115 lines
3.3 KiB
Plaintext
115 lines
3.3 KiB
Plaintext
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'\" te
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'\"! tbl|eqn | mmdoc
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'\"macro stdmacro
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.ds Vn Version 1.2
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.ds Dt 24 September 1999
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.ds Re Release 1.2.1
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.ds Dp Jan 14 18:30
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.ds Dm 01 alphafunc
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.ds Xs 25246 5 alphafunc.gl
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.TH GLALPHAFUNC 3G
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.SH NAME
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.B "glAlphaFunc
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\- specify the alpha test function
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.SH C SPECIFICATION
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void \f3glAlphaFunc\fP(
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GLenum \fIfunc\fP,
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.nf
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.ta \w'\f3void \fPglAlphaFunc( 'u
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GLclampf \fIref\fP )
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.fi
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.SH PARAMETERS
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.TP \w'\f2func\fP\ \ 'u
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\f2func\fP
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Specifies the alpha comparison function.
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Symbolic constants
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\%\f3GL_NEVER\fP,
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\%\f3GL_LESS\fP,
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\%\f3GL_EQUAL\fP,
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\%\f3GL_LEQUAL\fP,
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\%\f3GL_GREATER\fP,
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\%\f3GL_NOTEQUAL\fP,
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\%\f3GL_GEQUAL\fP, and
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\%\f3GL_ALWAYS\fP are accepted. The initial value is \%\f3GL_ALWAYS\fP.
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.TP
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\f2ref\fP
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Specifies the reference value that incoming alpha values are compared to.
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This value is clamped to the range [0,\ 1],
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where 0 represents the lowest possible alpha value
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and 1 the highest possible value.
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The initial reference value is 0.
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.SH DESCRIPTION
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The alpha test discards fragments depending on the outcome of a comparison
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between an incoming fragment's alpha value and a constant reference value.
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\%\f3glAlphaFunc\fP specifies the reference value and the comparison function.
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The comparison is performed only if alpha testing is enabled. By
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default, it is not enabled.
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(See
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\%\f3glEnable\fP and \%\f3glDisable\fP of \%\f3GL_ALPHA_TEST\fP.)
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.P
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\f2func\fP and \f2ref\fP specify the conditions under which
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the pixel is drawn.
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The incoming alpha value is compared to \f2ref\fP
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using the function specified by \f2func\fP.
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If the value passes the comparison,
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the incoming fragment is drawn
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if it also passes subsequent stencil and depth buffer tests.
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If the value fails the comparison,
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no change is made to the frame buffer at that pixel location. The
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comparison functions are as follows:
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.TP 18
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\%\f3GL_NEVER\fP
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Never passes.
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.TP
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\%\f3GL_LESS\fP
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Passes if the incoming alpha value is less than the reference value.
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.TP
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\%\f3GL_EQUAL\fP
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Passes if the incoming alpha value is equal to the reference value.
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.TP
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\%\f3GL_LEQUAL\fP
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Passes if the incoming alpha value is less than or equal to the reference value.
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.TP
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\%\f3GL_GREATER\fP
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Passes if the incoming alpha value is greater than the reference value.
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.TP
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\%\f3GL_NOTEQUAL\fP
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Passes if the incoming alpha value is not equal to the reference value.
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.TP
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\%\f3GL_GEQUAL\fP
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Passes if the incoming alpha value is greater than or equal to
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the reference value.
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.TP
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\%\f3GL_ALWAYS\fP
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Always passes (initial value).
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.P
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\%\f3glAlphaFunc\fP operates on all pixel write operations,
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including those resulting from the scan conversion of points,
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lines,
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polygons,
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and bitmaps,
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and from pixel draw and copy operations.
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\%\f3glAlphaFunc\fP does not affect screen clear operations.
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.SH NOTES
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Alpha testing is performed only in RGBA mode.
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.SH ERRORS
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\%\f3GL_INVALID_ENUM\fP is generated if \f2func\fP is not an accepted value.
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.P
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\%\f3GL_INVALID_OPERATION\fP is generated if \%\f3glAlphaFunc\fP
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is executed between the execution of \%\f3glBegin\fP
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and the corresponding execution of \%\f3glEnd\fP.
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.SH ASSOCIATED GETS
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\%\f3glGet\fP with argument \%\f3GL_ALPHA_TEST_FUNC\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_ALPHA_TEST_REF\fP
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.br
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\%\f3glIsEnabled\fP with argument \%\f3GL_ALPHA_TEST\fP
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.SH SEE ALSO
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\%\f3glBlendFunc(3G)\fP,
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\%\f3glClear(3G)\fP,
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\%\f3glDepthFunc(3G)\fP,
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\%\f3glEnable(3G)\fP,
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\%\f3glStencilFunc(3G)\fP
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