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<!DOCTYPE refentry [ <!ENTITY % mathent SYSTEM "math.ent"> %mathent; ]>
<!-- Converted by db4-upgrade version 1.1 -->
<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="glStencilOpSeparate">
<info>
<copyright>
<year>1991-2006</year>
<holder>Silicon Graphics, Inc.</holder>
</copyright>
</info>
<refmeta>
<refentrytitle>glStencilOpSeparate</refentrytitle>
<manvolnum>3G</manvolnum>
</refmeta>
<refnamediv>
<refname>glStencilOpSeparate</refname>
<refpurpose>set front and/or back stencil test actions</refpurpose>
</refnamediv>
<refsynopsisdiv><title>C Specification</title>
<funcsynopsis>
<funcprototype>
<funcdef>void <function>glStencilOpSeparate</function></funcdef>
<paramdef>GLenum <parameter>face</parameter></paramdef>
<paramdef>GLenum <parameter>sfail</parameter></paramdef>
<paramdef>GLenum <parameter>dpfail</parameter></paramdef>
<paramdef>GLenum <parameter>dppass</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 xml:id="parameters"><title>Parameters</title>
<variablelist>
<varlistentry>
<term><parameter>face</parameter></term>
<listitem>
<para>
Specifies whether front and/or back stencil state is updated.
Three symbolic constants are valid:
<constant>GL_FRONT</constant>,
<constant>GL_BACK</constant>, and
<constant>GL_FRONT_AND_BACK</constant>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>sfail</parameter></term>
<listitem>
<para>
Specifies the action to take when the stencil test fails.
Eight symbolic constants are accepted:
<constant>GL_KEEP</constant>,
<constant>GL_ZERO</constant>,
<constant>GL_REPLACE</constant>,
<constant>GL_INCR</constant>,
<constant>GL_INCR_WRAP</constant>,
<constant>GL_DECR</constant>,
<constant>GL_DECR_WRAP</constant>, and
<constant>GL_INVERT</constant>. The initial value is <constant>GL_KEEP</constant>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>dpfail</parameter></term>
<listitem>
<para>
Specifies the stencil action when the stencil test passes,
but the depth test fails.
<parameter>dpfail</parameter> accepts the same symbolic constants as <parameter>sfail</parameter>. The initial value
is <constant>GL_KEEP</constant>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>dppass</parameter></term>
<listitem>
<para>
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.
<parameter>dppass</parameter> accepts the same symbolic constants as <parameter>sfail</parameter>. The initial value
is <constant>GL_KEEP</constant>.
</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.
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.
</para>
<para>
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 <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>
and <!-- FIX: link to glEnable --> <function>glDisable</function> with argument
<constant>GL_STENCIL_TEST</constant>; to control it, call
<citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry> or
<citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>.
</para>
<para>
There can be two separate sets of <parameter>sfail</parameter>, <parameter>dpfail</parameter>, and
<parameter>dppass</parameter> parameters; one affects back-facing polygons, and the other
affects front-facing polygons as well as other non-polygon primitives.
<citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry> sets both front
and back stencil state to the same values, as if
<citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry> were called
with <parameter>face</parameter> set to <constant>GL_FRONT_AND_BACK</constant>.
</para>
<para>
<function>glStencilOpSeparate</function> 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 <parameter>sfail</parameter> specifies what happens to the stencil buffer contents.
The following eight actions are possible.
</para>
<variablelist>
<varlistentry>
<term><constant>GL_KEEP</constant></term>
<listitem>
<para>
Keeps the current value.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_ZERO</constant></term>
<listitem>
<para>
Sets the stencil buffer value to 0.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_REPLACE</constant></term>
<listitem>
<para>
Sets the stencil buffer value to <emphasis>ref</emphasis>,
as specified by <citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_INCR</constant></term>
<listitem>
<para>
Increments the current stencil buffer value.
Clamps to the maximum representable unsigned value.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_INCR_WRAP</constant></term>
<listitem>
<para>
Increments the current stencil buffer value.
Wraps stencil buffer value to zero when incrementing the maximum
representable unsigned value.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_DECR</constant></term>
<listitem>
<para>
Decrements the current stencil buffer value.
Clamps to 0.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_DECR_WRAP</constant></term>
<listitem>
<para>
Decrements the current stencil buffer value.
Wraps stencil buffer value to the maximum representable unsigned value when
decrementing a stencil buffer value of zero.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_INVERT</constant></term>
<listitem>
<para>
Bitwise inverts the current stencil buffer value.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Stencil buffer values are treated as unsigned integers.
When incremented and decremented,
values are clamped to 0 and
<inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<!-- eqn: 2 sup n - 1:-->
<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: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 value returned by querying <constant>GL_STENCIL_BITS</constant>.
</para>
<para>
The other two arguments to <function>glStencilOpSeparate</function> specify stencil buffer actions
that depend on whether subsequent depth buffer tests succeed (<parameter>dppass</parameter>)
or fail (<parameter>dpfail</parameter>) (see
<citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>).
The actions are specified using the same eight symbolic constants as <parameter>sfail</parameter>.
Note that <parameter>dpfail</parameter> is ignored when there is no depth buffer,
or when the depth buffer is not enabled.
In these cases, <parameter>sfail</parameter> and <parameter>dppass</parameter> specify stencil action when the
stencil test fails and passes,
respectively.
</para>
</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>
</refsect1>
<refsect1 xml:id="errors"><title>Errors</title>
<para>
<constant>GL_INVALID_ENUM</constant> is generated if <parameter>face</parameter> is any value
other than <constant>GL_FRONT</constant>, <constant>GL_BACK</constant>, or <constant>GL_FRONT_AND_BACK</constant>.
</para>
<para>
<constant>GL_INVALID_ENUM</constant> is generated if <parameter>sfail</parameter>,
<parameter>dpfail</parameter>, or <parameter>dppass</parameter> is any value other than the eight defined constant values.
</para>
</refsect1>
<refsect1 xml:id="associatedgets"><title>Associated Gets</title>
<para>
<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument
<constant>GL_STENCIL_FAIL</constant>, <constant>GL_STENCIL_PASS_DEPTH_PASS</constant>,
<constant>GL_STENCIL_PASS_DEPTH_FAIL</constant>, <constant>GL_STENCIL_BACK_FAIL</constant>,
<constant>GL_STENCIL_BACK_PASS_DEPTH_PASS</constant>, <constant>GL_STENCIL_BACK_PASS_DEPTH_FAIL</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="versions"><title>Version Support</title>
<informaltable>
<tgroup cols="13" align="left">
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="apifunchead.xml" xpointer="xpointer(/*/*)"/>
<tbody>
<row>
<entry><function>glStencilOpSeparate</function></entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="apiversion.xml" xpointer="xpointer(/*/*[@role='20']/*)"/>
</row>
</tbody>
</tgroup>
</informaltable>
</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>glStencilFunc</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glStencilMask</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glStencilMaskSeparate</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry>
</para>
</refsect1>
<refsect1 xml:id="Copyright"><title>Copyright</title>
<para>
Copyright <trademark class="copyright"/> 2006 Khronos Group.
This material may be distributed subject to the terms and conditions set forth in
the Open Publication License, v 1.0, 8 June 1999.
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://opencontent.org/openpub/">http://opencontent.org/openpub/</link>.
</para>
</refsect1>
</refentry>
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