Blending) with the following:

"Blending is controlled by the blend equations, defined by the command

    void BlendEquationOES(enum mode);
    
<mode> determines the blend equation.  <mode> must be one of FUNC_ADD_OES,
FUNC_SUBTRACT_OES, or FUNC_REVERSE_SUBTRACT_OES."

Replace the last paragraph of the "Blend Equation" discussion in section 
4.1.7 (Blending) with the following:

"Table 4.blendeq provides the corresponding per-component blend equations 
for each mode.  In the table, the s subscript on a color component 
abbreviation (R, G, B, or A) refers to the source color component for an 
incoming fragment and the d subscript on a color component abbreviation 
refers to the destination color component at the corresponding framebuffer 
location.  A color component abbreviation without a subscript refers to the
new color component resulting from blending.  Additionally, Sr, Sg, Sb, and
Sa are the red, green, blue, and alpha components of the source weighting 
factors determined by the source blend function, and Dr, Dg , Db, and Da 
are the red, green, blue, and alpha components of the destination weighting
factors determined by the destination blend function.  Blend functions are 
described below.

Mode                       Equation
-------------------------  ---------------------
FUNC_ADD_OES               R = Rs * Sr + Rd * Dr
                           G = Gs * Sg + Gd * Dg
                           B = Bs * Sb + Bd * Db
                           A = As * Sa + Ad * Da
-------------------------  ---------------------
FUNC_SUBTRACT_OES          R = Rs * Sr - Rd * Dr
                           G = Gs * Sg - Gd * Dg
                           B = Bs * Sb - Bd * Db
                           A = As * Sa - Ad * Da
-------------------------  ---------------------
FUNC_REVERSE_SUBTRACT_OES  R = Rd * Dr - Rs * Sr
                           G = Gd * Dg - Gs * Sg
                           B = Bd * Db - Bs * Sb
                           A = Ad * Da - As * Sa
-------------------------  ---------------------

Table 4.blendeq:  Blend equations."

In the "Blending State" paragraph, insert the following:

"The state required for blending is... one integer indicating the
blend equation...  The initial blending equation is FUNC_ADD_OES."

None

None

INVALID_ENUM is generated by BlendEquationOES if its single parameter
is not FUNC_ADD_OES, FUNC_SUBTRACT_OES, or FUNC_REVERSE_SUBTRACT_OES.

Get Value           Get Command     Type    Initial Value
---------           -----------     ----    -------------
BLEND_EQUATION_OES  GetIntegerv     Z3      FUNC_ADD_OES

None

2009/05/19    Benj Lipchak    First draft of true extension specification