


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 com
	     bined.  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  correspond
	      ing  components  of the source and destination col
	      ors.

       GL_MAX sets the blend equation so that each  component  of
	      the  result color is the maximum of the correspond
	      ing components of the source and	destination  col
	      ors.

       The  remaining blend equations use the source and destina
       tion  blend  factors  specified	 by   glBlendFunc.    See
       glBlendFunc  for	 a  description of the various blend fac
       tors.

       In the equations that follow, source and destination color
       components   are	  referred   to	  as   (Rs,Gs,Bs,As)  and
       (Rd,Gd,Bd,Ad), respectively.  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 components
       are understood to have values in the range [0, 1].

       GL_FUNC_ADD
	      sets the blend equation so that the source and des
	      tination	data  are  added.   Each component of the
	      source color is  multiplied  by  the  corresponding
	      source  factor, then each component of the destina
	      tion color is multiplied by the corresponding  des
	      tination	factor.	  The result is the componentwise
	      sum of the  two  products,  clamped  to  the  range
	      [0, 1].


		     Rr=min(1,RssR+RddR)



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GLBLENDEQUATION(3G)			      GLBLENDEQUATION(3G)


		     Gr=min(1,GssG+GddG)
		     Br=min(1,BssB+BddB)
		     Ar=min(1,AssA+AddA)

       GL_FUNC_SUBTRACT
	      Is  like GL_FUNC_ADD except the product of the des
	      tination factor and the destination color is compo
	      nentwise	subtracted from the product of the source
	      factor and the source color.  The result is clamped
	      to the range [0, 1].

		     Rr=max(0,RssR-RddR)
		     Gr=max(0,GssG-GddG)
		     Br=max(0,BssB-BddB)
		     Ar=max(0,AssA-AddA)

       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 fac
	      tor and  the  destination	 color.	  The  result  is
	      clamped to the range [0, 1].

		     Rr=max(0,RddR-RssR)
		     Gr=max(0,GddG-GssG)
		     Br=max(0,BddB-BssB)
		     Ar=max(0,AddA-AssA)

       The  GL_MIN  and	 GL_MAX equations are useful for applica
       tions 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  col
       ors.


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



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GLBLENDEQUATION(3G)			      GLBLENDEQUATION(3G)


       executed between the execution of glBegin and  the  corre
       sponding 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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