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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="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><=</mml:mo>
<mml:mi mathvariant="italic">i</mml:mi>
<mml:mo><</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><=</mml:mo>
<mml:mi mathvariant="italic">j</mml:mi>
<mml:mo><</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>⁢</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>⁢</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>⁢</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>⁢</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>⁢</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>⁢</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>⁢</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>