glBlendEquationEXT(3G)	       OpenGL Reference		glBlendEquationEXT(3G)



NAME
     glBlendEquationEXT	- set the blend	equation


C SPECIFICATION
     void glBlendEquationEXT( GLenum mode )


PARAMETERS
     mode  specifies how source	and destination	colors are combined.  It must
	   be GL_FUNC_ADD_EXT, GL_FUNC_SUBTRACT_EXT,
	   GL_FUNC_REVERSE_SUBTRACT_EXT, GL_MIN_EXT, GL_MAX_EXT, or
	   GL_LOGIC_OP.


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_EXT
	  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_EXT
	  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_LOGIC_OP
	  sets the blend equation so that each component of the	result color
	  is a bitwise logical combination of the corresponding	components of
	  the source and destination colors.  Use the glLogicOp	command	to
	  specify the logical operator.

     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 (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
     between 0.0 and 1.0, inclusive.

     GL_FUNC_ADD_EXT
	  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



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glBlendEquationEXT(3G)	       OpenGL Reference		glBlendEquationEXT(3G)



	  result is the	componentwise sum of the two products, clamped to the
	  range	[0,1].

			     Rr	= min(1,  Rs sR	 +  Rd dR)
			     Gr	= min(1,  Gs sG	 +  Gd dG)
			     Br	= min(1,  Bs sB	 +  Bd dB)
			     Ar	= min(1,  As sA	 +  Ad dA)

     GL_FUNC_SUBTRACT_EXT
	  Is like GL_FUNC_ADD_EXT 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)

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


     The GL_MIN_EXT and	GL_MAX_EXT equations are useful	for applications that
     analyze image data	(image thresholding against a constant color, for
     example).	The GL_LOGIC_OP	equation is most commonly used with GL_XOR for
     reversible	drawing.  The GL_FUNC_ADD_EXT equation is useful for anti-
     aliasing and transparency,	among other things.

     Initially the blend equation is set to GL_FUNC_ADD_EXT.

NOTES
     The GL_MIN_EXT, GL_MAX_EXT, and GL_LOGIC_OP 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_EXT,
     GL_FUNC_SUBTRACT_EXT, GL_FUNC_REVERSE_SUBTRACT_EXT, GL_MAX_EXT,
     GL_MIN_EXT, or GL_LOGIC_OP.

     GL_INVALID_OPERATION is generated if glBlendEquationEXT is	executed
     between the execution of glBegin and the corresponding execution of
     glEnd.






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glBlendEquationEXT(3G)	       OpenGL Reference		glBlendEquationEXT(3G)



ASSOCIATED GETS
     glGet with	an argument of GL_BLEND_EQUATION_EXT.

     glGetString with an argument of GL_EXTENSIONS.
	  This feature is not part of standard GL. The extensions string of
	  glGetString will contain the name blend_minmax if GL_MIN_EXT and
	  GL_MAX_EXT are supported.  It	will contain the name blend_logic_op
	  if GL_LOGIC_OP is supported in RGB mode. It will contain the name
	  blend_subtract if GL_FUNC_SUBTRACT_EXT and
	  GL_FUNC_REVERSE_SUBTRACT_EXT are supported.


MACHINE	DEPENDENCIES
     RealityEngine, RealityEngine2, and	VTX systems do not support the use of
     blend factor GL_SRC_ALPHA_SATURATE	with blend equation
     GL_FUNC_REVERSE_SUBTRACT_EXT; this	combination produces incorrect
     results.


SEE ALSO
     glGetString, glBlendColorEXT, glBlendFunc.


































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