void glBlendFunc(GLenum sfactor,
GLenum dfactor)
| 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, GL_CONSTANT_COLOR_EXT, GL_ONE_MINUS_CONSTANT_COLOR_EXT, GL_CONSTANT_ALPHA_EXT, GL_ONE_MINUS_CONSTANT_ALPHA_EXT, 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_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, GL_CONSTANT_COLOR_EXT, GL_ONE_MINUS_CONSTANT_COLOR_EXT, GL_CONSTANT_ALPHA_EXT, and GL_ONE_MINUS_CONSTANT_ALPHA_EXT. The initial value is GL_ZERO. |
glBlendFunc defines the operation of blending when it is enabled. sfactor specifies which of the methods is used to scale the source color components. dfactor specifies which of eight methods 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
kc = 2mc -1
and
Constant color components are referred to as
Source and destination scale factors are referred to as
(sR, sG, sB, sA) and
(dR, dG, dB, dA).
The scale factors described in the table, denoted
(fR, fG, fB, fA),
represent either source or destination factors. All scale factors
have range
| Parameter | (fR, fG, fB, fA) |
|---|---|
| GL_ZERO | (0,0,0,0) |
| GL_ONE | (1,1,1,1) |
| GL_SRC_COLOR | (Rs/kR, Gs/kG, Bs/kB, As/kA) |
| GL_ONE_MINUS_SRC_COLOR | (1,1,1,1) - (Rs/kR, Gs/kG, Bs/kB, As/kA) |
| GL_DST_COLOR | (Rd/kR, Gd/kG, Bd/kB, Ad/kA) |
| GL_ONE_MINUS_DST_COLOR | (1,1,1,1) - (Rd/kR, Gd/kG, Bd/kB, Ad/kA) |
| GL_SRC_ALPHA | (As/kA, As/kA, As/kA, As/kA) |
| GL_ONE_MINUS_SRC_ALPHA | (1,1,1,1) - (As/kA, As/kA, As/kA, As/kA) |
| GL_DST_ALPHA | (Ad/kA, Ad/kA, Ad/kA, Ad/kA) |
| GL_ONE_MINUS_DST_ALPHA | (1,1,1,1) - (Ad/kA, Ad/kA, Ad/kA, Ad/kA) |
| GL_SRC_ALPHA_SATURATE | (i,i,i,1) |
| GL_CONSTANT_COLOR_EXT | (Rc, Gc, Bc, Ac) |
| GL_ONE_MINUS_CONSTANT_COLOR_EXT | (1,1,1,1) - (Rc, Gc, Bc, Ac) |
| GL_CONSTANT_ALPHA_EXT | (Ac, Ac, Ac, Ac) |
| GL_ONE_MINUS_CONSTANT_ALPHA_EXT | (1,1,1,1) - (Ac, Ac, Ac, Ac) |
In the table,
i = min(As, kA -Ad) / kA
To determine the blended RGBA values of a pixel when drawing in RGBA mode, the system uses the following equations (if the blend equation extension is not supported):
Rd = min(kR, RssR+RddR)
Gd = min(kG, GssG+GddR)
Bd = min(kB, BssB+BddB)
Ad = min(kA, AssB+AddA)
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 As is equal to kA , the equations reduce to simple replacement:
Rd = Rs
Gd = Gs
Bd = Bs
Ad = As
If the blend equation extension is supported, the blended RGBA values of a pixel are computed based on the current blend equation (see glBlendEquationEXT.
Blend function (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) is also useful for rendering antialiased points and lines in arbitrary order.
Polygon antialiasing is optimized using blend function (GL_SRC_ALPHA_SATURATE, GL_ONE) with polygons sorted from nearest to farthest. (See the glEnable, glDisable reference page and the GL_POLYGON_SMOOTH argument for information on polygon antialiasing.) Destination alpha bitplanes, which must be present for this blend function to operate correctly, store the accumulated coverage.
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_INVALID_OPERATION is generated if glBlendFunc is executed between the execution of glBegin and the corresponding execution of glEnd.