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<?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="glGetActiveUniform">
<refmeta>
<refentrytitle>glGetActiveUniform</refentrytitle>
<manvolnum>3G</manvolnum>
</refmeta>
<refnamediv>
<refname>glGetActiveUniform</refname>
<refpurpose>Returns information about an active uniform variable for the specified program object</refpurpose>
</refnamediv>
<refsynopsisdiv><title>C Specification</title>
<funcsynopsis>
<funcprototype>
<funcdef>void <function>glGetActiveUniform</function></funcdef>
<paramdef>GLuint <parameter>program</parameter></paramdef>
<paramdef>GLuint <parameter>index</parameter></paramdef>
<paramdef>GLsizei <parameter>bufSize</parameter></paramdef>
<paramdef>GLsizei *<parameter>length</parameter></paramdef>
<paramdef>GLint *<parameter>size</parameter></paramdef>
<paramdef>GLenum *<parameter>type</parameter></paramdef>
<paramdef>GLchar *<parameter>name</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 id="parameters"><title>Parameters</title>
<variablelist>
<varlistentry>
<term><parameter>program</parameter></term>
<listitem>
<para>Specifies the program object to be
queried.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>index</parameter></term>
<listitem>
<para>Specifies the index of the uniform variable to
be queried.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>bufSize</parameter></term>
<listitem>
<para>Specifies the maximum number of characters
OpenGL is allowed to write in the character buffer
indicated by <parameter>name</parameter>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>length</parameter></term>
<listitem>
<para>Returns the number of characters actually
written by OpenGL in the string indicated by
<parameter>name</parameter> (excluding the null
terminator) if a value other than
<constant>NULL</constant> is passed.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>size</parameter></term>
<listitem>
<para>Returns the size of the uniform
variable.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>type</parameter></term>
<listitem>
<para>Returns the data type of the uniform
variable.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>name</parameter></term>
<listitem>
<para>Returns a null terminated string containing
the name of the uniform variable.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id="description"><title>Description</title>
<para><function>glGetActiveUniform</function> returns
information about an active uniform variable in the program
object specified by <parameter>program</parameter>. The number
of active uniform variables can be obtained by calling
<citerefentry><refentrytitle>glGetProgram</refentrytitle></citerefentry>
with the value <constant>GL_ACTIVE_UNIFORMS</constant>. A value
of 0 for <parameter>index</parameter> selects the first active
uniform variable. Permissible values for
<parameter>index</parameter> range from 0 to the number of
active uniform variables minus 1.</para>
<para>Shaders may use either built-in uniform variables,
user-defined uniform variables, or both. Built-in uniform
variables have a prefix of "gl_" and reference
existing OpenGL state or values derived from such state (e.g.,
<parameter>gl_Fog</parameter>,
<parameter>gl_ModelViewMatrix</parameter>, etc., see the OpenGL
Shading Language specification for a complete list.)
User-defined uniform variables have arbitrary names and obtain
their values from the application through calls to
<citerefentry><refentrytitle>glUniform</refentrytitle></citerefentry>.
A uniform variable (either built-in or user-defined) is
considered active if it is determined during the link operation
that it may be accessed during program execution. Therefore,
<parameter>program</parameter> should have previously been the
target of a call to
<citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry>,
but it is not necessary for it to have been linked
successfully.</para>
<para>The size of the character buffer required to store the
longest uniform variable name in <parameter>program</parameter>
can be obtained by calling
<citerefentry><refentrytitle>glGetProgram</refentrytitle></citerefentry>
with the value
<constant>GL_ACTIVE_UNIFORM_MAX_LENGTH</constant>. This value
should be used to allocate a buffer of sufficient size to store
the returned uniform variable name. The size of this character
buffer is passed in <parameter>bufSize</parameter>, and a
pointer to this character buffer is passed in
<parameter>name.</parameter></para>
<para><function>glGetActiveUniform</function> returns the name
of the uniform variable indicated by
<parameter>index</parameter>, storing it in the character buffer
specified by <parameter>name</parameter>. The string returned
will be null terminated. The actual number of characters written
into this buffer is returned in <parameter>length</parameter>,
and this count does not include the null termination character.
If the length of the returned string is not required, a value of
<constant>NULL</constant> can be passed in the
<parameter>length</parameter> argument.</para>
<para>The <parameter>type</parameter>
argument will return a pointer to the uniform variable's data
type. The symbolic constants
<constant>GL_FLOAT</constant>,
<constant>GL_FLOAT_VEC2</constant>,
<constant>GL_FLOAT_VEC3</constant>,
<constant>GL_FLOAT_VEC4</constant>,
<constant>GL_INT</constant>,
<constant>GL_INT_VEC2</constant>,
<constant>GL_INT_VEC3</constant>,
<constant>GL_INT_VEC4</constant>,
<constant>GL_BOOL</constant>,
<constant>GL_BOOL_VEC2</constant>,
<constant>GL_BOOL_VEC3</constant>,
<constant>GL_BOOL_VEC4</constant>,
<constant>GL_FLOAT_MAT2</constant>,
<constant>GL_FLOAT_MAT3</constant>,
<constant>GL_FLOAT_MAT4</constant>,
<constant>GL_FLOAT_MAT2x3</constant>,
<constant>GL_FLOAT_MAT2x4</constant>,
<constant>GL_FLOAT_MAT3x2</constant>,
<constant>GL_FLOAT_MAT3x4</constant>,
<constant>GL_FLOAT_MAT4x2</constant>,
<constant>GL_FLOAT_MAT4x3</constant>,
<constant>GL_SAMPLER_1D</constant>,
<constant>GL_SAMPLER_2D</constant>,
<constant>GL_SAMPLER_3D</constant>,
<constant>GL_SAMPLER_CUBE</constant>,
<constant>GL_SAMPLER_1D_SHADOW</constant>, or
<constant>GL_SAMPLER_2D_SHADOW</constant>
may be returned.</para>
<para>If one or more elements of an array are active, the name
of the array is returned in <parameter>name</parameter>, the
type is returned in <parameter>type</parameter>, and the
<parameter>size</parameter> parameter returns the highest array
element index used, plus one, as determined by the compiler
and/or linker. Only one active uniform variable will be reported
for a uniform array.</para>
<para>Uniform variables that are declared as structures or
arrays of structures will not be returned directly by this
function. Instead, each of these uniform variables will be
reduced to its fundamental components containing the
"." and "[]" operators such that each of the
names is valid as an argument to
<citerefentry><refentrytitle>glGetUniformLocation</refentrytitle></citerefentry>.
Each of these reduced uniform variables is counted as one active
uniform variable and is assigned an index. A valid name cannot
be a structure, an array of structures, or a subcomponent of a
vector or matrix.</para>
<para>The size of the uniform variable will be returned in
<parameter>size</parameter>. Uniform variables other than arrays
will have a size of 1. Structures and arrays of structures will
be reduced as described earlier, such that each of the names
returned will be a data type in the earlier list. If this
reduction results in an array, the size returned will be as
described for uniform arrays; otherwise, the size returned will
be 1.</para>
<para>The list of active uniform variables may include both
built-in uniform variables (which begin with the prefix
"gl_") as well as user-defined uniform variable
names.</para>
<para>This function will return as much information as it can
about the specified active uniform variable. If no information
is available, <parameter>length</parameter> will be 0, and
<parameter>name</parameter> will be an empty string. This
situation could occur if this function is called after a link
operation that failed. If an error occurs, the return values
<parameter>length</parameter>, <parameter>size</parameter>,
<parameter>type</parameter>, and <parameter>name</parameter>
will be unmodified.</para>
</refsect1>
<refsect1 id="notes"><title>Notes</title>
<para><function>glGetActiveUniform</function> is available only
if the GL version is 2.0 or greater.</para>
<para><constant>GL_FLOAT_MAT2x3</constant>,
<constant>GL_FLOAT_MAT2x4</constant>,
<constant>GL_FLOAT_MAT3x2</constant>,
<constant>GL_FLOAT_MAT3x4</constant>,
<constant>GL_FLOAT_MAT4x2</constant>, and
<constant>GL_FLOAT_MAT4x3</constant>
will only be returned as a <parameter>type</parameter>
if the GL version is 2.1 or greater.</para>
</refsect1>
<refsect1 id="errors"><title>Errors</title>
<para><constant>GL_INVALID_VALUE</constant> is generated if
<parameter>program</parameter> is not a value generated by
OpenGL.</para>
<para><constant>GL_INVALID_OPERATION</constant> is generated if
<parameter>program</parameter> is not a program object.</para>
<para><constant>GL_INVALID_VALUE</constant> is generated if
<parameter>index</parameter> is greater than or equal to the
number of active uniform variables in
<parameter>program</parameter>.</para>
<para><constant>GL_INVALID_OPERATION</constant> is generated if
<function>glGetActiveUniform</function> is executed between the
execution of
<citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
and the corresponding execution of
<citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.</para>
<para><constant>GL_INVALID_VALUE</constant> is generated if
<parameter>bufSize</parameter> is less than 0.</para>
</refsect1>
<refsect1 id="associatedgets"><title>Associated Gets</title>
<para><citerefentry><refentrytitle>glGet</refentrytitle></citerefentry>
with argument <constant>GL_MAX_VERTEX_UNIFORM_COMPONENTS
</constant> or
<constant>GL_MAX_FRAGMENT_UNIFORM_COMPONENTS</constant>.</para>
<para><citerefentry><refentrytitle>glGetProgram</refentrytitle></citerefentry>
with argument <constant>GL_ACTIVE_UNIFORMS</constant> or
<constant>GL_ACTIVE_UNIFORM_MAX_LENGTH</constant>.</para>
<para><citerefentry><refentrytitle>glIsProgram</refentrytitle></citerefentry></para>
</refsect1>
<refsect1 id="seealso"><title>See Also</title>
<para><citerefentry><refentrytitle>glGetUniform</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glGetUniformLocation</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glUniform</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glUseProgram</refentrytitle></citerefentry></para>
</refsect1>
<refsect1 id="Copyright"><title>Copyright</title>
<para>
Copyright <trademark class="copyright"></trademark> 2003-2005 3Dlabs Inc. Ltd.
This material may be distributed subject to the terms and conditions set forth in
the Open Publication License, v 1.0, 8 June 1999.
<ulink url="http://opencontent.org/openpub/">http://opencontent.org/openpub/</ulink>.
</para>
</refsect1>
</refentry>