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glBlendFunc

Name

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_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.

dfactor

Specifies how the red, green, blue, and alpha 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.

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.

glBlendFunc defines the operation of blending when it is enabled. sfactor specifies which method is used to scale the source color components. dfactor specifies which method is used to scale the destination color components. 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 . 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_CONSTANT_COLOR R c G c B c A c
GL_ONE_MINUS_CONSTANT_COLOR 1 1 1 1 - R c G c B c A c
GL_CONSTANT_ALPHA A c A c A c A c
GL_ONE_MINUS_CONSTANT_ALPHA 1 1 1 1 - A c A c A c A c
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 RGBA values of a pixel, the system uses one of the equations set by glBlendEquation or glBlendEquationSeparate.

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.

Transparency is best implemented using blend function (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 frame buffer.

Errors

GL_INVALID_ENUM is generated if either sfactor or dfactor is not an accepted value.

Associated Gets

glGet with argument GL_BLEND_SRC_RGB or GL_BLEND_SRC_ALPHA

glGet with argument GL_BLEND_DST_RGB or GL_BLEND_DST_ALPHA

glIsEnabled with argument GL_BLEND

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