<div class="refentry" title="glSampleCoverage"><a id="glSampleCoverage"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>glSampleCoverage — specify mask to modify multisampled pixel fragments</p></div><div class="refsynopsisdiv" title="C Specification"><h2>C Specification</h2><div class="funcsynopsis"><table class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">glSampleCoverage</b>(</code></td><td>GLclampf <var class="pdparam">value</var>, </td></tr><tr><td> </td><td>GLboolean <var class="pdparam">invert</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">glSampleCoveragex</b>(</code></td><td>GLclampx <var class="pdparam">value</var>, </td></tr><tr><td> </td><td>GLboolean <var class="pdparam">invert</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Parameters"><a id="parameters"></a><h2>Parameters</h2><div class="variablelist"><dl><dt><span class="term">

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

</span></dt><dd><p>Specifies the coverage of the modification mask.

The value is clamped to the range [0, 1], where 0 represents

no coverage and 1 full coverage. The initial value is 1.</p></dd><dt><span class="term">

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

</span></dt><dd><p>Specifies whether the modification mask implied by

<em class="parameter"><code>value</code></em> is inverted or not.

The initial value is <code class="constant">GL_FALSE</code>.

</p></dd></dl></div></div><div class="refsect1" title="Description"><a id="description"></a><h2>Description</h2><p><code class="function">glSampleCoverage</code> defines a mask to modify

the coverage of multisampled pixel fragments. This capability is used

for antialiased screen-door transparency and smooth transitions between

two renderings of an object (often for level-of-detail management in

simulation systems).</p><p>

When multisampling is enabled (see

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

with argument

<code class="constant">GL_MULTISAMPLE</code>) a ``fragment mask'' is computed for

each fragment

generated by a primitive. This mask reflects the amount of the pixel

covered by the fragment, and determines the frame buffer samples that may

be affected by the fragment.</p><p>

If conversion of alpha values to masks is enabled

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

with argument

<code class="constant">GL_SAMPLE_ALPHA_TO_COVERAGE</code>),

the fragment alpha value is used to

generate a temporary modification mask which is then ANDed with the

fragment mask. One way to interpret this is as a form of dithering: a

multivalued alpha (coverage or opacity) for the whole fragment is

converted to simple binary values of coverage at many locations (the

samples).</p><p>

After conversion of alpha values to masks, if replacement of alpha values

is enabled

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

with argument

<code class="constant">GL_SAMPLE_ALPHA_TO_ONE</code>), the

fragment's alpha is set to the maximum allowable value.</p><p>

Finally, if fragment mask modification is enabled

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

with argument

<code class="constant">GL_SAMPLE_COVERAGE</code>),

<code class="function">glSampleCoverage</code> defines an additional modification

mask. value is used to generate a modification mask in much the same way

alpha was used above. If invert is <code class="constant">GL_TRUE</code>,

then the modification mask

specified by value will be inverted. The final modification mask will

then be ANDed with the fragment mask resulting from the previous steps.

This can be viewed as an ``override'' control that selectively fades the

effects of multisampled fragments.</p><p>Note that

<code class="function">glSampleCoverage</code>(value, <code class="constant">GL_TRUE</code>)

is not necessarily equivalent to

<code class="function">glSampleCoverage</code>(1.0 - value, <code class="constant">GL_FALSE</code>);

due to round-off and other

issues, complementing the coverage will not necessarily yield an inverted

modification mask.</p></div><div class="refsect1" title="Associated Gets"><a id="associatedgets"></a><h2>Associated Gets</h2><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_SAMPLE_COVERAGE_VALUE</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_SAMPLE_COVERAGE_INVERT</code>

</p><p>

<a class="citerefentry" href="glIsEnabled"><span class="citerefentry"><span class="refentrytitle">glIsEnabled</span></span></a> with argument <code class="constant">GL_SAMPLE_ALPHA_TO_COVERAGE</code>

</p><p>

<a class="citerefentry" href="glIsEnabled"><span class="citerefentry"><span class="refentrytitle">glIsEnabled</span></span></a> with argument <code class="constant">GL_SAMPLE_COVERAGE</code>

</p></div><div class="refsect1" title="See Also"><a id="seealso"></a><h2>See Also</h2><p>

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

</p></div><div class="refsect1" title="Copyright"><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>