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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="glStencilFunc">
    <info>
        <copyright>
            <year>1991-2006</year>
            <holder>Silicon Graphics, Inc.</holder>
        </copyright>
        <copyright>
            <year>2010-2013</year>
            <holder>Khronos Group</holder>
        </copyright>
    </info>
    <refmeta>
        <refentrytitle>glStencilFunc</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glStencilFunc</refname>
        <refpurpose>set front and back function and reference value for stencil testing</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glStencilFunc</function></funcdef>
                <paramdef>GLenum <parameter>func</parameter></paramdef>
                <paramdef>GLint <parameter>ref</parameter></paramdef>
                <paramdef>GLuint <parameter>mask</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <refsect1 xml:id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>func</parameter></term>
            <listitem>
                <para>
                    Specifies the test function.
                    Eight symbolic constants are valid:
                    <constant>GL_NEVER</constant>,
                    <constant>GL_LESS</constant>,
                    <constant>GL_LEQUAL</constant>,
                    <constant>GL_GREATER</constant>,
                    <constant>GL_GEQUAL</constant>,
                    <constant>GL_EQUAL</constant>,
                    <constant>GL_NOTEQUAL</constant>, and
                    <constant>GL_ALWAYS</constant>. The initial value is <constant>GL_ALWAYS</constant>.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>ref</parameter></term>
            <listitem>
                <para>
                    Specifies the reference value for the stencil test.
                    <parameter>ref</parameter> is clamped to the range
                    <inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
                        <!-- eqn: [0, 2 sup n - 1]:-->
                        <mml:mfenced open="[" close="]">
                            <mml:mn>0</mml:mn>
                            <mml:mrow>
                                <mml:msup><mml:mn>2</mml:mn>
                                <mml:mi mathvariant="italic">n</mml:mi>
                                </mml:msup>
                                <mml:mo>-</mml:mo>
                                <mml:mn>1</mml:mn>
                            </mml:mrow>
                        </mml:mfenced>
                    </mml:math></inlineequation>,
                    where
                    <inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation>
                    is the number of bitplanes in the stencil buffer. The
                    initial value is 0.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>mask</parameter></term>
            <listitem>
                <para>
                    Specifies a mask that is ANDed with both the reference value
                    and the stored stencil value when the test is done. The initial value
                    is all 1's.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 xml:id="description"><title>Description</title>
        <para>
            Stenciling,
            like depth-buffering,
            enables and disables drawing on a per-pixel basis.
            Stencil planes are first drawn into using GL drawing primitives, then
            geometry and images are rendered 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.
        </para>
        <para>
            The stencil test conditionally eliminates a pixel based on the outcome
            of a comparison between the reference value
            and the value in the stencil buffer.
            To enable and disable the test, call <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry>
            with argument <constant>GL_STENCIL_TEST</constant>.
            To specify actions based on the outcome of the stencil test, call
            <citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry> or
            <citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry>.
        </para>
        <para>
            There can be two separate sets of <parameter>func</parameter>, <parameter>ref</parameter>, and
            <parameter>mask</parameter> parameters; one affects back-facing polygons, and the other
            affects front-facing polygons as well as other non-polygon primitives.
            <citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry> sets both front
            and back stencil state to the same values. Use <citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>
            to set front and back stencil state to different values.
        </para>
        <para>
            <parameter>func</parameter> is a symbolic constant that determines the stencil comparison function.
            It accepts one of eight values,
            shown in the following list.
            <parameter>ref</parameter> is an integer reference value that is used in the stencil comparison.
            It is clamped to the range
            <inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
                <!-- eqn: [0, 2 sup n - 1]:-->
                <mml:mfenced open="[" close="]">
                    <mml:mn>0</mml:mn>
                    <mml:mrow>
                        <mml:msup><mml:mn>2</mml:mn>
                        <mml:mi mathvariant="italic">n</mml:mi>
                        </mml:msup>
                        <mml:mo>-</mml:mo>
                        <mml:mn>1</mml:mn>
                    </mml:mrow>
                </mml:mfenced>
            </mml:math></inlineequation>,
            where
            <inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation>
            is the number of bitplanes in the stencil buffer.
            <parameter>mask</parameter> is bitwise ANDed with both the reference value
            and the stored stencil value,
            with the ANDed values participating in the comparison.
        </para>
        <para>
            If <emphasis>stencil</emphasis> represents the value stored in the corresponding
            stencil buffer location,
            the following list shows the effect of each comparison function
            that can be specified by <parameter>func</parameter>.
            Only if the comparison succeeds is the pixel passed through
            to the next stage in the rasterization process
            (see <citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry>).
            All tests treat <emphasis>stencil</emphasis> values as unsigned integers in the range
            <inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
                <!-- eqn: [0, 2 sup n - 1]:-->
                <mml:mfenced open="[" close="]">
                    <mml:mn>0</mml:mn>
                    <mml:mrow>
                        <mml:msup><mml:mn>2</mml:mn>
                        <mml:mi mathvariant="italic">n</mml:mi>
                        </mml:msup>
                        <mml:mo>-</mml:mo>
                        <mml:mn>1</mml:mn>
                    </mml:mrow>
                </mml:mfenced>
            </mml:math></inlineequation>,
            where
            <inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation>
            is the number of bitplanes in the stencil buffer.
        </para>
        <para>
            The following values are accepted by <parameter>func</parameter>:
        </para>
        <variablelist>
            <varlistentry>
                <term><constant>GL_NEVER</constant></term>
                <listitem>
                    <para>
                        Always fails.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_LESS</constant></term>
                <listitem>
                    <para>
                        Passes if ( <parameter>ref</parameter> &amp; <parameter>mask</parameter> ) &lt; ( <emphasis>stencil</emphasis> &amp; <parameter>mask</parameter> ).
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_LEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if ( <parameter>ref</parameter> &amp; <parameter>mask</parameter> ) &lt;= ( <emphasis>stencil</emphasis> &amp; <parameter>mask</parameter> ).
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_GREATER</constant></term>
                <listitem>
                    <para>
                        Passes if ( <parameter>ref</parameter> &amp; <parameter>mask</parameter> ) &gt; ( <emphasis>stencil</emphasis> &amp; <parameter>mask</parameter> ).
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_GEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if ( <parameter>ref</parameter> &amp; <parameter>mask</parameter> ) &gt;= ( <emphasis>stencil</emphasis> &amp; <parameter>mask</parameter> ).
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_EQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if ( <parameter>ref</parameter> &amp; <parameter>mask</parameter> ) = ( <emphasis>stencil</emphasis> &amp; <parameter>mask</parameter> ).
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_NOTEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if ( <parameter>ref</parameter> &amp; <parameter>mask</parameter> ) != ( <emphasis>stencil</emphasis> &amp; <parameter>mask</parameter> ).
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_ALWAYS</constant></term>
                <listitem>
                    <para>
                        Always passes.
                    </para>
                </listitem>
            </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 xml:id="notes"><title>Notes</title>
        <para>
            Initially, the stencil test is disabled.
            If there is no stencil buffer,
            no stencil modification can occur and it is as if
            the stencil test always passes.
        </para>
        <para>
            <citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry> is the same as
            calling <citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>
            with <parameter>face</parameter> set to <constant>GL_FRONT_AND_BACK</constant>.
        </para>
    </refsect1>
    <refsect1 xml:id="errors"><title>Errors</title>
        <para>
            <constant>GL_INVALID_ENUM</constant> is generated if <parameter>func</parameter> is not one of the eight
            accepted values.
        </para>
    </refsect1>
    <refsect1 xml:id="associatedgets"><title>Associated Gets</title>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument
            <constant>GL_STENCIL_FUNC</constant>, <constant>GL_STENCIL_VALUE_MASK</constant>,
            <constant>GL_STENCIL_REF</constant>, <constant>GL_STENCIL_BACK_FUNC</constant>,
            <constant>GL_STENCIL_BACK_VALUE_MASK</constant>, <constant>GL_STENCIL_BACK_REF</constant>,
            or <constant>GL_STENCIL_BITS</constant>
        </para>
        <para>
            <citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_STENCIL_TEST</constant>
        </para>
    </refsect1>
    <refsect1 xml:id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glBlendFunc</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glLogicOp</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glStencilMask</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glStencilMaskSeparate</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry>
        </para>
    </refsect1>
    <refsect1 xml:id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"/> 1991-2006 Silicon Graphics, Inc.
            Copyright <trademark class="copyright"/> 2010-2013 Khronos Group.
            This document is licensed under the SGI Free Software B License.
            For details, see
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</link>.
        </para>
    </refsect1>
</refentry>
