1991-2006
Silicon Graphics, Inc.
glBlendFuncSeparate
3G

# glBlendFuncSeparate

specify pixel arithmetic for RGB and alpha components separately

## C Specification

                    void glBlendFuncSeparate
                    GLenum srcRGB
                    GLenum dstRGB
                    GLenum srcAlpha
                    GLenum dstAlpha

## Parameters

`srcRGB`

  Specifies how the red, green, and blue blending factors are computed. The following symbolic constants are accepted: `GL_ZERO`, `GL_ONE`, `GL_SRC_COLOR`, `GL_ONE_MINUS_SRC_COLOR`, `GL_DST_COLOR`, `GL_ONE_MINUS_DST_COLOR`, `GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`, `GL_DST_ALPHA`, `GL_ONE_MINUS_DST_ALPHA`, `GL_CONSTANT_COLOR`, `GL_ONE_MINUS_CONSTANT_COLOR`, `GL_CONSTANT_ALPHA`, `GL_ONE_MINUS_CONSTANT_ALPHA`, and `GL_SRC_ALPHA_SATURATE`. The initial value is `GL_ONE`.

`dstRGB`

  Specifies how the red, green, and blue destination blending factors are computed. The following symbolic constants are accepted: `GL_ZERO`, `GL_ONE`, `GL_SRC_COLOR`, `GL_ONE_MINUS_SRC_COLOR`, `GL_DST_COLOR`, `GL_ONE_MINUS_DST_COLOR`, `GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`, `GL_DST_ALPHA`, `GL_ONE_MINUS_DST_ALPHA`. `GL_CONSTANT_COLOR`, `GL_ONE_MINUS_CONSTANT_COLOR`, `GL_CONSTANT_ALPHA`, and `GL_ONE_MINUS_CONSTANT_ALPHA`. The initial value is `GL_ZERO`.

`srcAlpha`

  Specified how the alpha source blending factor is computed. The same symbolic constants are accepted as for `srcRGB`. The initial value is `GL_ONE`.

`dstAlpha`

  Specified how the alpha destination blending factor is computed. The same symbolic constants are accepted as for `dstRGB`. The initial value is `GL_ZERO`.

## Description

In RGBA mode, pixels can be drawn using a function that blends the incoming (source) RGBA values with the RGBA values that are already in the frame buffer (the destination values). Blending is initially disabled. Use glEnable and glDisable with argument `GL_BLEND` to enable and disable blending.

`glBlendFuncSeparate` defines the operation of blending when it is enabled. `srcRGB` specifies which method is used to scale the source RGB-color components. `dstRGB` specifies which method is used to scale the destination RGB-color components. Likewise, `srcAlpha` specifies which method is used to scale the source alpha color component, and `dstAlpha` specifies which method is used to scale the destination alpha component. The possible methods are described in the following table. Each method defines four scale factors, one each for red, green, blue, and alpha.

In the table and in subsequent equations, source and destination color components are referred to as R s G s B s A s and R d G d B d A d . The color specified by glBlendColor is referred to as R c G c B c A c . They are understood to have integer values between 0 and k R k G k B k A , where

k c = 2 m c - 1

and m R m G m B m A is the number of red, green, blue, and alpha bitplanes.

Source and destination scale factors are referred to as s R s G s B s A and d R d G d B d A . All scale factors have range 0 1 .

Parameter | RGB Factor | Alpha Factor
--- | --- | ---
`GL_ZERO` | 0 0 0 | 0
`GL_ONE` | 1 1 1 | 1
`GL_SRC_COLOR` | R s k R G s k G B s k B | A s k A
`GL_ONE_MINUS_SRC_COLOR` | 1 1 1 1 - R s k R G s k G B s k B | 1 - A s k A
`GL_DST_COLOR` | R d k R G d k G B d k B | A d k A
`GL_ONE_MINUS_DST_COLOR` | 1 1 1 - R d k R G d k G B d k B | 1 - A d k A
`GL_SRC_ALPHA` | A s k A A s k A A s k A | A s k A
`GL_ONE_MINUS_SRC_ALPHA` | 1 1 1 - A s k A A s k A A s k A | 1 - A s k A
`GL_DST_ALPHA` | A d k A A d k A A d k A | A d k A
`GL_ONE_MINUS_DST_ALPHA` | 1 1 1 - A d k A A d k A A d k A | 1 - A d k A
`GL_CONSTANT_COLOR` | R c G c B c | A c
`GL_ONE_MINUS_CONSTANT_COLOR` | 1 1 1 - R c G c B c | 1 - A c
`GL_CONSTANT_ALPHA` | A c A c A c | A c
`GL_ONE_MINUS_CONSTANT_ALPHA` | 1 1 1 - A c A c A c | 1 - A c
`GL_SRC_ALPHA_SATURATE` | i i i | 1

In the table,

i = min &af; A s 1 - A d

To determine the blended RGBA values of a pixel when drawing in RGBA mode, the system uses the following equations:

R d = min &af; k R R s &it; s R + R d &it; d R G d = min &af; k G G s &it; s G + G d &it; d G B d = min &af; k B B s &it; s B + B d &it; d B A d = min &af; k A A s &it; s A + A d &it; d A

Despite the apparent precision of the above equations, blending arithmetic is not exactly specified, because blending operates with imprecise integer color values. However, a blend factor that should be equal to 1 is guaranteed not to modify its multiplicand, and a blend factor equal to 0 reduces its multiplicand to 0. For example, when `srcRGB` is `GL_SRC_ALPHA`, `dstRGB` is `GL_ONE_MINUS_SRC_ALPHA`, and A s is equal to k A , the equations reduce to simple replacement:

R d = R s G d = G s B d = B s A d = A s

## Notes

`glBlendFuncSeparate` is available only if the GL version is 1.4 or greater.

Incoming (source) alpha is correctly thought of as a material opacity, ranging from 1.0 ( K A ), representing complete opacity, to 0.0 (0), representing complete transparency.

When more than one color buffer is enabled for drawing, the GL performs blending separately for each enabled buffer, using the contents of that buffer for destination color. (See glDrawBuffer.)

Blending affects only RGBA rendering. It is ignored by color index renderers.

`GL_CONSTANT_COLOR`, `GL_ONE_MINUS_CONSTANT_COLOR`, `GL_CONSTANT_ALPHA`, `GL_ONE_MINUS_CONSTANT_ALPHA` are available only if the GL version is 1.4 or greater or if the `ARB_imaging` is supported by your implementation.

`GL_SRC_COLOR` and `GL_ONE_MINUS_SRC_COLOR` are valid only for `srcRGB` if the GL version is 1.4 or greater.

`GL_DST_COLOR` and `GL_ONE_MINUS_DST_COLOR` are valid only for `dstRGB` if the GL version is 1.4 or greater.

## Errors

`GL_INVALID_ENUM` is generated if either `srcRGB` or `dstRGB` is not an accepted value.

`GL_INVALID_OPERATION` is generated if `glBlendFuncSeparate` is executed between the execution of glBegin and the corresponding execution of glEnd.

## Associated Gets

glGet with argument `GL_BLEND_SRC_RGB`

glGet with argument `GL_BLEND_SRC_ALPHA`

glGet with argument `GL_BLEND_DST_RGB`

glGet with argument `GL_BLEND_DST_ALPHA`

glIsEnabled with argument `GL_BLEND`

## See Also

glAlphaFunc, glBlendColor, glBlendFunc, glBlendEquation, glClear, glDrawBuffer, glEnable, glLogicOp, glStencilFunc

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.