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<refentry id="glReadPixels">
    <refmeta>
        <refmetainfo>
            <copyright>
                <year>1991-2006</year>
                <holder>Silicon Graphics, Inc.</holder>
            </copyright>
        </refmetainfo>
        <refentrytitle>glReadPixels</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glReadPixels</refname>
        <refpurpose>read a block of pixels from the frame buffer</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glReadPixels</function></funcdef>
                <paramdef>GLint <parameter>x</parameter></paramdef>
                <paramdef>GLint <parameter>y</parameter></paramdef>
                <paramdef>GLsizei <parameter>width</parameter></paramdef>
                <paramdef>GLsizei <parameter>height</parameter></paramdef>
                <paramdef>GLenum <parameter>format</parameter></paramdef>
                <paramdef>GLenum <parameter>type</parameter></paramdef>
                <paramdef>GLvoid * <parameter>data</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <!-- eqn: ignoring delim $$ -->
    <refsect1 id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>x</parameter></term>
            <term><parameter>y</parameter></term>
            <listitem>
                <para>
                    Specify the window coordinates of the first pixel
                    that is read from the frame buffer.
                    This location is the lower left corner of a rectangular block of pixels.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>width</parameter></term>
            <term><parameter>height</parameter></term>
            <listitem>
                <para>
                    Specify the dimensions of the pixel rectangle.
                    <parameter>width</parameter> and <parameter>height</parameter> of one correspond to a single pixel.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>format</parameter></term>
            <listitem>
                <para>
                    Specifies the format of the pixel data.
                    The following symbolic values are accepted:
                    <constant>GL_COLOR_INDEX</constant>,
                    <constant>GL_STENCIL_INDEX</constant>,
                    <constant>GL_DEPTH_COMPONENT</constant>,
                    <constant>GL_RED</constant>,
                    <constant>GL_GREEN</constant>,
                    <constant>GL_BLUE</constant>,
                    <constant>GL_ALPHA</constant>,
                    <constant>GL_RGB</constant>,
                    <constant>GL_BGR</constant>,
                    <constant>GL_RGBA</constant>,
                    <constant>GL_BGRA</constant>,
                    <constant>GL_LUMINANCE</constant>, and
                    <constant>GL_LUMINANCE_ALPHA</constant>.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>type</parameter></term>
            <listitem>
                <para>
                    Specifies the data type of the pixel data.
                    Must be one of
                    <constant>GL_UNSIGNED_BYTE</constant>,
                    <constant>GL_BYTE</constant>,
                    <constant>GL_BITMAP</constant>,
                    <constant>GL_UNSIGNED_SHORT</constant>,
                    <constant>GL_SHORT</constant>,
                    <constant>GL_UNSIGNED_INT</constant>,
                    <constant>GL_INT</constant>,
                    <constant>GL_FLOAT</constant>,
                    <constant>GL_UNSIGNED_BYTE_3_3_2</constant>,
                    <constant>GL_UNSIGNED_BYTE_2_3_3_REV</constant>,
                    <constant>GL_UNSIGNED_SHORT_5_6_5</constant>,
                    <constant>GL_UNSIGNED_SHORT_5_6_5_REV</constant>,
                    <constant>GL_UNSIGNED_SHORT_4_4_4_4</constant>,
                    <constant>GL_UNSIGNED_SHORT_4_4_4_4_REV</constant>,
                    <constant>GL_UNSIGNED_SHORT_5_5_5_1</constant>,
                    <constant>GL_UNSIGNED_SHORT_1_5_5_5_REV</constant>,
                    <constant>GL_UNSIGNED_INT_8_8_8_8</constant>,
                    <constant>GL_UNSIGNED_INT_8_8_8_8_REV</constant>,
                    <constant>GL_UNSIGNED_INT_10_10_10_2</constant>, or
                    <constant>GL_UNSIGNED_INT_2_10_10_10_REV</constant>.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>data</parameter></term>
            <listitem>
                <para>
                    Returns the pixel data.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 id="description"><title>Description</title>
        <para>
            <function>glReadPixels</function> returns pixel data from the frame buffer,
            starting with the pixel whose lower left corner
            is at location (<parameter>x</parameter>, <parameter>y</parameter>),
            into client memory starting at location <parameter>data</parameter>.
            Several parameters control the processing of the pixel data before
            it is placed into client memory.
            These parameters are set with three commands:
            <citerefentry><refentrytitle>glPixelStore</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glPixelTransfer</refentrytitle></citerefentry>, and
            <citerefentry><refentrytitle>glPixelMap</refentrytitle></citerefentry>.
            This reference page describes the effects on <function>glReadPixels</function> of most,
            but not all of the parameters specified by these three commands.
        </para>
        <para>
            If a non-zero named buffer object is bound to the <constant>GL_PIXEL_PACK_BUFFER</constant> target 
            (see <citerefentry><refentrytitle>glBindBuffer</refentrytitle></citerefentry>) while a block of pixels is
            requested, <parameter>data</parameter> is treated as a byte offset into the buffer object's data store
            rather than a pointer to client memory.
        </para>
        <para>
            When the <code>ARB_imaging</code> extension is supported, the pixel data may
            be processed by additional operations including color table lookup,
            color matrix transformations, convolutions, histograms, and minimum and
            maximum pixel value computations.
        </para>
        <para>
            <function>glReadPixels</function> returns values from each pixel with lower left corner at
            <inlineequation><mml:math>
                <!-- eqn: (x + i, y + j):-->
                <mml:mfenced open="(" close=")">
                    <mml:mrow>
                        <mml:mi mathvariant="italic">x</mml:mi>
                        <mml:mo>+</mml:mo>
                        <mml:mi mathvariant="italic">i</mml:mi>
                    </mml:mrow>
                    <mml:mrow>
                        <mml:mi mathvariant="italic">y</mml:mi>
                        <mml:mo>+</mml:mo>
                        <mml:mi mathvariant="italic">j</mml:mi>
                    </mml:mrow>
                </mml:mfenced>
            </mml:math></inlineequation>
            for 
            <inlineequation><mml:math>
                <!-- eqn: 0 <= i < width:-->
                <mml:mrow>
                    <mml:mn>0</mml:mn>
                    <mml:mo>&lt;=</mml:mo>
                    <mml:mi mathvariant="italic">i</mml:mi>
                    <mml:mo>&lt;</mml:mo>
                    <mml:mi mathvariant="italic">width</mml:mi>
                </mml:mrow>
            </mml:math></inlineequation>
            and
            <inlineequation><mml:math>
                <!-- eqn: 0 <= j < height:-->
                <mml:mrow>
                    <mml:mn>0</mml:mn>
                    <mml:mo>&lt;=</mml:mo>
                    <mml:mi mathvariant="italic">j</mml:mi>
                    <mml:mo>&lt;</mml:mo>
                    <mml:mi mathvariant="italic">height</mml:mi>
                </mml:mrow>
            </mml:math></inlineequation>.
            This pixel is said to be the 
            <inlineequation><mml:math><mml:mi mathvariant="italic">i</mml:mi></mml:math></inlineequation>th
            pixel in the 
            <inlineequation><mml:math><mml:mi mathvariant="italic">j</mml:mi></mml:math></inlineequation>th
            row.
            Pixels are returned in row order from the lowest to the highest row,
            left to right in each row.
        </para>
        <para>
            <parameter>format</parameter> specifies the format for the returned pixel values;
            accepted values are:
        </para>
        <variablelist>
            <varlistentry>
                <term><constant>GL_COLOR_INDEX</constant></term>
                <listitem>
                    <para>
                        Color indices are read from the color buffer
                        selected by <citerefentry><refentrytitle>glReadBuffer</refentrytitle></citerefentry>.
                        Each index is converted to fixed point,
                        shifted left or right depending on the value and sign of <constant>GL_INDEX_SHIFT</constant>,
                        and added to <constant>GL_INDEX_OFFSET</constant>.
                        If <constant>GL_MAP_COLOR</constant> is <constant>GL_TRUE</constant>,
                        indices are replaced by their mappings in the table <constant>GL_PIXEL_MAP_I_TO_I</constant>.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_STENCIL_INDEX</constant></term>
                <listitem>
                    <para>
                        Stencil values are read from the stencil buffer.
                        Each index is converted to fixed point,
                        shifted left or right depending on the value and sign of <constant>GL_INDEX_SHIFT</constant>,
                        and added to <constant>GL_INDEX_OFFSET</constant>.
                        If <constant>GL_MAP_STENCIL</constant> is <constant>GL_TRUE</constant>,
                        indices are replaced by their mappings in the table <constant>GL_PIXEL_MAP_S_TO_S</constant>.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_DEPTH_COMPONENT</constant></term>
                <listitem>
                    <para>
                        Depth values are read from the depth buffer.
                        Each component is converted to floating point such that the minimum depth
                        value maps to 0 and the maximum value maps to 1.
                        Each component is then multiplied by <constant>GL_DEPTH_SCALE</constant>,
                        added to <constant>GL_DEPTH_BIAS</constant>,
                        and finally 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>.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_RED</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_GREEN</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_BLUE</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_ALPHA</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_RGB</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_BGR</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_RGBA</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_BGRA</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_LUMINANCE</constant></term>
                <listitem>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GL_LUMINANCE_ALPHA</constant></term>
                <listitem>
                    <para>
                        Processing differs depending on whether color buffers store color indices
                        or RGBA color components.
                        If color indices are stored,
                        they are read from the color buffer selected by <citerefentry><refentrytitle>glReadBuffer</refentrytitle></citerefentry>.
                        Each index is converted to fixed point,
                        shifted left or right depending on the value and sign of <constant>GL_INDEX_SHIFT</constant>,
                        and added to <constant>GL_INDEX_OFFSET</constant>.
                        Indices are then replaced by the red,
                        green,
                        blue,
                        and alpha values obtained by indexing the tables
                        <constant>GL_PIXEL_MAP_I_TO_R</constant>,
                        <constant>GL_PIXEL_MAP_I_TO_G</constant>,
                        <constant>GL_PIXEL_MAP_I_TO_B</constant>, and
                        <constant>GL_PIXEL_MAP_I_TO_A</constant>.
                        Each table must be of size 
                        <inlineequation><mml:math>
                            <!-- eqn: 2 sup n:-->
                            <mml:msup><mml:mn>2</mml:mn>
                            <mml:mi mathvariant="italic">n</mml:mi>
                            </mml:msup>
                        </mml:math></inlineequation>,
                        but 
                        <inlineequation><mml:math><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation>
                        may be different for
                        different tables.
                        Before an index is used to look up a value in a table of
                        size 
                        <inlineequation><mml:math>
                            <!-- eqn: 2 sup n:-->
                            <mml:msup><mml:mn>2</mml:mn>
                            <mml:mi mathvariant="italic">n</mml:mi>
                            </mml:msup>
                        </mml:math></inlineequation>,
                        it must be masked against 
                        <inlineequation><mml:math>
                            <!-- 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>.
                    </para>
                    <para>
                        If RGBA color components are stored in the color buffers,
                        they are read from the color buffer selected by <citerefentry><refentrytitle>glReadBuffer</refentrytitle></citerefentry>.
                        Each color component is converted to floating point such that zero intensity
                        maps to 0.0 and full intensity maps to 1.0.
                        Each component is then multiplied by <constant>GL_c_SCALE</constant> and
                        added to <constant>GL_c_BIAS</constant>,
                        where <emphasis>c</emphasis> is RED, GREEN, BLUE, or ALPHA.
                        Finally,
                        if <constant>GL_MAP_COLOR</constant> is <constant>GL_TRUE</constant>,
                        each component 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>,
                        scaled to the size of its corresponding table, and is then
                        replaced by its mapping in the table
                        <constant>GL_PIXEL_MAP_c_TO_c</constant>,
                        where <emphasis>c</emphasis> is R, G, B, or A.
                    </para>
                    <para>
                        Unneeded data is then discarded.
                        For example,
                        <constant>GL_RED</constant> discards the green, blue, and alpha components,
                        while <constant>GL_RGB</constant> discards only the alpha component.
                        <constant>GL_LUMINANCE</constant> computes a single-component value as the sum of
                        the red,
                        green,
                        and blue components,
                        and <constant>GL_LUMINANCE_ALPHA</constant> does the same,
                        while keeping alpha as a second value.
                        The final values are 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>.
                    </para>
                </listitem>
            </varlistentry>
        </variablelist>
        <para>
            The shift,
            scale,
            bias,
            and lookup factors just described are all specified by
            <citerefentry><refentrytitle>glPixelTransfer</refentrytitle></citerefentry>.
            The lookup table contents themselves are specified by <citerefentry><refentrytitle>glPixelMap</refentrytitle></citerefentry>.
        </para>
        <para>
            Finally, the indices or components
            are converted to the proper format,
            as specified by <parameter>type</parameter>.
            If <parameter>format</parameter> is <constant>GL_COLOR_INDEX</constant> or <constant>GL_STENCIL_INDEX</constant>
            and <parameter>type</parameter> is not <constant>GL_FLOAT</constant>,
            each index is masked with the mask value given in the following table.
            If <parameter>type</parameter> is <constant>GL_FLOAT</constant>, then each integer index is converted to
            single-precision floating-point format.
        </para>
        <para>
            If <parameter>format</parameter> is
            <constant>GL_RED</constant>,
            <constant>GL_GREEN</constant>,
            <constant>GL_BLUE</constant>,
            <constant>GL_ALPHA</constant>,
            <constant>GL_RGB</constant>,
            <constant>GL_BGR</constant>,
            <constant>GL_RGBA</constant>,
            <constant>GL_BGRA</constant>,
            <constant>GL_LUMINANCE</constant>, or
            <constant>GL_LUMINANCE_ALPHA</constant> and <parameter>type</parameter> is not <constant>GL_FLOAT</constant>,
            each component is multiplied by the multiplier shown in the following table.
            If type is <constant>GL_FLOAT</constant>, then each component is passed as is
            (or converted to the client's single-precision floating-point format if
            it is different from the one used by the GL).
        </para>
        <para>
        </para>
        <informaltable frame="topbot">
            <tgroup cols="3" align="left">
                <colspec/>
                <colspec align="center"/>
                <colspec align="center"/>
                <thead>
                    <row>
                        <entry rowsep="1" align="left">
                        <parameter>type</parameter>
                        </entry>
                        <entry rowsep="1" align="center"><emphasis role="bold">
                        Index Mask
                        </emphasis></entry>
                        <entry rowsep="1" align="center"><emphasis role="bold">
                        Component Conversion
                        </emphasis></entry>
                    </row>
                </thead>
                <tbody>
                    <row>
                        <entry align="left">
                        <constant>GL_UNSIGNED_BYTE</constant>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 2 sup 8 - 1:-->
                            <mml:mrow>
                                <mml:msup><mml:mn>2</mml:mn>
                                <mml:mn>8</mml:mn>
                                </mml:msup>
                                <mml:mo>-</mml:mo>
                                <mml:mn>1</mml:mn>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: (2 sup 8 - 1) c:-->
                            <mml:mrow>
                                <mml:mfenced open="(" close=")">
                                    <mml:mrow>
                                        <mml:msup><mml:mn>2</mml:mn>
                                        <mml:mn>8</mml:mn>
                                        </mml:msup>
                                        <mml:mo>-</mml:mo>
                                        <mml:mn>1</mml:mn>
                                    </mml:mrow>
                                </mml:mfenced>
                                <mml:mo>&it;</mml:mo>
                                <mml:mi mathvariant="italic">c</mml:mi>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_BYTE</constant>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 2 sup 7 - 1:-->
                            <mml:mrow>
                                <mml:msup><mml:mn>2</mml:mn>
                                <mml:mn>7</mml:mn>
                                </mml:msup>
                                <mml:mo>-</mml:mo>
                                <mml:mn>1</mml:mn>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: {(2 sup 8 - 1) c - 1} / 2:-->
                            <mml:mfrac>
                                <mml:mfenced open="" close="">
                                    <mml:mrow>
                                        <mml:mfenced open="(" close=")">
                                            <mml:mrow>
                                                <mml:msup><mml:mn>2</mml:mn>
                                                <mml:mn>8</mml:mn>
                                                </mml:msup>
                                                <mml:mo>-</mml:mo>
                                                <mml:mn>1</mml:mn>
                                            </mml:mrow>
                                        </mml:mfenced>
                                        <mml:mo>&it;</mml:mo>
                                        <mml:mi mathvariant="italic">c</mml:mi>
                                        <mml:mo>-</mml:mo>
                                        <mml:mn>1</mml:mn>
                                    </mml:mrow>
                                </mml:mfenced>
                                <mml:mn>2</mml:mn>
                            </mml:mfrac>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_BITMAP</constant>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 1:-->
                            <mml:mn>1</mml:mn>
                        </mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 1:-->
                            <mml:mn>1</mml:mn>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_UNSIGNED_SHORT</constant>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 2 sup 16 - 1:-->
                            <mml:mrow>
                                <mml:msup><mml:mn>2</mml:mn>
                                <mml:mn>16</mml:mn>
                                </mml:msup>
                                <mml:mo>-</mml:mo>
                                <mml:mn>1</mml:mn>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: (2 sup 16 - 1) c:-->
                            <mml:mrow>
                                <mml:mfenced open="(" close=")">
                                    <mml:mrow>
                                        <mml:msup><mml:mn>2</mml:mn>
                                        <mml:mn>16</mml:mn>
                                        </mml:msup>
                                        <mml:mo>-</mml:mo>
                                        <mml:mn>1</mml:mn>
                                    </mml:mrow>
                                </mml:mfenced>
                                <mml:mo>&it;</mml:mo>
                                <mml:mi mathvariant="italic">c</mml:mi>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_SHORT</constant>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 2 sup 15 - 1:-->
                            <mml:mrow>
                                <mml:msup><mml:mn>2</mml:mn>
                                <mml:mn>15</mml:mn>
                                </mml:msup>
                                <mml:mo>-</mml:mo>
                                <mml:mn>1</mml:mn>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: {(2 sup 16 - 1) c - 1} / 2:-->
                            <mml:mfrac>
                                <mml:mfenced open="" close="">
                                    <mml:mrow>
                                        <mml:mfenced open="(" close=")">
                                            <mml:mrow>
                                                <mml:msup><mml:mn>2</mml:mn>
                                                <mml:mn>16</mml:mn>
                                                </mml:msup>
                                                <mml:mo>-</mml:mo>
                                                <mml:mn>1</mml:mn>
                                            </mml:mrow>
                                        </mml:mfenced>
                                        <mml:mo>&it;</mml:mo>
                                        <mml:mi mathvariant="italic">c</mml:mi>
                                        <mml:mo>-</mml:mo>
                                        <mml:mn>1</mml:mn>
                                    </mml:mrow>
                                </mml:mfenced>
                                <mml:mn>2</mml:mn>
                            </mml:mfrac>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_UNSIGNED_INT</constant>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 2 sup 32 - 1:-->
                            <mml:mrow>
                                <mml:msup><mml:mn>2</mml:mn>
                                <mml:mn>32</mml:mn>
                                </mml:msup>
                                <mml:mo>-</mml:mo>
                                <mml:mn>1</mml:mn>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: (2 sup 32 - 1) c:-->
                            <mml:mrow>
                                <mml:mfenced open="(" close=")">
                                    <mml:mrow>
                                        <mml:msup><mml:mn>2</mml:mn>
                                        <mml:mn>32</mml:mn>
                                        </mml:msup>
                                        <mml:mo>-</mml:mo>
                                        <mml:mn>1</mml:mn>
                                    </mml:mrow>
                                </mml:mfenced>
                                <mml:mo>&it;</mml:mo>
                                <mml:mi mathvariant="italic">c</mml:mi>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_INT</constant>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 2 sup 31 - 1:-->
                            <mml:mrow>
                                <mml:msup><mml:mn>2</mml:mn>
                                <mml:mn>31</mml:mn>
                                </mml:msup>
                                <mml:mo>-</mml:mo>
                                <mml:mn>1</mml:mn>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: {(2 sup 32 - 1) c - 1} / 2:-->
                            <mml:mfrac>
                                <mml:mfenced open="" close="">
                                    <mml:mrow>
                                        <mml:mfenced open="(" close=")">
                                            <mml:mrow>
                                                <mml:msup><mml:mn>2</mml:mn>
                                                <mml:mn>32</mml:mn>
                                                </mml:msup>
                                                <mml:mo>-</mml:mo>
                                                <mml:mn>1</mml:mn>
                                            </mml:mrow>
                                        </mml:mfenced>
                                        <mml:mo>&it;</mml:mo>
                                        <mml:mi mathvariant="italic">c</mml:mi>
                                        <mml:mo>-</mml:mo>
                                        <mml:mn>1</mml:mn>
                                    </mml:mrow>
                                </mml:mfenced>
                                <mml:mn>2</mml:mn>
                            </mml:mfrac>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_FLOAT</constant>
                        </entry>
                        <entry align="center">
                        none
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math><mml:mi mathvariant="italic">c</mml:mi></mml:math></inlineequation>
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
        <para>
            Return values are placed in memory as follows.
            If <parameter>format</parameter> is
            <constant>GL_COLOR_INDEX</constant>,
            <constant>GL_STENCIL_INDEX</constant>,
            <constant>GL_DEPTH_COMPONENT</constant>,
            <constant>GL_RED</constant>,
            <constant>GL_GREEN</constant>,
            <constant>GL_BLUE</constant>,
            <constant>GL_ALPHA</constant>, or
            <constant>GL_LUMINANCE</constant>,
            a single value is returned and the data for the 
            <inlineequation><mml:math><mml:mi mathvariant="italic">i</mml:mi></mml:math></inlineequation>th
            pixel in the 
            <inlineequation><mml:math><mml:mi mathvariant="italic">j</mml:mi></mml:math></inlineequation>th
            row
            is placed in location 
            <inlineequation><mml:math>
                <!-- eqn: (j) width + i:-->
                <mml:mrow>
                    <mml:mfenced open="(" close=")">
                        <mml:mi mathvariant="italic">j</mml:mi>
                    </mml:mfenced>
                    <mml:mo>&it;</mml:mo>
                    <mml:mi mathvariant="italic">width</mml:mi>
                    <mml:mo>+</mml:mo>
                    <mml:mi mathvariant="italic">i</mml:mi>
                </mml:mrow>
            </mml:math></inlineequation>.
            <constant>GL_RGB</constant> and <constant>GL_BGR</constant> return three values,
            <constant>GL_RGBA</constant> and <constant>GL_BGRA</constant> return four values,
            and <constant>GL_LUMINANCE_ALPHA</constant> returns two values for each pixel,
            with all values corresponding to a single pixel occupying contiguous space
            in <parameter>data</parameter>.
            Storage parameters set by <citerefentry><refentrytitle>glPixelStore</refentrytitle></citerefentry>,
            such as <constant>GL_PACK_LSB_FIRST</constant> and <constant>GL_PACK_SWAP_BYTES</constant>,
            affect the way that data is written into memory.
            See <citerefentry><refentrytitle>glPixelStore</refentrytitle></citerefentry> for a description.
        </para>
    </refsect1>
    <refsect1 id="notes"><title>Notes</title>
        <para>
            Values for pixels that lie outside the window
            connected to the current GL context are undefined.
        </para>
        <para>
            If an error is generated,
            no change is made to the contents of <parameter>data</parameter>.
        </para>
    </refsect1>
    <refsect1 id="errors"><title>Errors</title>
        <para>
            <constant>GL_INVALID_ENUM</constant> is generated if <parameter>format</parameter> or <parameter>type</parameter> is not an
            accepted value.
        </para>
        <para>
            <constant>GL_INVALID_ENUM</constant> is generated if <parameter>type</parameter> is <constant>GL_BITMAP</constant> and <parameter>format</parameter> is
            not <constant>GL_COLOR_INDEX</constant> or <constant>GL_STENCIL_INDEX</constant>.
        </para>
        <para>
            <constant>GL_INVALID_VALUE</constant> is generated if either <parameter>width</parameter> or <parameter>height</parameter> is negative.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <parameter>format</parameter> is <constant>GL_COLOR_INDEX</constant>
            and the color buffers store RGBA color components.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <parameter>format</parameter> is <constant>GL_STENCIL_INDEX</constant>
            and there is no stencil buffer.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <parameter>format</parameter> is <constant>GL_DEPTH_COMPONENT</constant>
            and there is no depth buffer.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <parameter>type</parameter> is one of
            <constant>GL_UNSIGNED_BYTE_3_3_2</constant>,
            <constant>GL_UNSIGNED_BYTE_2_3_3_REV</constant>,
            <constant>GL_UNSIGNED_SHORT_5_6_5</constant>, or
            <constant>GL_UNSIGNED_SHORT_5_6_5_REV</constant>
            and <parameter>format</parameter> is not <constant>GL_RGB</constant>.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <parameter>type</parameter> is one of
            <constant>GL_UNSIGNED_SHORT_4_4_4_4</constant>,
            <constant>GL_UNSIGNED_SHORT_4_4_4_4_REV</constant>,
            <constant>GL_UNSIGNED_SHORT_5_5_5_1</constant>,
            <constant>GL_UNSIGNED_SHORT_1_5_5_5_REV</constant>,
            <constant>GL_UNSIGNED_INT_8_8_8_8</constant>,
            <constant>GL_UNSIGNED_INT_8_8_8_8_REV</constant>,
            <constant>GL_UNSIGNED_INT_10_10_10_2</constant>, or
            <constant>GL_UNSIGNED_INT_2_10_10_10_REV</constant>
            and <parameter>format</parameter> is neither <constant>GL_RGBA</constant> nor <constant>GL_BGRA</constant>.
        </para>
        <para>
            The formats <constant>GL_BGR</constant>, and <constant>GL_BGRA</constant> and types
            <constant>GL_UNSIGNED_BYTE_3_3_2</constant>,
            <constant>GL_UNSIGNED_BYTE_2_3_3_REV</constant>,
            <constant>GL_UNSIGNED_SHORT_5_6_5</constant>,
            <constant>GL_UNSIGNED_SHORT_5_6_5_REV</constant>,
            <constant>GL_UNSIGNED_SHORT_4_4_4_4</constant>,
            <constant>GL_UNSIGNED_SHORT_4_4_4_4_REV</constant>,
            <constant>GL_UNSIGNED_SHORT_5_5_5_1</constant>,
            <constant>GL_UNSIGNED_SHORT_1_5_5_5_REV</constant>,
            <constant>GL_UNSIGNED_INT_8_8_8_8</constant>,
            <constant>GL_UNSIGNED_INT_8_8_8_8_REV</constant>,
            <constant>GL_UNSIGNED_INT_10_10_10_2</constant>, and
            <constant>GL_UNSIGNED_INT_2_10_10_10_REV</constant> are available only if the GL version
            is 1.2 or greater.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if a non-zero buffer object name is bound to the
            <constant>GL_PIXEL_PACK_BUFFER</constant> target and the buffer object's data store is currently mapped.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if a non-zero buffer object name is bound to the
            <constant>GL_PIXEL_PACK_BUFFER</constant> target and the data would be packed to the buffer 
            object such that the memory writes required would exceed the data store size.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if a non-zero buffer object name is bound to the
            <constant>GL_PIXEL_PACK_BUFFER</constant> target and <parameter>data</parameter> is not evenly divisible 
            into the number of bytes needed to store in memory a datum indicated by <parameter>type</parameter>.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <function>glReadPixels</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_INDEX_MODE</constant>
        </para>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_PIXEL_PACK_BUFFER_BINDING</constant>
        </para>
    </refsect1>
    <refsect1 id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glCopyPixels</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glDrawPixels</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glPixelMap</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glPixelStore</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glPixelTransfer</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glReadBuffer</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>
