[clinton/guile-figl.git] / upstream-doc / man2 / glAlphaFunc.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
              "http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
<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>
Commit	Line	Data
7faf1d71 AW	1	<?xml version="1.0" encoding="UTF-8"?>
	2	<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
	3	"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
	4	<refentry id="glAlphaFunc">
	5	<refmeta>
	6	<refmetainfo>
	7	<copyright>
	8	<year>1991-2006</year>
	9	<holder>Silicon Graphics, Inc.</holder>
	10	</copyright>
	11	</refmetainfo>
	12	<refentrytitle>glAlphaFunc</refentrytitle>
	13	<manvolnum>3G</manvolnum>
	14	</refmeta>
	15	<refnamediv>
	16	<refname>glAlphaFunc</refname>
	17	<refpurpose>specify the alpha test function</refpurpose>
	18	</refnamediv>
	19	<refsynopsisdiv><title>C Specification</title>
	20	<funcsynopsis>
	21	<funcprototype>
	22	<funcdef>void <function>glAlphaFunc</function></funcdef>
	23	<paramdef>GLenum <parameter>func</parameter></paramdef>
	24	<paramdef>GLclampf <parameter>ref</parameter></paramdef>
	25	</funcprototype>
	26	</funcsynopsis>
	27	</refsynopsisdiv>
	28	<refsect1 id="parameters"><title>Parameters</title>
	29	<variablelist>
	30	<varlistentry>
	31	<term><parameter>func</parameter></term>
	32	<listitem>
	33	<para>
	34	Specifies the alpha comparison function.
	35	Symbolic constants
	36	<constant>GL_NEVER</constant>,
	37	<constant>GL_LESS</constant>,
	38	<constant>GL_EQUAL</constant>,
	39	<constant>GL_LEQUAL</constant>,
	40	<constant>GL_GREATER</constant>,
	41	<constant>GL_NOTEQUAL</constant>,
	42	<constant>GL_GEQUAL</constant>, and
	43	<constant>GL_ALWAYS</constant> are accepted. The initial value is <constant>GL_ALWAYS</constant>.
	44	</para>
	45	</listitem>
	46	</varlistentry>
	47	<varlistentry>
	48	<term><parameter>ref</parameter></term>
	49	<listitem>
	50	<para>
	51	Specifies the reference value that incoming alpha values are compared to.
	52	This value is clamped to the range
	53	<inlineequation><mml:math>
	54	<!-- eqn: [0,1]:-->
	55	<mml:mfenced open="[" close="]">
	56	<mml:mn>0</mml:mn>
	57	<mml:mn>1</mml:mn>
	58	</mml:mfenced>
	59	</mml:math></inlineequation>,
	60	where 0 represents the lowest possible alpha value
	61	and 1 the highest possible value.
	62	The initial reference value is 0.
	63	</para>
	64	</listitem>
65	</varlistentry>
66	</variablelist>
67	</refsect1>
68	<refsect1 id="description"><title>Description</title>
69	<para>
70	The alpha test discards fragments depending on the outcome of a comparison
71	between an incoming fragment's alpha value and a constant reference value.
72	<function>glAlphaFunc</function> specifies the reference value and the comparison function.
73	The comparison is performed only if alpha testing is enabled. By
74	default, it is not enabled.
75	(See
76	<citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry> of <constant>GL_ALPHA_TEST</constant>.)
77	</para>
78	<para>
79	<parameter>func</parameter> and <parameter>ref</parameter> specify the conditions under which
80	the pixel is drawn.
81	The incoming alpha value is compared to <parameter>ref</parameter>
82	using the function specified by <parameter>func</parameter>.
83	If the value passes the comparison,
84	the incoming fragment is drawn
85	if it also passes subsequent stencil and depth buffer tests.
86	If the value fails the comparison,
87	no change is made to the frame buffer at that pixel location. The
88	comparison functions are as follows:
89	</para>
90	<variablelist>
91	<varlistentry>
92	<term><constant>GL_NEVER</constant></term>
93	<listitem>
94	<para>
95	Never passes.
96	</para>
97	</listitem>
98	</varlistentry>
99	<varlistentry>
100	<term><constant>GL_LESS</constant></term>
101	<listitem>
102	<para>
103	Passes if the incoming alpha value is less than the reference value.
104	</para>
105	</listitem>
106	</varlistentry>
107	<varlistentry>
108	<term><constant>GL_EQUAL</constant></term>
109	<listitem>
110	<para>
111	Passes if the incoming alpha value is equal to the reference value.
112	</para>
113	</listitem>
114	</varlistentry>
115	<varlistentry>
116	<term><constant>GL_LEQUAL</constant></term>
117	<listitem>
118	<para>
119	Passes if the incoming alpha value is less than or equal to the reference value.
120	</para>
121	</listitem>
122	</varlistentry>
123	<varlistentry>
124	<term><constant>GL_GREATER</constant></term>
125	<listitem>
126	<para>
127	Passes if the incoming alpha value is greater than the reference value.
128	</para>
129	</listitem>
130	</varlistentry>
131	<varlistentry>
132	<term><constant>GL_NOTEQUAL</constant></term>
133	<listitem>
134	<para>
135	Passes if the incoming alpha value is not equal to the reference value.
136	</para>
137	</listitem>
138	</varlistentry>
139	<varlistentry>
140	<term><constant>GL_GEQUAL</constant></term>
141	<listitem>
142	<para>
143	Passes if the incoming alpha value is greater than or equal to
144	the reference value.
145	</para>
146	</listitem>
147	</varlistentry>
148	<varlistentry>
149	<term><constant>GL_ALWAYS</constant></term>
150	<listitem>
151	<para>
152	Always passes (initial value).
153	</para>
154	</listitem>
155	</varlistentry>
156	</variablelist>
157	<para>
158	<function>glAlphaFunc</function> operates on all pixel write operations,
159	including those resulting from the scan conversion of points,
160	lines,
161	polygons,
162	and bitmaps,
163	and from pixel draw and copy operations.
164	<function>glAlphaFunc</function> does not affect screen clear operations.
165	</para>
166	</refsect1>
167	<refsect1 id="notes"><title>Notes</title>
168	<para>
169	Alpha testing is performed only in RGBA mode.
170	</para>
171	</refsect1>
172	<refsect1 id="errors"><title>Errors</title>
173	<para>
174	<constant>GL_INVALID_ENUM</constant> is generated if <parameter>func</parameter> is not an accepted value.
175	</para>
176	<para>
177	<constant>GL_INVALID_OPERATION</constant> is generated if <function>glAlphaFunc</function>
178	is executed between the execution of <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
179	and the corresponding execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
180	</para>
181	</refsect1>
182	<refsect1 id="associatedgets"><title>Associated Gets</title>
183	<para>
184	<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ALPHA_TEST_FUNC</constant>
185	</para>
186	<para>
187	<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ALPHA_TEST_REF</constant>
188	</para>
189	<para>
190	<citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_ALPHA_TEST</constant>
191	</para>
192	</refsect1>
193	<refsect1 id="seealso"><title>See Also</title>
194	<para>
195	<citerefentry><refentrytitle>glBlendFunc</refentrytitle></citerefentry>,
196	<citerefentry><refentrytitle>glClear</refentrytitle></citerefentry>,
197	<citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>,
198	<citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>,
199	<citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry>
200	</para>
201	</refsect1>
202	<refsect1 id="Copyright"><title>Copyright</title>
203	<para>
204	Copyright <trademark class="copyright"></trademark> 1991-2006
205	Silicon Graphics, Inc. This document is licensed under the SGI
206	Free Software B License. For details, see
207	<ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
208	</para>
209	</refsect1>
210	</refentry>