glBlendFunc man page on OpenBSD

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GLBLENDFUNC(3G)						       GLBLENDFUNC(3G)

NAME
       glBlendFunc - specify pixel arithmetic

C SPECIFICATION
       void glBlendFunc( GLenum sfactor,
	    GLenum dfactor )

       delim $$

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
       disabled.  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 $(R sub s , G sub s , B sub s , A sub s
       )$ and $(R sub d , G sub d , B sub d , A sub d )$.  The color specified
       by glBlendColor is referred to as $(R sub c , G sub c , B sub c , A sub
       c )$.  They are understood to have integer values between 0 and $(k sub
       R , k sub G , k sub B , k sub A )$, where
	      $k sub c ~=~ 2 sup m sub c - 1$

       and $(m sub R , m sub G , m sub B , m sub A )$ is the number of red,
       green, blue, and alpha bitplanes.

       Source and destination scale factors are referred to as $(s sub R , s
       sub G , s sub B , s sub A )$ and $(d sub R , d sub G , d sub B , d sub
       A )$.  The scale factors described in the table, denoted $(f sub R , f
       sub G , f sub B , f sub A )$, represent either source or destination
       factors.	 All scale factors have range [0, 1].

       -----------------------------------------------------------------------------------------------------------------------------------------
       Parameter						$(f sub R , ~~ f sub G , ~~ f sub B , ~~ f sub A )$
       -----------------------------------------------------------------------------------------------------------------------------------------
       GL_ZERO									 $(0, ~0, ~0, ~0 )$
       GL_ONE									 $(1, ~1, ~1, ~1 )$
       GL_SRC_COLOR			       $(R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$
       GL_ONE_MINUS_SRC_COLOR	     $(1, ~1, ~1, ~1 ) ~-~ (R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$
       GL_DST_COLOR			       $(R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$
       GL_ONE_MINUS_DST_COLOR	     $(1, ~1, ~1, ~1 ) ~-~ (R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$
       GL_SRC_ALPHA			       $(A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$
       GL_ONE_MINUS_SRC_ALPHA	     $(1, ~1, ~1, ~1 ) ~-~ (A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$
       GL_DST_ALPHA			       $(A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$
       GL_ONE_MINUS_DST_ALPHA	     $(1, ~1, ~1, ~1 ) ~-~ (A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$
       GL_SRC_ALPHA_SATURATE							 $(i, ~i, ~i, ~1 )$
       GL_CONSTANT_COLOR					       $(R sub c, G sub c, B sub c, A sub c)$
       GL_ONE_MINUS_CONSTANT_COLOR			    $(1, ~1, ~1, ~1 ) ~-~ (R sub c, G sub c, B sub c, A sub c)$
       GL_CONSTANT_ALPHA					       $(A sub c, A sub c, A sub c, A sub c)$
       GL_ONE_MINUS_CONSTANT_ALPHA			    $(1, ~1, ~1, ~1 ) ~-~ (A sub c, A sub c, A sub c, A sub c)$
       -----------------------------------------------------------------------------------------------------------------------------------------

       In the table,

		     $i ~=~  min (A sub s , ~k sub A ~-~ A sub d ) ~/~ k sub A$

       To determine the blended RGBA values of a pixel when drawing in RGBA
       mode, the system uses the following equations:

		     $R sub d ~=~ mark	 min ( k sub R, ~R sub s~s sub R~+~R sub d~d sub R )$
		     $G sub d ~=~ lineup min ( k sub G, ~G sub s~s sub G~+~G sub d~d sub G )$
		     $B sub d ~=~ lineup min ( k sub B, ~B sub s~s sub B~+~B sub d~d sub B )$
		     $A sub d ~=~ lineup min ( k sub A, ~A sub s~s sub A~+~A sub d~d sub A )$

       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 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 $A sub
       s$ is equal to $k sub A$, the equations reduce to simple replacement:

		     $R sub d ~=~ mark	 R sub s$
		     $G sub d ~=~ lineup G sub s$
		     $B sub d ~=~ lineup B sub s$
		     $A sub d ~=~ lineup A sub s$

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.

       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.

NOTES
       Incoming (source) alpha is correctly thought of as a material opacity,
       ranging from 1.0 ($K sub A$), representing complete opacity, to 0.0
       (0), representing 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(3G), glBlendColor(3G), glBlendEquation(3G), glClear(3G),
       glDrawBuffer(3G), glEnable(3G), glLogicOp(3G), glStencilFunc(3G)

								 March 1, 2011
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