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<STRONG>NAME</STRONG>
<STRONG>glAlphaFunc</STRONG> - specify the alpha test function
<STRONG>C</STRONG> <STRONG>SPECIFICATION</STRONG>
void <STRONG>glAlphaFunc</STRONG>( GLenum <EM>func</EM>,
GLclampf <EM>ref</EM> )
<STRONG>PARAMETERS</STRONG>
<EM>func</EM> Specifies the alpha comparison function. Symbolic
constants <STRONG>GL_NEVER</STRONG>, <STRONG>GL_LESS</STRONG>, <STRONG>GL_EQUAL</STRONG>, <STRONG>GL_LEQUAL</STRONG>,
<STRONG>GL_GREATER</STRONG>, <STRONG>GL_NOTEQUAL</STRONG>, <STRONG>GL_GEQUAL</STRONG>, and <STRONG>GL_ALWAYS</STRONG> are
accepted. The initial value is <STRONG>GL_ALWAYS</STRONG>.
<EM>ref</EM> Specifies the reference value that incoming alpha
values are compared to. This value is clamped to the
range 0 through 1, where 0 represents the lowest
possible alpha value and 1 the highest possible value.
The initial reference value is 0.
<STRONG>DESCRIPTION</STRONG>
The alpha test discards fragments depending on the outcome
of a comparison between an incoming fragment's alpha value
and a constant reference value. <STRONG>glAlphaFunc</STRONG> 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 <STRONG>glEnable</STRONG> and <STRONG>glDisable</STRONG> of
<STRONG>GL_ALPHA_TEST</STRONG>.)
<EM>func</EM> and <EM>ref</EM> specify the conditions under which the pixel is
drawn. The incoming alpha value is compared to <EM>ref</EM> using
the function specified by <EM>func</EM>. 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:
<STRONG>GL_NEVER</STRONG> Never passes.
<STRONG>GL_LESS</STRONG> Passes if the incoming alpha value is less
than the reference value.
<STRONG>GL_EQUAL</STRONG> Passes if the incoming alpha value is
equal to the reference value.
<STRONG>GL_LEQUAL</STRONG> Passes if the incoming alpha value is less
than or equal to the reference value.
<STRONG>GL_GREATER</STRONG> Passes if the incoming alpha value is
greater than the reference value.
<STRONG>GL_NOTEQUAL</STRONG> Passes if the incoming alpha value is not
equal to the reference value.
<STRONG>GL_GEQUAL</STRONG> Passes if the incoming alpha value is
greater than or equal to the reference
value.
<STRONG>GL_ALWAYS</STRONG> Always passes (initial value).
<STRONG>glAlphaFunc</STRONG> 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. <STRONG>glAlphaFunc</STRONG> does not affect screen
clear operations.
<STRONG>NOTES</STRONG>
Alpha testing is performed only in RGBA mode.
<STRONG>ERRORS</STRONG>
<STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>func</EM> is not an accepted
value.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <STRONG>glAlphaFunc</STRONG> is executed
between the execution of <STRONG>glBegin</STRONG> and the corresponding
execution of <STRONG>glEnd</STRONG>.
<STRONG>ASSOCIATED</STRONG> <STRONG>GETS</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_ALPHA_TEST_FUNC</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_ALPHA_TEST_REF</STRONG>
<STRONG>glIsEnabled</STRONG> with argument <STRONG>GL_ALPHA_TEST</STRONG>
<STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>
<STRONG>glBlendFunc</STRONG>, <STRONG>glClear</STRONG>, <STRONG>glDepthFunc</STRONG>, <STRONG>glEnable</STRONG>, <STRONG>glStencilFunc</STRONG>
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