specifies how source and destination colors are combined.
It must be GL_FUNC_ADD, GL_FUNC_SUBTRACT,
GL_FUNC_REVERSE_SUBTRACT, GL_MIN, GL_MAX.
DESCRIPTION
The blend equation determines how a new pixel (the ``source'' color)
is combined with a pixel already in the framebuffer (the ``destination''
color).
GL_MIN
sets the blend equation so that each component of the result color
is the minimum of the corresponding components of the source and destination
colors.
GL_MAX
sets the blend equation so that each component of the result color
is the maximum of the corresponding components of the source and destination
colors.
The remaining blend equations use the source and destination blend factors
specified by glBlendFunc.
See glBlendFunc for a description of the various blend factors.
In the equations that follow, source and destination
color components are referred to as
$(R sub s, G sub s, B sub s, A sub s )$
and
$(R sub d, G sub d, B sub d, A sub d )$,
respectively.
The result color is referred to as
$(R sub r, G sub r, B sub r, A sub r )$.
The source and destination blend factors are denoted
$(s sub R, s sub G, s sub B, s sub A )$ and
$(d sub R, d sub G, d sub B, d sub A )$, respectively.
For these equations all color components are understood to have values
in the range [0, 1].
GL_FUNC_ADD
sets the blend equation so that the source and destination
data are added.
Each component of the source color is
multiplied by the corresponding source factor, then
each component of the destination color is multiplied
by the corresponding destination factor.
The result is the componentwise sum of the two products, clamped to the
range [0, 1].
$Rr ~=~ mark min (1, ~R sub s~s sub R ~+~ R sub d~d sub R )$
$Gr ~=~ lineup min (1, ~G sub s~s sub G ~+~ G sub d~d sub G )$
$Br ~=~ lineup min (1, ~B sub s~s sub B ~+~ B sub d~d sub B )$
$Ar ~=~ lineup min (1, ~A sub s~s sub A ~+~ A sub d~d sub A )$
GL_FUNC_SUBTRACT
Is like GL_FUNC_ADD except the product of the destination factor and
the destination color is componentwise subtracted from the product of the
source factor and the source color.
The result is clamped to the range [0, 1].
$Rr ~=~ mark max (0 , ~R sub s~s sub R ~-~ R sub d~d sub R )$
$Gr ~=~ lineup max (0 , ~G sub s~s sub G ~-~ G sub d~d sub G )$
$Br ~=~ lineup max (0 , ~B sub s~s sub B ~-~ B sub d~d sub B )$
$Ar ~=~ lineup max (0 , ~A sub s~s sub A ~-~ A sub d~d sub A )$
GL_FUNC_REVERSE_SUBTRACT
Is like GL_FUNC_ADD except the product of the source factor and the
source color is componentwise subtracted from the product of the destination
factor and the destination color.
The result is clamped to the range [0, 1].
$Rr ~=~ mark max (0 , ~R sub d~d sub R ~-~ R sub s~s sub R )$
$Gr ~=~ lineup max (0 , ~G sub d~d sub G ~-~ G sub s~s sub G )$
$Br ~=~ lineup max (0 , ~B sub d~d sub B ~-~ B sub s~s sub B )$
$Ar ~=~ lineup max (0 , ~A sub d~d sub A ~-~ A sub s~s sub A )$
The GL_MIN and GL_MAX equations are useful for applications
that analyze
image data (image thresholding against a constant color,
for example).
The GL_FUNC_ADD equation is useful
for antialiasing and transparency, among other things.
Initially, the blend equation is set to GL_FUNC_ADD.
NOTES
glBlendEquation is part of the GL_ARB_imaging subset. glBlendEquation is present only
if GL_ARB_imaging is returned when glGetString is called with
GL_EXTENSIONS as its argument.
The GL_MIN, and GL_MAX equations do not use
the source or destination factors, only the source and destination colors.
ERRORS
GL_INVALID_ENUM is generated if mode is not one of
GL_FUNC_ADD, GL_FUNC_SUBTRACT, GL_FUNC_REVERSE_SUBTRACT,
GL_MAX, or GL_MIN.
GL_INVALID_OPERATION is generated if glBlendEquation is executed
between the execution of glBegin and the corresponding
execution of glEnd.
