Name

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 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 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

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/.