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+_C_ License Applicability. Except to the extent portions of this file are
+_C_ made subject to an alternative license as permitted in the SGI Free
+_C_ Software License B, Version 1.1 (the "License"), the contents of this
+_C_ file are subject only to the provisions of the License. You may not use
+_C_ this file except in compliance with the License. You may obtain a copy
+_C_ of the License at Silicon Graphics, Inc., attn: Legal Services, 1600
+_C_ Amphitheatre Parkway, Mountain View, CA 94043-1351, or at:
+_C_ 
+_C_ http://oss.sgi.com/projects/FreeB
+_C_ 
+_C_ Note that, as provided in the License, the Software is distributed on an
+_C_ "AS IS" basis, with ALL EXPRESS AND IMPLIED WARRANTIES AND CONDITIONS
+_C_ DISCLAIMED, INCLUDING, WITHOUT LIMITATION, ANY IMPLIED WARRANTIES AND
+_C_ CONDITIONS OF MERCHANTABILITY, SATISFACTORY QUALITY, FITNESS FOR A
+_C_ PARTICULAR PURPOSE, AND NON-INFRINGEMENT.
+_C_ 
+_C_ Original Code. The Original Code is: OpenGL Sample Implementation,
+_C_ Version 1.2.1, released January 26, 2000, developed by Silicon Graphics,
+_C_ Inc. The Original Code is Copyright (c) 1991-2000 Silicon Graphics, Inc.
+_C_ Copyright in any portions created by third parties is as indicated
+_C_ elsewhere herein. All Rights Reserved.
+_C_ 
+_C_ Additional Notice Provisions: The application programming interfaces
+_C_ established by SGI in conjunction with the Original Code are The
+_C_ OpenGL(R) Graphics System: A Specification (Version 1.2.1), released
+_C_ April 1, 1999; The OpenGL(R) Graphics System Utility Library (Version
+_C_ 1.3), released November 4, 1998; and OpenGL(R) Graphics with the X
+_C_ Window System(R) (Version 1.3), released October 19, 1998. This software
+_C_ was created using the OpenGL(R) version 1.2.1 Sample Implementation
+_C_ published by SGI, but has not been independently verified as being
+_C_ compliant with the OpenGL(R) version 1.2.1 Specification.
+_C_
+_C_ The first character in this file must be an '_'!
+_C_ Anything on a line after _C_ is ignored
+_define(_filters,tbl|eqn)_C_
+_C_      eqn is automatically replaced with neqn for nroff
+_header(BlendEquation, set the blend equation)
+_names(BlendEquation)
+.EQ
+delim $$
+.EN
+.SH PARAMETERS
+_phead(_param1)
+specifies how source and destination colors are combined.
+It must be _const(FUNC_ADD), _const(FUNC_SUBTRACT),
+_const(FUNC_REVERSE_SUBTRACT), _const(MIN), _const(MAX).
+
+.SH DESCRIPTION
+The blend equation determines how a new pixel (the ``source'' color)
+is combined with a pixel already in the framebuffer (the ``destination''
+color).
+.TP
+_const(MIN)
+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.
+.TP
+_const(MAX)
+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.
+.P
+The remaining blend equations use the source and destination blend factors
+specified by _cmnd(BlendFunc).
+See _cmnd(BlendFunc) for a description of the various blend factors. 
+.P
+In the equations that follow, source and destination
+color components are referred to as
+$(R sub s, G sub s, B sub s, A sub s )$ 
+and
+$(R sub d, G sub d, B sub d, A sub d )$,
+respectively.
+The result color is referred to as 
+$(R sub r, G sub r, B sub r, A sub r )$.
+The source and destination blend factors are denoted 
+$(s sub R, s sub G, s sub B, s sub A )$ and 
+$(d sub R, d sub G, d sub B, d sub A )$, respectively.
+For these equations all color components are understood to have values
+in the range [0,\ 1].
+.TP
+_const(FUNC_ADD)
+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 result is the componentwise sum of the two products, clamped to the
+range [0,\ 1].
+.P
+.RS
+.nf
+.IP
+$Rr ~=~ mark   min (1, ~R sub s~s sub R ~+~ R sub d~d sub R )$
+$Gr ~=~ lineup min (1, ~G sub s~s sub G ~+~ G sub d~d sub G )$
+$Br ~=~ lineup min (1, ~B sub s~s sub B ~+~ B sub d~d sub B )$
+$Ar ~=~ lineup min (1, ~A sub s~s sub A ~+~ A sub d~d sub A )$
+.fi
+.RE
+.TP
+_const(FUNC_SUBTRACT)
+Is like _const(FUNC_ADD) 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].
+.RS
+.nf
+.IP
+$Rr ~=~ mark   max (0 , ~R sub s~s sub R ~-~ R sub d~d sub R )$
+$Gr ~=~ lineup max (0 , ~G sub s~s sub G ~-~ G sub d~d sub G )$
+$Br ~=~ lineup max (0 , ~B sub s~s sub B ~-~ B sub d~d sub B )$
+$Ar ~=~ lineup max (0 , ~A sub s~s sub A ~-~ A sub d~d sub A )$
+.fi
+.RE
+.TP
+_const(FUNC_REVERSE_SUBTRACT)
+Is like _const(FUNC_ADD) 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].
+.RS
+.nf
+.IP
+$Rr ~=~ mark   max (0 , ~R sub d~d sub R ~-~ R sub s~s sub R )$
+$Gr ~=~ lineup max (0 , ~G sub d~d sub G ~-~ G sub s~s sub G )$
+$Br ~=~ lineup max (0 , ~B sub d~d sub B ~-~ B sub s~s sub B )$
+$Ar ~=~ lineup max (0 , ~A sub d~d sub A ~-~ A sub s~s sub A )$
+.fi
+.RE
+.P
+The _const(MIN) and _const(MAX) equations are useful for applications
+that analyze
+.br
+image data (image thresholding against a constant color,
+for example).
+The _const(FUNC_ADD) equation is useful
+for antialiasing and transparency, among other things.
+.P
+Initially, the blend equation is set to _const(FUNC_ADD).
+.P
+.SH NOTES
+_cmnd is part of the _arbstring(imaging) subset. _cmnd is present only
+if _arbstring(imaging) is returned when _cmnd(GetString) is called with
+_const(EXTENSIONS) as its argument.
+.P
+The _const(MIN), and _const(MAX) equations do not use
+the source or destination factors, only the source and destination colors.
+.P
+.SH ERRORS
+_const(INVALID_ENUM) is generated if _param1 is not one of 
+_const(FUNC_ADD), _const(FUNC_SUBTRACT), _const(FUNC_REVERSE_SUBTRACT),
+_const(MAX), or _const(MIN).
+.P
+_const(INVALID_OPERATION) is generated if _cmnd is executed
+between the execution of _cmnd(Begin) and the corresponding
+execution of _cmnd(End).
+.SH ASSOCIATED GETS
+.TP
+_cmnd(Get) with an argument of _const(BLEND_EQUATION)
+.SH SEE ALSO
+_cmnd(GetString),
+_cmnd(BlendColor),
+_cmnd(BlendFunc)