glBlendFunc man page on Darwin

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GLBLENDFUNC(3G)						       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.  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,	   and
		GL_SRC_ALPHA_SATURATE.	The initial value is GL_ONE.

		Additionally, if the GL_ARB_imaging  extension	is  supported,
		the   following	 constants  are	 accepted:  GL_CONSTANT_COLOR,
		GL_ONE_MINUS_CONSTANT_COLOR,		    GL_CONSTANT_ALPHA,
		GL_ONE_MINUS_CONSTANT_ALPHA.

       dfactor	Specifies  how	the  red,  green,  blue, and alpha destination
		blending factors are computed.	Eight 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,  and  GL_ONE_MINUS_DST_ALPHA.  The initial value
		is GL_ZERO.

		Additionally, if the GL_ARB_imaging  extension	is  supported,
		the   following	 constants  are	 accepted:  GL_CONSTANT_COLOR,
		GL_ONE_MINUS_CONSTANT_COLOR,		    GL_CONSTANT_ALPHA,
		GL_ONE_MINUS_CONSTANT_ALPHA.

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 nine methods is used	to  scale  the	source
       color  components.  dfactor specifies which of eight methods is used to
       scale the destination color components.	The  eleven  possible  methods
       are  described  in the following table.	Each method defines four scale
       factors, one each for red, green, blue, and alpha.

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

				      kc = (2^^ mc) -1

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

       Source and destination scale factors are referred to as (sR , sG , sB ,
       sA ) and (dR , dG , dB , dA ).  The scale factors described in the  ta‐
       ble,  denoted (fR , fG , fB , fA ), represent either source or destina‐
       tion 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	     (Rc, Gc, Bc, Ac)
	GL_ONE_MINUS_CONSTANT_COLOR(1, 1, 1, 1 ) - (Rc, Gc, Bc, Ac)
	GL_CONSTANT_ALPHA	     (Ac, Ac, Ac, Ac)
	GL_ONE_MINUS_CONSTANT_ALPHA(1, 1, 1, 1 ) - (Ac, Ac, Ac, Ac)

       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 following equations:

		     Rd = min ( kR, Rs sR + Rd dR )
		     Gd = min ( kG, Gs sG + Gd dG )
		     Bd = min ( kB, Bs sB + Bd dB )
		     Ad = min ( kA, As sA + Ad dA )

       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_POLYGON_SMOOTH  argument  for	 information on polygon antialiasing.)
       Destination alpha bitplanes, which must be present for this blend func‐
       tion 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.

       GL_CONSTANT_COLOR,   GL_ONE_MINUS_CONSTANT_COLOR,    GL_CONSTANT_ALPHA,
       GL_ONE_MINUS_CONSTANT_ALPHA are only available if the GL_ARB_imaging is
       supported by your implementation.

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,  glBlendColor,  glBlendEquation,  glClear,	 glDrawBuffer,
       glEnable, glLogicOp, glStencilFunc

							       GLBLENDFUNC(3G)
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