glBlendFuncSeparate - OpenGL 4 Reference Pages

## Name

glBlendFuncSeparate — specify pixel arithmetic for RGB and alpha components separately

## C Specification

`void glBlendFuncSeparate(` | GLenum srcRGB,
| GLenum dstRGB,
| GLenum srcAlpha,
| GLenum dstAlpha`)`;

`void glBlendFuncSeparatei(` | GLuint buf,
| GLenum srcRGB,
| GLenum dstRGB,
| GLenum srcAlpha,
| GLenum dstAlpha`)`;

## Parameters

`buf`

  For `glBlendFuncSeparatei`, specifies the index of the draw buffer for which to set the blend functions.

`srcRGB`

  Specifies how the red, green, and blue blending factors are computed. The initial value is `GL_ONE`.

`dstRGB`

  Specifies how the red, green, and blue destination blending factors are computed. The initial value is `GL_ZERO`.

`srcAlpha`

  Specified how the alpha source blending factor is computed. The initial value is `GL_ONE`.

`dstAlpha`

  Specified how the alpha destination blending factor is computed. 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 for all draw buffers when it is enabled. `glBlendFuncSeparatei` defines the operation of blending for a single draw buffer specified by `buf` when enabled for that draw buffer. `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, first source, second source and destination color components are referred to as R s0 G s0 B s0 A s0 , R s1 G s1 B s1 A s1 , and R d G d B d A d , respectively. 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 s0 k R G s0 k G B s0 k B | A s0 k A
`GL_ONE_MINUS_SRC_COLOR` | 1 1 1 - R s0 k R G s0 k G B s0 k B | 1 - A s0 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 s0 k A A s0 k A A s0 k A | A s0 k A
`GL_ONE_MINUS_SRC_ALPHA` | 1 1 1 - A s0 k A A s0 k A A s0 k A | 1 - A s0 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
`GL_SRC1_COLOR` | R s1 k R G s1 k G B s1 k B | A s1 k A
`GL_ONE_MINUS_SRC1_COLOR` | 1 1 1 1 - R s1 k R G s1 k G B s1 k B | 1 - A s1 k A
`GL_SRC1_ALPHA` | A s1 k A A s1 k A A s1 k A | A s1 k A
`GL_ONE_MINUS_SRC1_ALPHA` | 1 1 1 - A s1 k A A s1 k A A s1 k A | 1 - A s1 k A

In the table,

i = min ⁡ A s 1 - A d

To determine the blended RGBA values of a pixel, the system uses the following equations:

R d = min ⁡ k R R s ⁢ s R + R d ⁢ d R G d = min ⁡ k G G s ⁢ s G + G d ⁢ d G B d = min ⁡ k B B s ⁢ s B + B d ⁢ d B A d = min ⁡ k A A s ⁢ s A + A d ⁢ 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

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.)

When dual source blending is enabled (i.e., one of the blend factors requiring the second color input is used), the maximum number of enabled draw buffers is given by `GL_MAX_DUAL_SOURCE_DRAW_BUFFERS`, which may be lower than `GL_MAX_DRAW_BUFFERS`.

## Errors

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

`GL_INVALID_VALUE` is generated by `glBlendFuncSeparatei` if `buf` is greater than or equal to the value of `GL_MAX_DRAW_BUFFERS`.

## 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`

## Version Support

| OpenGL Version
--- | ---
Function / Feature Name | 2.0 | 2.1 | 3.0 | 3.1 | 3.2 | 3.3 | 4.0 | 4.1 | 4.2 | 4.3 | 4.4 | 4.5
`glBlendFuncSeparate` | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔
`glBlendFuncSeparatei` | - | - | - | - | - | - | ✔ | ✔ | ✔ | ✔ | ✔ | ✔

## See Also

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

## Copyright

Copyright © 1991-2006 Silicon Graphics, Inc. Copyright © 2010-2018 Khronos Group. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.