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glBlendEquation(3G)	       OpenGL Reference		   glBlendEquation(3G)


NAME    [Toc]    [Back]

     glBlendEquation - set the blend equation

C SPECIFICATION    [Toc]    [Back]

     void glBlendEquation( GLenum mode )

PARAMETERS    [Toc]    [Back]

     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, GL_ALPHA_MIN_SGIX, or GL_ALPHA_MAX_SGIX.

DESCRIPTION    [Toc]    [Back]

     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    [Toc]    [Back]
	  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    [Toc]    [Back]
	  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.

     GL_ALPHA_MIN_SGIX    [Toc]    [Back]
	  sets the blend equation so that the result color is either the
	  source or destination	color, according to whether the	alpha
	  component of the source color	is less	than, or greater than or equal
	  to, the alpha	component of the destination color, respectively.

     GL_ALPHA_MAX_SGIX    [Toc]    [Back]
	  sets the blend equation so that the result color is either the
	  source or destination	color, according to whether the	alpha
	  component of the source color	is greater than, or less than or equal
	  to, the alpha	component of the destination color, respectively.

     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	G  B  A	) and (R  G  B	A ), respectively.  The	result
		      s, s, s, s	d, d, d, d
     color is referred to as (R	 G  B  A ).  The source	and destination	blend
			       r, r, r,	r
     factors are denoted (s  s	s  s ) and (d  d  d  d ), respectively.	 For
			   R, G, B, A	     R,	G, B, A
     these equations all color components are understood to have values	in the
     range [0, 1].




									Page 1






glBlendEquation(3G)	       OpenGL Reference		   glBlendEquation(3G)



     GL_FUNC_ADD    [Toc]    [Back]
	  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 = min(1, R  s	 + R  d	)
			    s  R    d  R
	       Gr = min(1, G  s	 + G  d	)
			    s  G    d  G
	       Br = min(1, B  s	 + B  d	)
			    s  B    d  B
	       Ar = min(1, A  s	 + A  d	)
			    s  A    d  A

     GL_FUNC_SUBTRACT    [Toc]    [Back]
	  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 = max(0, R  s	 - R  d	)
			    s  R    d  R
	       Gr = max(0, G  s	 - G  d	)
			    s  G    d  G
	       Br = max(0, B  s	 - B  d	)
			    s  B    d  B
	       Ar = max(0, A  s	 - A  d	)
			    s  A    d  A

     GL_FUNC_REVERSE_SUBTRACT    [Toc]    [Back]
	  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 = max(0, R  d	 - R  s	)
			    d  R    s  R
	       Gr = max(0, G  d	 - G  s	)
			    d  G    s  G
	       Br = max(0, B  d	 - B  s	)
			    d  B    s  B
	       Ar = max(0, A  d	 - A  s	)
			    d  A    s  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    [Toc]    [Back]

     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, GL_MAX, GL_ALPHA_MIN_SGIX and GL_ALPHA_MAX_SGIX equations do
     not use the source	or destination factors,	only the source	and
     destination colors.





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glBlendEquation(3G)	       OpenGL Reference		   glBlendEquation(3G)


ERRORS    [Toc]    [Back]

     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    [Toc]    [Back]

     glGet with	an argument of GL_BLEND_EQUATION


MACHINE	DEPENDENCIES
     The SGIX_blend_alpha_minmax extension is supported	only on	Octane2	VPro
     systems.

SEE ALSO    [Toc]    [Back]

      
      
     glGetString, glBlendColor,	glBlendFunc


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