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<?xml-stylesheet type="text/xsl" href="mathml.xsl"?><html xmlns="http://www.w3.org/1999/xhtml" xmlns:pref="http://www.w3.org/2002/Math/preference" pref:renderer="mathplayer-dl"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><link rel="stylesheet" type="text/css" href="opengl-man.css" /><title>glStencilOp - OpenGL 4 Reference Pages</title><meta name="generator" content="DocBook XSL Stylesheets V1.69.1" /></head><body><div class="refentry" lang="en" xml:lang="en"><a id="glStencilOp"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>glStencilOp — set front and back stencil test actions</p></div><div class="refsynopsisdiv"><h2>C Specification</h2><div class="funcsynopsis"><table border="0" summary="Function synopsis" cellspacing="0" cellpadding="0"><tr><td><code class="funcdef">void <b class="fsfunc">glStencilOp</b>(</code></td><td>GLenum  </td><td><var class="pdparam">sfail</var>, </td></tr><tr><td> </td><td>GLenum  </td><td><var class="pdparam">dpfail</var>, </td></tr><tr><td> </td><td>GLenum  </td><td><var class="pdparam">dppass</var><code>)</code>;</td></tr></table></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>sfail</code></em></span></dt><dd><p>
                    Specifies the action to take when the stencil test fails.
                    Eight symbolic constants are accepted:
                    <code class="constant">GL_KEEP</code>,
                    <code class="constant">GL_ZERO</code>,
                    <code class="constant">GL_REPLACE</code>,
                    <code class="constant">GL_INCR</code>,
                    <code class="constant">GL_INCR_WRAP</code>,
                    <code class="constant">GL_DECR</code>,
                    <code class="constant">GL_DECR_WRAP</code>, and
                    <code class="constant">GL_INVERT</code>. The initial value is <code class="constant">GL_KEEP</code>.
                </p></dd><dt><span class="term"><em class="parameter"><code>dpfail</code></em></span></dt><dd><p>
                    Specifies the stencil action when the stencil test passes,
                    but the depth test fails.
                    <em class="parameter"><code>dpfail</code></em> accepts the same symbolic constants as <em class="parameter"><code>sfail</code></em>. The initial value
                    is <code class="constant">GL_KEEP</code>.
                </p></dd><dt><span class="term"><em class="parameter"><code>dppass</code></em></span></dt><dd><p>
                    Specifies the stencil action when both the stencil test and the depth
                    test pass, or when the stencil test passes and either there is no
                    depth buffer or depth testing is not enabled.
                    <em class="parameter"><code>dppass</code></em> accepts the same symbolic constants as <em class="parameter"><code>sfail</code></em>. The initial value
                    is <code class="constant">GL_KEEP</code>.
                </p></dd></dl></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="description"></a><h2>Description</h2><p>
            Stenciling,
            like depth-buffering,
            enables and disables drawing on a per-pixel basis.
            You draw into the stencil planes using GL drawing primitives,
            then render geometry and images,
            using the stencil planes to mask out portions of the screen.
            Stenciling is typically used in multipass rendering algorithms
            to achieve special effects,
            such as decals,
            outlining,
            and constructive solid geometry rendering.
        </p><p>
            The stencil test conditionally eliminates a pixel based on the outcome
            of a comparison between the value in the stencil buffer and a
            reference value. To enable and disable the test, call <a href="glEnable.xml"><span class="citerefentry"><span class="refentrytitle">glEnable</span></span></a>
            and <a href="glDisable.xml"><span class="citerefentry"><span class="refentrytitle">glDisable</span></span></a> with argument
            <code class="constant">GL_STENCIL_TEST</code>; to control it, call 
            <a href="glStencilFunc.xml"><span class="citerefentry"><span class="refentrytitle">glStencilFunc</span></span></a> or 
            <a href="glStencilFuncSeparate.xml"><span class="citerefentry"><span class="refentrytitle">glStencilFuncSeparate</span></span></a>.
        </p><p>
            There can be two separate sets of <em class="parameter"><code>sfail</code></em>, <em class="parameter"><code>dpfail</code></em>, and 
            <em class="parameter"><code>dppass</code></em> parameters; one affects back-facing polygons, and the other
            affects front-facing polygons as well as other non-polygon primitives. 
            <a href="glStencilOp.xml"><span class="citerefentry"><span class="refentrytitle">glStencilOp</span></span></a> sets both front
            and back stencil state to the same values. Use <a href="glStencilOpSeparate.xml"><span class="citerefentry"><span class="refentrytitle">glStencilOpSeparate</span></span></a>
            to set front and back stencil state to different values.
        </p><p>
            <code class="function">glStencilOp</code> takes three arguments that indicate what happens
            to the stored stencil value while stenciling is enabled.
            If the stencil test fails,
            no change is made to the pixel's color or depth buffers,
            and <em class="parameter"><code>sfail</code></em> specifies what happens to the stencil buffer contents.
            The following eight actions are possible.
        </p><div class="variablelist"><dl><dt><span class="term"><code class="constant">GL_KEEP</code></span></dt><dd><p>
                        Keeps the current value.
                    </p></dd><dt><span class="term"><code class="constant">GL_ZERO</code></span></dt><dd><p>
                        Sets the stencil buffer value to 0.
                    </p></dd><dt><span class="term"><code class="constant">GL_REPLACE</code></span></dt><dd><p>
                        Sets the stencil buffer value to <span class="emphasis"><em>ref</em></span>,
                        as specified by <a href="glStencilFunc.xml"><span class="citerefentry"><span class="refentrytitle">glStencilFunc</span></span></a>.
                    </p></dd><dt><span class="term"><code class="constant">GL_INCR</code></span></dt><dd><p>
                        Increments the current stencil buffer value.
                        Clamps to the maximum representable unsigned value.
                    </p></dd><dt><span class="term"><code class="constant">GL_INCR_WRAP</code></span></dt><dd><p>
                        Increments the current stencil buffer value.
                        Wraps stencil buffer value to zero when incrementing the maximum
                        representable unsigned value.
                    </p></dd><dt><span class="term"><code class="constant">GL_DECR</code></span></dt><dd><p>
                        Decrements the current stencil buffer value.
                        Clamps to 0.
                    </p></dd><dt><span class="term"><code class="constant">GL_DECR_WRAP</code></span></dt><dd><p>
                        Decrements the current stencil buffer value.
                        Wraps stencil buffer value to the maximum representable unsigned value when
                        decrementing a stencil buffer value of zero.
                    </p></dd><dt><span class="term"><code class="constant">GL_INVERT</code></span></dt><dd><p>
                        Bitwise inverts the current stencil buffer value.
                    </p></dd></dl></div><p>
            Stencil buffer values are treated as unsigned integers.
            When incremented and decremented,
            values are clamped to 0 and 
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            where 
            <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mi mathvariant="italic">n</mml:mi></mml:math>
            is the value returned by querying <code class="constant">GL_STENCIL_BITS</code>.
        </p><p>
            The other two arguments to <code class="function">glStencilOp</code> specify stencil buffer actions
            that depend on whether subsequent depth buffer tests succeed (<em class="parameter"><code>dppass</code></em>)
            or fail (<em class="parameter"><code>dpfail</code></em>) (see
            <a href="glDepthFunc.xml"><span class="citerefentry"><span class="refentrytitle">glDepthFunc</span></span></a>).
            The actions are specified using the same eight symbolic constants as <em class="parameter"><code>sfail</code></em>.
            Note that <em class="parameter"><code>dpfail</code></em> is ignored when there is no depth buffer,
            or when the depth buffer is not enabled.
            In these cases, <em class="parameter"><code>sfail</code></em> and <em class="parameter"><code>dppass</code></em> specify stencil action when the
            stencil test fails and passes,
            respectively.
        </p></div><div class="refsect1" lang="en" xml:lang="en"><a id="notes"></a><h2>Notes</h2><p>
            Initially the stencil test is disabled.
            If there is no stencil buffer,
            no stencil modification can occur
            and it is as if the stencil tests always pass,
            regardless of any call to <code class="function">glStencilOp</code>.
        </p><p>
            <a href="glStencilOp.xml"><span class="citerefentry"><span class="refentrytitle">glStencilOp</span></span></a> is the same as
            calling <a href="glStencilOpSeparate.xml"><span class="citerefentry"><span class="refentrytitle">glStencilOpSeparate</span></span></a>
            with <em class="parameter"><code>face</code></em> set to <code class="constant">GL_FRONT_AND_BACK</code>.
        </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>sfail</code></em>,
            <em class="parameter"><code>dpfail</code></em>, or <em class="parameter"><code>dppass</code></em> is any value other than the defined constant values.
        </p></div><div class="refsect1" lang="en" xml:lang="en"><a id="associatedgets"></a><h2>Associated Gets</h2><p>
            <a href="glGet.xml"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument 
            <code class="constant">GL_STENCIL_FAIL</code>, <code class="constant">GL_STENCIL_PASS_DEPTH_PASS</code>, 
            <code class="constant">GL_STENCIL_PASS_DEPTH_FAIL</code>, <code class="constant">GL_STENCIL_BACK_FAIL</code>, 
            <code class="constant">GL_STENCIL_BACK_PASS_DEPTH_PASS</code>, <code class="constant">GL_STENCIL_BACK_PASS_DEPTH_FAIL</code>,
            or <code class="constant">GL_STENCIL_BITS</code>
        </p><p>
            <a href="glIsEnabled.xml"><span class="citerefentry"><span class="refentrytitle">glIsEnabled</span></span></a> with argument <code class="constant">GL_STENCIL_TEST</code>
        </p></div><div class="refsect1" lang="en" xml:lang="en"><a id="seealso"></a><h2>See Also</h2><p>
            <a href="glBlendFunc.xml"><span class="citerefentry"><span class="refentrytitle">glBlendFunc</span></span></a>,
            <a href="glDepthFunc.xml"><span class="citerefentry"><span class="refentrytitle">glDepthFunc</span></span></a>,
            <a href="glEnable.xml"><span class="citerefentry"><span class="refentrytitle">glEnable</span></span></a>,
            <a href="glLogicOp.xml"><span class="citerefentry"><span class="refentrytitle">glLogicOp</span></span></a>,
            <a href="glStencilFunc.xml"><span class="citerefentry"><span class="refentrytitle">glStencilFunc</span></span></a>,
            <a href="glStencilFuncSeparate.xml"><span class="citerefentry"><span class="refentrytitle">glStencilFuncSeparate</span></span></a>,
            <a href="glStencilMask.xml"><span class="citerefentry"><span class="refentrytitle">glStencilMask</span></span></a>,
            <a href="glStencilMaskSeparate.xml"><span class="citerefentry"><span class="refentrytitle">glStencilMaskSeparate</span></span></a>,
            <a href="glStencilOpSeparate.xml"><span class="citerefentry"><span class="refentrytitle">glStencilOpSeparate</span></span></a>
        </p></div><div class="refsect1" lang="en" xml:lang="en"><a id="Copyright"></a><h2>Copyright</h2><p>
            Copyright <span class="trademark"></span>© 1991-2006
            Silicon Graphics, Inc. This document is licensed under the SGI
            Free Software B License. For details, see
            <a href="http://oss.sgi.com/projects/FreeB/" target="_top">http://oss.sgi.com/projects/FreeB/</a>.
        </p></div></div></body></html>
