The glBlendFunc function specifies pixel arithmetic.
void glBlendFunc(
GLenum sfactor,
GLenum dfactor
);
In RGB 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). By default, blending is disabled. Use glEnable and glDisable with the GL_BLEND argument to enable and disable blending.
When enabled, glBlendFunc defines the operation of blending. The sfactor parameter specifies which of nine methods is used to scale the source color components. The dfactor parameter specifies which of eight 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 zero 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) |
| GL_SRC_ALPHA | (R (d) /k (R) ,G (d) /k (G) ,B (d) /k (B) ,A (d) /k (A) ) (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) ) / kA
To determine the blended RGBA values of a pixel when drawing in RGB mode, the system uses the following equations:
R (d) = min(kR,RssR+RddR)
G (d) = min(kG,GssG+GddG)
B (d) = min(kB,BssB+BddB)
A (d) = min(kA,AssA+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 one is guaranteed not to modify its multiplicand, and a blend factor equal to zero reduces its multiplicand to zero. Thus, 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
)
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 frame buffer.
You can also use glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) for rendering antialiased points and lines in arbitrary order.
To optimize polygon antialiasing, use glBlendFunc(GL_SRC_ALPHA_SATURATE, GL_ONE) with polygons sorted from nearest to farthest. (See the GL_POLYGON_SMOOTH argument in glEnable for information on polygon antialiasing.) Destination alpha bitplanes, which must be present for this blend function to operate correctly, store the accumulated coverage.
Incoming (source) alpha is a material opacity, ranging from 1.0 (K (A) ), representing complete opacity, to 0.0 (0), representing complete transparency.
When you enable more than one color buffer for drawing, each enabled buffer is blended separately, and the contents of the buffer is used for destination color. (See glDrawBuffer.)
Blending affects only RGB rendering. It is ignored by color-index renderers.
The following functions retrieve information related to glBlendFunc:
glGet with argument GL_BLEND_SRC
glGet with argument GL_BLEND_DST
glIsEnabled with argument GL_BLEND
The following are the error codes generated and their conditions.
| Error Code | Condition |
|---|---|
| GL_INVALID_ENUM | Either sfactor or dfactor was not an accepted value. |
| GL_INVALID_OPERATION | glBlendFunc was called between a call to glBegin and the corresponding call to glEnd. |
Windows NT: Use version 3.5 and later.
Windows: Use Windows 95 and later.
Windows CE: Unsupported.
Header: Declared in gl.h.
Import Library: Link with opengl32.lib.
glAlphaFunc, glBegin, glClear, glDisable, glDrawBuffer, glEnable, glGet, glIsEnabled, glLogicOp, glStencilFunc