glBlendFunc - specify pixel arithmetic
void glBlendFunc( GLenum sfactor, GLenum dfactor )
sfactor Specifies how the red, green, blue, and alpha source blending factors are computed. Nine 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, and GL_SRC_ALPHA_SATURATE. dfactor Specifies how the red, green, blue, and alpha destination blending factors are computed. Eight 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, and GL_ONE_MINUS_DST_ALPHA.
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 argument GL_BLEND to enable and disable
blending.
glBlendFunc defines the operation of blending when it is enabled. sfactor
specifies which of nine methods is used to scale the source color
components. dfactor specifies which of eight methods is used to scale the
destination color components. The eleven possible methods are described in
the table below. 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 ,G ,B ,A ) and (R ,G ,B ,A ). They are
s s s s d d d d
understood to have integer values between zero and (k ,k ,k ,k ), where
R G B A
mc
k = 2 - 1
c
and (mR,mG,mB,mA) is the number of red, green, blue, and alpha bitplanes.
Source and destination scale factors are referred to as (s ,s ,s ,s ) and
R G B A
(d ,d ,d ,d ). The scale factors described in the table, denoted
R G B A
(f ,f ,f ,f ), represent either source or destination factors. All scale
R G B A
factors have range [0,1].
-----------------------------------------------------------------------
| parameter | (f , f , f , f ) |
| | R G B A |
-----------------------------------------------------------------------
| | |
| GL_ZERO | (0, 0, 0, 0) |
| | |
| GL_ONE | (1, 1, 1, 1) |
| | |
| GL_SRC_COLOR | (R /k , G /k , B /k , A /k ) |
| | s R s G s B s A |
| | |
|GL_ONE_MINUS_SRC_COLOR | (1, 1, 1, 1) - (R /k , G /k , B /k , A /k ) |
| | s R s G s B s A |
| | |
| GL_DST_COLOR | (R /k , G /k , B /k , A /k ) |
| | d R d G d B d A |
| | |
|GL_ONE_MINUS_DST_COLOR | (1, 1, 1, 1) - (R /k , G /k , B /k , A /k ) |
| | d R d G d B d A |
| | |
| GL_SRC_ALPHA | (A /k , A /k , A /k , A /k ) |
| | s R s G s B s A |
| | |
| | |
|GL_ONE_MINUS_SRC_ALPHA | (1, 1, 1, 1) - (A /k , A /k , A /k , A /k ) |
| | s R s G s B s A |
| | |
| | |
| GL_DST_ALPHA | (A /k , A /k , A /k , A /k ) |
| | d R d G d B d A |
| | |
|GL_ONE_MINUS_DST_ALPHA | (1, 1, 1, 1) - (A /k , A /k , A /k , A /k ) |
| | d R d G d B d A |
| | |
| | |
|GL_SRC_ALPHA_SATURATE | (i, i, i, 1) |
| | |
-----------------------------------------------------------------------
In the table,
i = min(A , k -A ) / k
s A d A
To determine the blended RGBA values of a pixel when drawing in RGB mode,
the system uses the following equations:
R = min(k , R s +R d )
d R s R d R
G = min(k , G s +G d )
d G s G d G
B = min(k , B s +B d )
d B s B d B
A = min(k , A s +A d )
d A s A 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 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 is equal to k the
s A
equations reduce to simple replacement:
R = R
d s
G = G
d s
B = B
d s
A = A
d s
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. 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.
Incoming (source) alpha is correctly thought of as a material opacity,
ranging from 1.0 (K ), representing complete opacity, to 0.0 (0),
A
representing complete transparency.
When more than one color buffer is enabled for drawing, blending is done
separately for each enabled buffer, using for destination color the
contents of that buffer. (See glDrawBuffer.)
Blending affects only RGB rendering. It is ignored by color index
renderers.
GL_INVALID_ENUM is generated if either sfactor or dfactor is not an accepted value. GL_INVALID_OPERATION is generated if glBlendFunc is executed between the execution of glBegin and the corresponding execution of glEnd.
glGet with argument GL_BLEND_SRC glGet with argument GL_BLEND_DST glIsEnabled with argument GL_BLEND
glAlphaFunc, glClear, glDrawBuffer, glEnable, glLogicOp, glStencilFunc
Introduction | Alphabetic | Specification
Last Edited: Fri, May 26, 1995