
<refentry id="glAlphaFunc">
    <refmeta>
        <refmetainfo>
            <copyright>
                <year>1991-2006</year>
                <holder>Silicon Graphics, Inc.</holder>
            </copyright>
        </refmetainfo>
        <refentrytitle>glAlphaFunc</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glAlphaFunc</refname>
        <refpurpose>specify the alpha test function</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glAlphaFunc</function></funcdef>
                <paramdef>GLenum <parameter>func</parameter></paramdef>
                <paramdef>GLclampf <parameter>ref</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <refsect1 id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>func</parameter></term>
            <listitem>
                <para>
                    Specifies the alpha 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_ALWAYS</constant>.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>ref</parameter></term>
            <listitem>
                <para>
                    Specifies the reference value that incoming alpha values are compared to.
                    This value is clamped to the range 
                    <inlineequation><mml:math>
                        <!-- eqn: [0,1]:-->
                        <mml:mfenced open="[" close="]">
                            <mml:mn>0</mml:mn>
                            <mml:mn>1</mml:mn>
                        </mml:mfenced>
                    </mml:math></inlineequation>,
                    where 0 represents the lowest possible alpha value
                    and 1 the highest possible value.
                    The initial reference value is 0.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 id="description"><title>Description</title>
        <para>
            The alpha test discards fragments depending on the outcome of a comparison
            between an incoming fragment's alpha value and a constant reference value.
            <function>glAlphaFunc</function> specifies the reference value and the comparison function.
            The comparison is performed only if alpha testing is enabled. By
            default, it is not enabled.
            (See
            <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry> of <constant>GL_ALPHA_TEST</constant>.)
        </para>
        <para>
            <parameter>func</parameter> and <parameter>ref</parameter> specify the conditions under which
            the pixel is drawn.
            The incoming alpha value is compared to <parameter>ref</parameter>
            using the function specified by <parameter>func</parameter>.
            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:
        </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 alpha value is less than the reference value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_EQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming alpha value is equal to the reference value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_LEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming alpha value is less than or equal to the reference value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_GREATER</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming alpha value is greater than the reference value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_NOTEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming alpha value is not equal to the reference value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_GEQUAL</constant></term>
                <listitem>
                    <para>
                        Passes if the incoming alpha value is greater than or equal to
                        the reference value.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_ALWAYS</constant></term>
                <listitem>
                    <para>
                        Always passes (initial value).
                    </para>
                </listitem>
            </varlistentry>
        </variablelist>
        <para>
            <function>glAlphaFunc</function> operates on all pixel write operations,
            including those resulting from the scan conversion of points,
            lines,
            polygons,
            and bitmaps,
            and from pixel draw and copy operations.
            <function>glAlphaFunc</function> does not affect screen clear operations.
        </para>
    </refsect1>
    <refsect1 id="notes"><title>Notes</title>
        <para>
            Alpha testing is performed only in RGBA mode.
        </para>
    </refsect1>
    <refsect1 id="errors"><title>Errors</title>
        <para>
            <constant>GL_INVALID_ENUM</constant> is generated if <parameter>func</parameter> is not an accepted value.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <function>glAlphaFunc</function>
            is executed between the execution of <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
            and the corresponding execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
        </para>
    </refsect1>
    <refsect1 id="associatedgets"><title>Associated Gets</title>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ALPHA_TEST_FUNC</constant>
        </para>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ALPHA_TEST_REF</constant>
        </para>
        <para>
            <citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_ALPHA_TEST</constant>
        </para>
    </refsect1>
    <refsect1 id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glBlendFunc</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glClear</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry>
        </para>
    </refsect1>
    <refsect1 id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"></trademark> 1991-2006
            Silicon Graphics, Inc. This document is licensed under the SGI
            Free Software B License. For details, see
            <ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
        </para>
    </refsect1>
</refentry>
