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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(AlphaFunc,specify the alpha test function)
+_names(AlphaFunc)
+.SH PARAMETERS
+_phead(_param1)
+Specifies the alpha comparison function.
+Symbolic constants
+_const(NEVER),
+_const(LESS),
+_const(EQUAL),
+_const(LEQUAL),
+_const(GREATER),
+_const(NOTEQUAL),
+_const(GEQUAL), and
+_const(ALWAYS) are accepted. The initial value is _const(ALWAYS).
+_phead(_param2)
+Specifies the reference value that incoming alpha values are compared to.
+This value is clamped to the range [0,\ 1],
+where 0 represents the lowest possible alpha value
+and 1 the highest possible value.
+The initial reference value is 0.
+.SH DESCRIPTION
+The alpha test discards fragments depending on the outcome of a comparison
+between an incoming fragment's alpha value and a constant reference value.
+_cmnd specifies the reference value and the comparison function.
+The comparison is performed only if alpha testing is enabled. By
+default, it is not enabled. 
+(See 
+_cmnd(Enable) and _cmnd(Disable) of _const(ALPHA_TEST).)
+.P
+_param1 and _param2 specify the conditions under which
+the pixel is drawn.
+The incoming alpha value is compared to _param2
+using the function specified by _param1.
+If the value passes the comparison,
+the incoming fragment is drawn
+if it also passes subsequent stencil and depth buffer tests. 
+If the value fails the comparison,
+no change is made to the frame buffer at that pixel location. The
+comparison functions are as follows: 
+.TP 18
+_const(NEVER)
+Never passes. 
+.TP
+_const(LESS)
+Passes if the incoming alpha value is less than the reference value.
+.TP
+_const(EQUAL)
+Passes if the incoming alpha value is equal to the reference value.
+.TP
+_const(LEQUAL)
+Passes if the incoming alpha value is less than or equal to the reference value.
+.TP
+_const(GREATER) 
+Passes if the incoming alpha value is greater than the reference value.
+.TP
+_const(NOTEQUAL)
+Passes if the incoming alpha value is not equal to the reference value.
+.TP
+_const(GEQUAL)
+Passes if the incoming alpha value is greater than or equal to
+the reference value.
+.TP
+_const(ALWAYS)
+Always passes (initial value). 
+.P
+_cmnd operates on all pixel write operations,
+including those resulting from the scan conversion of points,
+lines,
+polygons,
+and bitmaps,
+and from pixel draw and copy operations.
+_cmnd does not affect screen clear operations.
+.SH NOTES
+Alpha testing is performed only in RGBA mode.
+.SH ERRORS
+_const(INVALID_ENUM) is generated if _param1 is not an accepted value.
+.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
+_cmnd(Get) with argument _const(ALPHA_TEST_FUNC)
+.br
+_cmnd(Get) with argument _const(ALPHA_TEST_REF)
+.br
+_cmnd(IsEnabled) with argument _const(ALPHA_TEST)
+.SH SEE ALSO
+_cmnd(BlendFunc),
+_cmnd(Clear),
+_cmnd(DepthFunc),
+_cmnd(Enable),
+_cmnd(StencilFunc)