glBlendFunc(3G)		       OpenGL Reference		       glBlendFunc(3G)



NAME
     glBlendFunc - specify pixel arithmetic


C SPECIFICATION
     void glBlendFunc( GLenum sfactor,
		       GLenum dfactor )


PARAMETERS
     sfactor  Specifies	how the	red, green, blue, and alpha source blending
	      factors are computed.  Thirteen symbolic constants are accepted:
	      GL_ZERO, GL_ONE, GL_DST_COLOR, GL_ONE_MINUS_DST_COLOR,
	      GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA,
	      GL_ONE_MINUS_DST_ALPHA, GL_SRC_ALPHA_SATURATE,
	      GL_CONSTANT_COLOR_EXT, GL_ONE_MINUS_CONSTANT_COLOR_EXT,
	      GL_CONSTANT_ALPHA_EXT, and GL_ONE_MINUS_CONSTANT_ALPHA_EXT.

     dfactor  Specifies	how the	red, green, blue, and alpha destination
	      blending factors are computed.  Twelve symbolic constants	are
	      accepted:	 GL_ZERO, GL_ONE, GL_SRC_COLOR,
	      GL_ONE_MINUS_SRC_COLOR, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA,
	      GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA, GL_CONSTANT_COLOR_EXT,
	      GL_ONE_MINUS_CONSTANT_COLOR_EXT, GL_CONSTANT_ALPHA_EXT, and
	      GL_ONE_MINUS_CONSTANT_ALPHA_EXT.


DESCRIPTION
     In	RGB mode, pixels can be	drawn using a function that blends the
     incoming (source) RGBA values with	the RGBA values	that are already in
     the frame buffer (the destination values).	 By default, blending is
     disabled.	Use glEnable and glDisable with	argument GL_BLEND to enable
     and disable blending.

     When blending is enabled, glBlendFunc, glBlendColorEXT, and
     glBlendEquationEXT	determine the blending operation.  sfactor and dfactor
     specify the scaling rules used for	scaling	the source and destination
     color components, respectively.  Each rule	defines	four scale factors,
     one each for red, green, blue, and	alpha.	The rules are described	in the
     table below.

     In	the following, source, destination, and	GL_BLEND_COLOR_EXT color
     components	are denoted by (Rs,Gs,Bs,As), (Rd,Gd,Bd,Ad) and	(Rc,Gc,Bc,Ac),
     respectively.  Color components are understood to have integer values
     between 0 and (kR,kG,kB,kA), where	kc = 2mc-1 with	mc being the number of
     bitplanes for the corresponding color component (c	is one of R, G,	B, or
     A). All scale factors are in the range [0,1].  Source and destination
     scale factors are denoted by (sR,sG,sB,sA)	and (dR,dG,dB,dA).  In the
     following table, (fR,fG,fB,fA) denotes either source or destination
     factors, and i = min(As, kA-Ad) / kA.





									Page 1






glBlendFunc(3G)		       OpenGL Reference		       glBlendFunc(3G)



	      parameter		   |		(fR,  fG,  fB,	fA)
   ________________________________|____________________________________________
	       GL_ZERO		   |		    (0,	0, 0, 0)
	       GL_ONE		   |		    (1,	1, 1, 1)
	    GL_SRC_COLOR	   |	    (Rs/kR, Gs/kG, Bs/kB, As/kA)
       GL_ONE_MINUS_SRC_COLOR	   | (1, 1, 1, 1) - (Rs/kR, Gs/kG, Bs/kB, As/kA)
	    GL_DST_COLOR	   |	    (Rd/kR, Gd/kG, Bd/kB, Ad/kA)
       GL_ONE_MINUS_DST_COLOR	   | (1, 1, 1, 1) - (Rd/kR, Gd/kG, Bd/kB, Ad/kA)
	    GL_SRC_ALPHA	   |	    (As/kA, As/kA, As/kA, As/kA)
       GL_ONE_MINUS_SRC_ALPHA	   | (1, 1, 1, 1) - (As/kA, As/kA, As/kA, As/kA)
	    GL_DST_ALPHA	   |	    (Ad/kA, Ad/kA, Ad/kA, Ad/kA)
       GL_ONE_MINUS_DST_ALPHA	   | (1, 1, 1, 1) - (Ad/kA, Ad/kA, Ad/kA, Ad/kA)
	GL_CONSTANT_COLOR_EXT	   |		  (Rc, Gc, Bc, Ac)
   GL_ONE_MINUS_CONSTANT_COLOR_EXT |	   (1, 1, 1, 1)	- (Rc, Gc, Bc, Ac)
	GL_CONSTANT_ALPHA_EXT	   |		  (Ac, Ac, Ac, Ac)
   GL_ONE_MINUS_CONSTANT_ALPHA_EXT |	   (1, 1, 1, 1)	- (Ac, Ac, Ac, Ac)
	GL_SRC_ALPHA_SATURATE	   |		    (i,	i, i, 1)

     Blending combines corresponding source and	destination color components
     according to the blending operation specified by GL_BLEND_EQUATION_EXT.
     The blending operations are:

		   GL_BLEND_EQUATION_EXT     |	Binary Operation
		_____________________________|____________________
		GL_FUNC_ADD_EXT		     | min(CsxsC+CdxdC,kC)
		GL_FUNC_SUBTRACT_EXT	     | max(CsxsC-CdxdC,0)
		GL_FUNC_REVERSE_SUBTRACT_EXT | max(CdxdC-CsxsC,0)
		GL_LOGIC_OP		     | CsxsC  Lop  CdxdC
		GL_MIN_EXT		     | min(CsxsC,CdxdC)
		GL_MAX_EXT		     | max(CsxsC,CdxdC)

     where C is	the relevant color component (R, G, B, or A), Cs and Cd	are
     the source	and destination	color components, respectively,	sC and sD are
     the source	and destination	scale factors, respectively, and Lop is	one of
     16	bitwise	operators specified by glLogicOp.

     Despite the apparent precision of the above equations, blending
     arithmetic	is not exactly specified, because blending operates with
     imprecise integer color values.  However, a blend factor that should be
     equal to one is guaranteed	not to modify its multiplicand,	and a blend
     factor equal to zero reduces its multiplicand to zero.  Thus, for
     example, when sfactor is GL_SRC_ALPHA, dfactor is GL_ONE_MINUS_SRC_ALPHA,
     GL_BLEND_EQUATION_EXT is GL_FUNC_ADD_EXT, and As is equal to kA, the
     equations reduce to simple	replacement:
     Rd	= Rs,  Gd = Gs,	 Bd = Bs,  Ad =	As.

EXAMPLES
     Transparency is best implemented using blend function (GL_SRC_ALPHA,
     GL_ONE_MINUS_SRC_ALPHA) with primitives sorted from farthest to nearest.
     Note that this transparency calculation does not require the presence of
     alpha bitplanes in	the frame buffer.




									Page 2






glBlendFunc(3G)		       OpenGL Reference		       glBlendFunc(3G)



     Blend function (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) is also useful for
     rendering antialiased points and lines in arbitrary order.

     Polygon antialiasing is optimized using blend function
     (GL_SRC_ALPHA_SATURATE, GL_ONE) with polygons sorted from nearest to
     farthest.	(See the glEnable, glDisable reference page and	the
     GL_POLYGON_SMOOTH argument	for information	on polygon antialiasing.)
     Destination alpha bitplanes, which	must be	present	for this blend
     function to operate correctly, store the accumulated coverage.

     To	blend two RGB (no alpha	component) images as (1-p)xImageA+pxImageB,
     one would disable blending, draw ImageA into the frame buffer, enable
     blending, set GL_BLEND_COLOR_EXT to (0,0,0,p), the	blend function to
     (GL_CONSTANT_ALPHA_EXT, GL_ONE_MINUS_CONSTANT_ALPHA_EXT), (and the	blend
     equation to the default GL_FUNC_ADD_EXT), and draw	ImageB.

NOTES
     Incoming (source) alpha is	correctly thought of as	a material opacity,
     ranging from 1.0 (KA), representing complete opacity, to 0.0 (0),
     representing complete transparency.

     When more than one	color buffer is	enabled	for drawing, blending is done
     separately	for each enabled buffer, using for destination color the
     contents of that buffer.  (See glDrawBuffer.)

     Blending affects only RGB rendering.  It is ignored by color index
     renderers.


ERRORS
     GL_INVALID_ENUM is	generated if either sfactor or dfactor is not an
     accepted value.

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

ASSOCIATED GETS
     glGet with	argument GL_BLEND_SRC, GL_BLEND_DST, GL_LOGIC_OP_MODE, or
     GL_BLEND_EQUATION_EXT, GL_BLEND_COLOR_EXT.

     glIsEnabled with argument GL_BLEND


MACHINE	DEPENDENCIES
     RealityEngine, RealityEngine2, and	VTX systems do not support the use of
     source blend factor GL_SRC_ALPHA_SATURATE and a destination factor	based
     on	the constant blend color (GL_.._CONSTANT_.._EXT); such combinations
     produce incorrect results.







									Page 3






glBlendFunc(3G)		       OpenGL Reference		       glBlendFunc(3G)



SEE ALSO
     glAlphaFunc, glClear, glDrawBuffer, glEnable, glLogicOp, glStencilFunc,
     glBlendColorEXT, glBlendEquationEXT.




















































									Page 4