GLBLENDEQUATION(3G)                                        GLBLENDEQUATION(3G)



NAME
       glBlendEquation - set the blend equation


C SPECIFICATION
       void glBlendEquation( GLenum mode )


PARAMETERS
       mode  specifies  how  source  and  destination colors are combined.  It
             must be GLFUNCAD, GLFUNCSUBTRACT,  GLFUNCREVERSESUBTRACT,
             GLMIN, GLMAX.


DESCRIPTION
       The blend equation determines how a new pixel (the ``source'' color) is
       combined with a pixel already in the framebuffer  (the  ``destination''
       color).

       GLMIN 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.

       GLMAX 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 (Rs, Gs, Bs, As ) and (Rd, Gd,  Bd,  Ad  ),  respec-
       tively.   The  result  color  is referred to as (Rr, Gr, Br, Ar ).  The
       source and destination blend factors are denoted (sR, sG, sB, sA )  and
       (dR,  dG, dB, dA ), respectively.  For these equations all color compo-
       nents are understood to have values in the range [0, 1].

       GLFUNCAD
              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=min(1,RssR]RddR)Gr = min (1, Gs sG ] Gd dG )
                     Br=min(1,BssB]BddB)Ar = min (1, As sA ] Ad dA )

       GLFUNCSUBTRACT
              Is like GLFUNCAD 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 = max (0 , Rs sR - Rd dR )
                     Gr = max (0 , Gs sG - Gd dG )
                     Br = max (0 , Bs sB - Bd dB )
                     Ar = max (0 , As sA - Ad dA )

       GLFUNCREVERSESUBTRACT
              Is like GLFUNCAD 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=max(0,RddR-RssR)Gr = max (0 , Gd dG - Gs sG )
                     Br=max(0,BddB-BssB)Ar = max (0 , Ad dA - As sA )

       The  GLMIN  and GLMAX equations are useful for applications that ana-
       lyze
       image data (image thresholding against a constant color, for  example).
       The  GLFUNCAD  equation is useful for antialiasing and transparency,
       among other things.

       Initially, the blend equation is set to GLFUNCAD.


NOTES
       glBlendEquation is part of the GLARBimaging  subset.  glBlendEquation
       is  present  only  if  GLARBimaging  is  returned when glGetString is
       called with GLEXTENSIONS as its argument.

       The GLMIN, and GLMAX equations do not use the source  or  destination
       factors, only the source and destination colors.


ERORS
       GLINVALIDENUM  is  generated  if  mode  is  not  one  of GLFUNCAD,
       GLFUNCSUBTRACT, GLFUNCREVERSESUBTRACT, GLMAX, or GLMIN.

       GLINVALIDOPERATION  is  generated  if  glBlendEquation  is   executed
       between  the  execution  of  glBegin and the corresponding execution of
       glEnd.

ASOCIATED GETS
       glGet with an argument of GLBLENDEQUATION

SEE ALSO
       glGetString, glBlendColor, glBlendFunc



                                                           GLBLENDEQUATION(3G)