<div class="refentry" lang="en" xml:lang="en"><a id="glAlphaFunc"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>glAlphaFunc — specify the alpha test function</p></div><div class="refsynopsisdiv"><h2>C Specification</h2><div class="funcsynopsis"><table class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">glAlphaFunc</b>(</code></td><td>GLenum <var class="pdparam">func</var>, </td></tr><tr><td> </td><td>GLclampf <var class="pdparam">ref</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">glAlphaFuncx</b>(</code></td><td>GLenum <var class="pdparam">func</var>, </td></tr><tr><td> </td><td>GLclampx <var class="pdparam">ref</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="parameters"></a><h2>Parameters</h2><div class="variablelist"><dl><dt><span class="term">

<em class="parameter"><code>func</code></em>

</span></dt><dd><p>Specifies the alpha comparison function. Symbolic

constants

<code class="constant">GL_NEVER</code>,

<code class="constant">GL_LESS</code>,

<code class="constant">GL_EQUAL</code>,

<code class="constant">GL_LEQUAL</code>,

<code class="constant">GL_GREATER</code>,

<code class="constant">GL_NOTEQUAL</code>,

<code class="constant">GL_GEQUAL</code>, and

<code class="constant">GL_ALWAYS</code>

are accepted. The initial value is

<code class="constant">GL_ALWAYS</code>.

</p></dd><dt><span class="term">

<em class="parameter"><code>ref</code></em>

</span></dt><dd><p>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.

</p></dd></dl></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="description"></a><h2>Description</h2><p>The alpha test discards fragments depending on the

outcome of a comparison between an incoming fragment's alpha

value and a constant reference value.

<code class="function">glAlphaFunc</code>

specifies the reference value and the comparison function. The

comparison is performed only if alpha testing is enabled.

To enable and disable alpha testing, call

<a class="citerefentry" href="glEnable"><span class="citerefentry"><span class="refentrytitle">glEnable</span></span></a> and

<a class="citerefentry" href="glDisable"><span class="citerefentry"><span class="refentrytitle">glDisable</span></span></a>

with argument <code class="constant">GL_ALPHA_TEST</code>.

Alpha testing is initially disabled. When disabled, it is as if the comparison

always passes.

</p><p>

<em class="parameter"><code>func</code></em> and <em class="parameter"><code>ref</code></em>

specify the conditions under which the pixel is drawn. The

incoming alpha value is compared to <em class="parameter"><code>ref</code></em>

using the function specified by <em class="parameter"><code>func</code></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:

</p><div class="variablelist"><dl><dt><span class="term">

<code class="constant">GL_NEVER</code>

</span></dt><dd><p>Never passes.</p></dd><dt><span class="term">

<code class="constant">GL_LESS</code>

</span></dt><dd><p>Passes if the incoming alpha value is less than the

reference value.</p></dd><dt><span class="term">

<code class="constant">GL_EQUAL</code>

</span></dt><dd><p>Passes if the incoming alpha value is equal to the

reference value.</p></dd><dt><span class="term">

<code class="constant">GL_LEQUAL</code>

</span></dt><dd><p>Passes if the incoming alpha value is less than or

equal to the reference value.</p></dd><dt><span class="term">

<code class="constant">GL_GREATER</code>

</span></dt><dd><p>Passes if the incoming alpha value is greater than

the reference value.</p></dd><dt><span class="term">

<code class="constant">GL_NOTEQUAL</code>

</span></dt><dd><p>Passes if the incoming alpha value is not equal to

the reference value.</p></dd><dt><span class="term">

<code class="constant">GL_GEQUAL</code>

</span></dt><dd><p>Passes if the incoming alpha value is greater than

or equal to the reference value.</p></dd><dt><span class="term">

<code class="constant">GL_ALWAYS</code>

</span></dt><dd><p>Always passes (initial value).</p></dd></dl></div><p>

<code class="function">glAlphaFunc</code>

operates on all pixel write operations, including those

resulting from the scan conversion of points, lines, and

polygons.

<code class="function">glAlphaFunc</code>

does not affect

<a class="citerefentry" href="glClear"><span class="citerefentry"><span class="refentrytitle">glClear</span></span></a>.

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="errors"></a><h2>Errors</h2><p>

<code class="constant">GL_INVALID_ENUM</code> is generated if

<em class="parameter"><code>func</code></em> is not an accepted value.

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="seealso"></a><h2>See Also</h2><p>

<a class="citerefentry" href="glBlendFunc"><span class="citerefentry"><span class="refentrytitle">glBlendFunc</span></span></a>,

<a class="citerefentry" href="glClear"><span class="citerefentry"><span class="refentrytitle">glClear</span></span></a>,

<a class="citerefentry" href="glDepthFunc"><span class="citerefentry"><span class="refentrytitle">glDepthFunc</span></span></a>,

<a class="citerefentry" href="glEnable"><span class="citerefentry"><span class="refentrytitle">glEnable</span></span></a>,

<a class="citerefentry" href="glStencilFunc"><span class="citerefentry"><span class="refentrytitle">glStencilFunc</span></span></a>

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="copyright"></a><h2>Copyright</h2><p>

Copyright © 2003-2004

Silicon Graphics, Inc. This document is licensed under the SGI

Free Software B License. For details, see

<a class="ulink" href="https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/" target="_top">https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/</a>.

</p></div></div>