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GLBLENDFUNC(3gl)					      GLBLENDFUNC(3gl)

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.  The following  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,	 GL_ONE_MINUS_CONSTANT_ALPHA_EXT.   If
		the device supports GL_SUN_blend_src_mult_dst_alpha extension,
		the   following	  symbolic   constants	 are   also  accepted:
		GL_SRC_ALPHA_MULT_DST_ALPHA_SUN,
		GL_SRC_ALPHA_MULT_ONE_MINUS_DST_ALPHA_SUN.   The initial value
		is GL_ONE.

       dfactor	Specifies how the red,	green,	blue,  and  alpha  destination
		blending  factors  are	computed.  The following symbolic con‐
		stants	 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,	  and	  GL_ONE_MINUS_DST_ALPHA,      GL_CON‐
		STANT_COLOR_EXT,    GL_ONE_MINUS_CONSTANT_COLOR_EXT,   GL_CON‐
		STANT_ALPHA_EXT, GL_ONE_MINUS_CONSTANT_ALPHA_EXT. The  initial
		value is GL_ZERO.

DESCRIPTION
       In  RGBA	 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).  Blending is initially dis‐
       abled.  Use glEnable and glDisable with argument GL_BLEND to enable and
       disable blending.

       glBlendFunc  defines  the  operation  of	 blending  when it is enabled.
       sfactor specifies which of the methods is  used	to  scale  the	source
       color  components.   dfactor  specifies which of the methods is used to
       scale the destination  color  components.   The	possible  methods  are
       described  in the following table.  Each method defines four scale fac‐
       tors, one each for red, green, blue, and alpha.

       In the table and in subsequent equations, source, destination, and con‐
       stant color components are referred to as (Rs,Gs,Bs,As) , (Rd,Gd,Bd,Ad)
       and (Rc,Gc,Bc,Ac) .  The source and destination	color  components  are
       understood to have integer values between 0 and (kR,kG,kB,kA), where

					  kc=2mc−1

       and  (mR,mG,mB,mA)  is  the  number of red, green, blue, and alpha bit‐
       planes.

       The constant color components are the  color  components	 specified  by
       glBlendColorEXT.	 These	color components are understood to be in range
       [0,1], and are not scaled by (kR,kG,kB,kA).

       Source and destination scale factors are referred to  as	 (sR,sG,sB,sA)
       and  (dR,dG,dB,dA).   The scale factors described in the table, denoted
       (fR,fG,fB,fA), represent either source  or  destination	factors.   All
       scale factors have range [0,1].

┌──────────────────────────────────────────┬───────────────────────────────────────────────────────────────────┐
│		 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_SRC_ALPHA_SATURATE	   │				 (i,i,i,1)			       │
│	   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_MULT_DST_ALPHA_SUN	   │	     (As/kA*Ad/kA,As/kA*Ad/kA,As/kA*Ad/kA,As/kA*Ad/kA)	       │
│GL_SRC_ALPHA_MULT_ONE_MINUS_DST_ALPHA_SUN │ (As/kA*(1−Ad/kA),As/kA*(1−Ad/kA),As/kA*(1−Ad/kA),As/kA*(1−Ad/kA)) │
└──────────────────────────────────────────┴───────────────────────────────────────────────────────────────────┘
       In the table,

		     i=min(As,kA−Ad)/kA

       To  determine  the  blended RGBA values of a pixel when drawing in RGBA
       mode, the system uses the  blend	 equation  specified  by  glBlendEqua‐
       tionEXT:

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

       and the following equations:

		     Rd=min(kR,Rr)
		     Gd=min(kG,Gr)
		     Bd=min(kB,Br)
		     Ad=min(kA,Ar)

       Despite	the apparent precision of the above equations, blending arith‐
       metic is not exactly specified, because blending operates  with	impre‐
       cise  integer  color  values.   However,	 a blend factor that should be
       equal to 1 is guaranteed not to modify its multiplicand,	 and  a	 blend
       factor  equal  to  0  reduces its multiplicand to 0.  For example, when
       sfactor is GL_SRC_ALPHA, dfactor is GL_ONE_MINUS_SRC_ALPHA, 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	 near‐
       est.   Note  that  this	transparency  calculation does not require the
       presence of alpha bitplanes in the frame buffer.

       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_POLY‐
       GON_SMOOTH argument for information on polygon antialiasing.)  Destina‐
       tion  alpha bitplanes, which must be present for this blend function to
       operate correctly, store the accumulated coverage.

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

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

       Blending affects only RGBA 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
       glGet with argument GL_BLEND_DST
       glIsEnabled with argument GL_BLEND

SEE ALSO
       glAlphaFunc,   glBlendColorEXT,	glBlendEquationEXT,  glClear,  glDraw‐
       Buffer, glEnable, glLogicOp, glStencilFunc

				  24 Sept 97		      GLBLENDFUNC(3gl)
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