2003-2004 Silicon Graphics, Inc.
glBlendFunc 3G
glBlendFunc specify pixel arithmetic

## C Specification

void `glBlendFunc` GLenum sfactor GLenum dfactor

## Parameters

sfactor

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

dfactor

  Specifies how the red, green, blue, and alpha destination blending factors are computed. Eight symbolic constants are accepted: `GL_ZERO`, `GL_ONE`, `GL_SRC_COLOR`, `GL_ONE_MINUS_SRC_COLOR`, `GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`, `GL_DST_ALPHA`, and `GL_ONE_MINUS_DST_ALPHA`. The initial value is `GL_ZERO`.

## Description

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

`glBlendFunc` defines the operation of blending when it is enabled. sfactor specifies which of eleven methods is used to scale the source color components. dfactor specifies which of ten methods is used to scale the destination color components. The eleven 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 ). 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 ). The scale factors described in the table, denoted ( f R, f G, f B, f A ), represent either source or destination factors. All scale factors have range [0, 1].

Parameter | ( f R, f G, f B, f A )
--- | ---
`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, 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, 1 ) - ( R d/k R, G d/k G, B d/k B, 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, 1 ) - ( A s/k A, A s/k A, A s/k A, 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, 1 ) - ( A d/k A, A d/k A, A d/k A, A d/k A )
`GL_SRC_ALPHA_SATURATE` | ( i, i, i, 1 )

In the table,

i=min( A s, k A-A d ) / k A

To determine the blended 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 sfactor is `GL_SRC_ALPHA`, dfactor 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

`glBlendFunc` operates on all pixel write operations, including the scan conversion of points, lines, and polygons. `glBlendFunc` does not affect glClear.

## Examples

Transparency is best implemented using `glBlendFunc`(`GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`) with primitives sorted from farthest to nearest. Note that this transparency calculation does not require the presence of alpha bitplanes in the color buffer.

`glBlendFunc`(`GL_SRC_ALPHA`, `GL_ONE_MINUS_SRC_ALPHA`) is also useful for rendering antialiased points and lines.

## 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 either sfactor or dfactor is not an accepted value.

## See Also

glAlphaFunc, glClear, glDepthFunc, glEnable, glLogicOp, glStencilFunc

## Copyright

Copyright 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.