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Name

glBlendEquation - set the blend equation

C Specification

void glBlendEquation( GLenum mode ) 


 eqn not supported


Parameters






mode 


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. 

Associated Gets






glGet with an argument of GL_BLEND_EQUATION 







See
Also

glGetString(3G)
, glBlendColor(3G)
, glBlendFunc(3G)
 




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