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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="glDepthFunc">
    <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>glDepthFunc</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glDepthFunc</refname>
        <refpurpose>specify the value used for depth buffer comparisons</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glDepthFunc</function></funcdef>
                <paramdef>GLenum <parameter>func</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <refsect1 xml:id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>func</parameter></term>
            <listitem>
                <para>
                    Specifies the depth comparison function.
                    Symbolic constants
                    <constant>GL_NEVER</constant>,
                    <constant>GL_LESS</constant>,
                    <constant>GL_EQUAL</constant>,
                    <constant>GL_LEQUAL</constant>,
                    <constant>GL_GREATER</constant>,
                    <constant>GL_NOTEQUAL</constant>,
                    <constant>GL_GEQUAL</constant>, and
                    <constant>GL_ALWAYS</constant> are accepted.
                    The initial value is <constant>GL_LESS</constant>.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 xml:id="description"><title>Description</title>
        <para>
            <function>glDepthFunc</function> specifies the function used to compare each incoming pixel depth value
            with the depth value present in the depth buffer.
            The comparison is performed only if depth testing is enabled.
            (See <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry> of <constant>GL_DEPTH_TEST</constant>.)
        </para>
        <para>
            <parameter>func</parameter> specifies the conditions under which the pixel will be drawn.
            The comparison functions are as follows:
        </para>
        <variablelist>
            <varlistentry>
                <term><constant>GL_NEVER</constant></term>
                <listitem>
                    <para>
                        Never passes.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_LESS</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming depth value is less than the stored depth value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_EQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming depth value is equal to the stored depth value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_LEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming depth value is less than or equal to
                        the stored depth value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_GREATER</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming depth value is greater than the stored depth value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_NOTEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming depth value is not equal to the stored depth value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_GEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming depth value is greater than or equal to
                        the stored depth value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_ALWAYS</constant></term>
                <listitem>
                    <para>
                        Always passes.
                    </para>
                </listitem>
            </varlistentry>
        </variablelist>
        <para>
            The initial value of <parameter>func</parameter> is <constant>GL_LESS</constant>.
            Initially, depth testing is disabled.  If depth testing is disabled or if no
            depth buffer exists, it is as if the depth test always passes.
        </para>
    </refsect1>
    <refsect1 xml:id="notes"><title>Notes</title>
        <para>
            Even if the depth buffer exists and the depth mask is non-zero, the
            depth buffer is not updated if the depth test is disabled. In order to
            unconditionally write to the depth buffer, the depth test should be enabled
            and set to <constant>GL_ALWAYS</constant>.
        </para>
    </refsect1>
    <refsect1 xml:id="errors"><title>Errors</title>
        <para>
            <constant>GL_INVALID_ENUM</constant> is generated if <parameter>func</parameter> is not an accepted value.
        </para>
    </refsect1>
    <refsect1 xml:id="associatedgets"><title>Associated Gets</title>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_DEPTH_FUNC</constant>
        </para>
        <para>
            <citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_DEPTH_TEST</constant>
        </para>
    </refsect1>
    <refsect1 xml:id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glDepthRange</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glPolygonOffset</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>
