Parameters

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 $\left({\mathit{R}}_{\mathit{s}},{\mathit{G}}_{\mathit{s}},{\mathit{B}}_{\mathit{s}},{\mathit{A}}_{\mathit{s}}\right)$ and $\left({\mathit{R}}_{\mathit{d}},{\mathit{G}}_{\mathit{d}},{\mathit{B}}_{\mathit{d}},{\mathit{A}}_{\mathit{d}}\right)$. The color specified by glBlendColor is referred to as $\left({\mathit{R}}_{\mathit{c}},{\mathit{G}}_{\mathit{c}},{\mathit{B}}_{\mathit{c}},{\mathit{A}}_{\mathit{c}}\right)$. They are understood to have integer values between 0 and $\left({\mathit{k}}_{\mathit{R}},{\mathit{k}}_{\mathit{G}},{\mathit{k}}_{\mathit{B}},{\mathit{k}}_{\mathit{A}}\right)$, where

${\mathit{k}}_{\mathit{c}}=2^{{\mathit{m}}_{\mathit{c}}}-1$

and $\left({\mathit{m}}_{\mathit{R}},{\mathit{m}}_{\mathit{G}},{\mathit{m}}_{\mathit{B}},{\mathit{m}}_{\mathit{A}}\right)$ is the number of red, green, blue, and alpha bitplanes.

Source and destination scale factors are referred to as $\left({\mathit{s}}_{\mathit{R}},{\mathit{s}}_{\mathit{G}},{\mathit{s}}_{\mathit{B}},{\mathit{s}}_{\mathit{A}}\right)$ and $\left({\mathit{d}}_{\mathit{R}},{\mathit{d}}_{\mathit{G}},{\mathit{d}}_{\mathit{B}},{\mathit{d}}_{\mathit{A}}\right)$. All scale factors have range $\left[0,1\right]$.

In the table,

$\mathit{i}=\mathit{min}\left({\mathit{A}}_{\mathit{s}},1-{\mathit{A}}_{\mathit{d}}\right)$

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 (${\mathit{K}}_{\mathit{A}}$), 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