- specify pixel arithmetic
C SPECIFICATION
PARAMETERS
_param1 Specifies how the red, green, blue, and alpha
source blending factors are computed. The fol-
lowing 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. The initial value is
GL_ONE.
Additionally, if the GL_ARB_imaging extension is
supported, the following constants are accepted:
GL_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR,
GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA.
_param2 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. The
initial value is GL_ZERO.
Additionally, if the GL_ARB_imaging extension is
supported, the following constants are accepted:
GL_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR,
GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA.
DESCRIPTION
In RGBA 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 destina-
tion 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. _param1 specifies which of nine methods is used
to scale the source color components. _param2 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 des-
tination color components are referred to as (Rs,Gs,Bs,As)
and (Rd,Gd,Bd,Ad). The color specified by glBlendColor is
------------------------------------------------------------------
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 (Rc,Gc,Bc,Ac)
GL_ONE_MINUS_CONSTANT_COLOR (1,1,1,1)-(Rc,Gc,Bc,Ac)
GL_CONSTANT_ALPHA (Ac,Ac,Ac,Ac)
GL_ONE_MINUS_CONSTANT_ALPHA (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 draw-
ing in RGBA mode, the system uses the following equations:
Rd=min(kR,RssR+RddR)
Gd=min(kG,GssG+GddG)
Bd=min(kB,BssB+BddB)
Ad=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 1 is guar-
anteed not to modify its multiplicand, and a blend factor
equal to 0 reduces its multiplicand to 0. For example,
when _param1 is GL_SRC_ALPHA, _param2 is
GL_ONE_MINUS_SRC_ALPHA, and As is equal to kA, the equa-
tions reduce to simple replacement:
Rd=Rs
Gd=Gs
Bd=Bs
Ad=As
EXAMPLES
tion on polygon antialiasing.) Destination alpha bit-
planes, which must be present for this blend function to
operate correctly, store the accumulated coverage.
NOTES
Incoming (source) alpha is correctly thought of as a mate-
rial opacity, ranging from 1.0 (KA), representing complete
opacity, to 0.0 (0), representing complete transparency.
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_CONSTANT_COLOR, GL_ONE_MINUS_CONSTANT_COLOR,
GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA are only
available if the GL_ARB_imaging is supported by your
implementation.
ERRORS
GL_INVALID_ENUM is generated if either _param1 or _param2
is not an accepted value.
GL_INVALID_OPERATION is generated if glBlendFunc is exe-
cuted between the execution of glBegin and the correspond-
ing execution of glEnd.
ASSOCIATED GETS
glGet with argument GL_BLEND_SRC
glGet with argument GL_BLEND_DST
glIsEnabled with argument GL_BLEND
SEE ALSO
glAlphaFunc, glBlendColor, glBlendEquation, glClear,
glDrawBuffer, glEnable, glLogicOp, glStencilFunc
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