glBlendEquation(3)

NAME

       glBlendEquation - set the blend equation

C SPECIFICATION

       void glBlendEquation( GLenum mode )

       delim $$

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 fac-
       tors 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 des-
              tination  color  is  multiplied by the corresponding destination
              factor.  The result is the componentwise sum of  the  two  prod-
              ucts, 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 ana-
       lyze
       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)

                                                           GLBLENDEQUATION(3G)