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

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, the system uses one of the equations set by glBlendEquation or glBlendEquationSeparate.

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.

## Notes

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.

## Errors

`GL_INVALID_ENUM` is generated if `srcRGB`, `dstRGB`, `srcAlpha`, or `dstAlpha` is not an accepted value.

## 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

glBlendColor, glBlendEquation, glBlendEquationSeparate, glBlendFunc, glClear, glEnable, 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/.