<div class="refentry"><a id="glReadPixels"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>glReadPixels — read a block of pixels from the frame buffer</p></div><div class="refsynopsisdiv"><h2>C Specification</h2><div class="funcsynopsis"><table class="funcprototype-table" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">void <strong class="fsfunc">glReadPixels</strong>(</code></td><td>GLint <var class="pdparam">x</var>, </td></tr><tr><td> </td><td>GLint <var class="pdparam">y</var>, </td></tr><tr><td> </td><td>GLsizei <var class="pdparam">width</var>, </td></tr><tr><td> </td><td>GLsizei <var class="pdparam">height</var>, </td></tr><tr><td> </td><td>GLenum <var class="pdparam">format</var>, </td></tr><tr><td> </td><td>GLenum <var class="pdparam">type</var>, </td></tr><tr><td> </td><td>GLvoid * <var class="pdparam">data</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1"><a id="parameters"></a><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="parameter"><code>x</code></em>, </span><span class="term"><em class="parameter"><code>y</code></em></span></dt><dd><p>

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.

</p></dd><dt><span class="term"><em class="parameter"><code>width</code></em>, </span><span class="term"><em class="parameter"><code>height</code></em></span></dt><dd><p>

Specify the dimensions of the pixel rectangle.

<em class="parameter"><code>width</code></em> and <em class="parameter"><code>height</code></em> of one correspond to a single pixel.

</p></dd><dt><span class="term"><em class="parameter"><code>format</code></em></span></dt><dd><p>

Specifies the format of the pixel data.

The following symbolic values are accepted:

<code class="constant">GL_ALPHA</code>,

<code class="constant">GL_RGB</code>, and

<code class="constant">GL_RGBA</code>.

</p></dd><dt><span class="term"><em class="parameter"><code>type</code></em></span></dt><dd><p>

Specifies the data type of the pixel data.

Must be one of

<code class="constant">GL_UNSIGNED_BYTE</code>,

<code class="constant">GL_UNSIGNED_SHORT_5_6_5</code>,

<code class="constant">GL_UNSIGNED_SHORT_4_4_4_4</code>, or

<code class="constant">GL_UNSIGNED_SHORT_5_5_5_1</code>.

</p></dd><dt><span class="term"><em class="parameter"><code>data</code></em></span></dt><dd><p>

Returns the pixel data.

</p></dd></dl></div></div><div class="refsect1"><a id="description"></a><h2>Description</h2><p>

<code class="function">glReadPixels</code> returns pixel data from the frame buffer,

starting with the pixel whose lower left corner

is at location (<em class="parameter"><code>x</code></em>, <em class="parameter"><code>y</code></em>),

into client memory starting at location <em class="parameter"><code>data</code></em>.

The <code class="constant">GL_PACK_ALIGNMENT</code> parameter, set with the

<a class="citerefentry" href="glPixelStorei"><span class="citerefentry"><span class="refentrytitle">glPixelStorei</span></span></a>

command, affects the processing of the pixel data before it is placed into

client memory.

</p><p>

<code class="function">glReadPixels</code> returns values from each pixel with lower left corner at

<math overflow="scroll">

<mfenced open="(" close=")">

<mrow>

<mi mathvariant="italic">x</mi>

<mo>+</mo>

<mi mathvariant="italic">i</mi>

</mrow>

<mrow>

<mi mathvariant="italic">y</mi>

<mo>+</mo>

<mi mathvariant="italic">j</mi>

</mrow>

</mfenced>

</math>

for

<math overflow="scroll">

<mrow>

<mn>0</mn>

<mo>&lt;=</mo>

<mi mathvariant="italic">i</mi>

<mo>&lt;</mo>

<mi mathvariant="italic">width</mi>

</mrow>

</math>

and

<math overflow="scroll">

<mrow>

<mn>0</mn>

<mo>&lt;=</mo>

<mi mathvariant="italic">j</mi>

<mo>&lt;</mo>

<mi mathvariant="italic">height</mi>

</mrow>

</math>.

This pixel is said to be the

<math overflow="scroll"><mi mathvariant="italic">i</mi></math>th

pixel in the

<math overflow="scroll"><mi mathvariant="italic">j</mi></math>th

row.

Pixels are returned in row order from the lowest to the highest row,

left to right in each row.

</p><p>

<em class="parameter"><code>format</code></em> specifies the format for the returned pixel values;

accepted values are:

</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="constant">GL_ALPHA</code></span></dt><dd></dd><dt><span class="term"><code class="constant">GL_RGB</code></span></dt><dd></dd><dt><span class="term"><code class="constant">GL_RGBA</code></span></dt><dd></dd></dl></div><p>

RGBA color components are read from the color buffer.

Each color component is converted to floating point such that zero intensity

maps to 0.0 and full intensity maps to 1.0.

</p><p>

Unneeded data is then discarded.

For example,

<code class="constant">GL_ALPHA</code> discards the red, green, and blue components,

while <code class="constant">GL_RGB</code> discards only the alpha component.

The final values are clamped to the range

<math overflow="scroll">

<mfenced open="[" close="]">

<mn>0</mn>

<mn>1</mn>

</mfenced>

</math>.

</p><p>

Finally, the components

are converted to the proper format,

as specified by <em class="parameter"><code>type</code></em>.

When <em class="parameter"><code>type</code></em> is <code class="constant">GL_UNSIGNED_BYTE</code>,

each component is multiplied by

<math overflow="scroll">

<mrow>

<msup><mn>2</mn>

<mn>8</mn>

</msup>

<mo>-</mo>

<mn>1</mn>

</mrow>

</math>.

When <em class="parameter"><code>type</code></em> is <code class="constant">GL_UNSIGNED_SHORT_5_6_5</code>,

<code class="constant">GL_UNSIGNED_SHORT_4_4_4_4</code>, or <code class="constant">GL_UNSIGNED_SHORT_5_5_5_1</code>,

each component is multiplied by

<math overflow="scroll">

<mrow>

<msup><mn>2</mn>

<mi>N</mi>

</msup>

<mo>-</mo>

<mn>1</mn>

</mrow>

</math>,

where <math overflow="scroll"><mi mathvariant="italic">N</mi></math>

is the number of bits in the bitfield.

</p><p>

Return values are placed in memory as follows.

If <em class="parameter"><code>format</code></em> is

<code class="constant">GL_ALPHA</code>,

a single value is returned and the data for the

<math overflow="scroll"><mi mathvariant="italic">i</mi></math>th

pixel in the

<math overflow="scroll"><mi mathvariant="italic">j</mi></math>th

row

is placed in location

<math overflow="scroll">

<mrow>

<mfenced open="(" close=")">

<mi mathvariant="italic">j</mi>

</mfenced>

<mo>⁢</mo>

<mi mathvariant="italic">width</mi>

<mo>+</mo>

<mi mathvariant="italic">i</mi>

</mrow>

</math>.

<code class="constant">GL_RGB</code> returns three values and

<code class="constant">GL_RGBA</code> returns four values for each pixel,

with all values corresponding to a single pixel occupying contiguous space

in <em class="parameter"><code>data</code></em>.

Storage parameter <code class="constant">GL_PACK_ALIGNMENT</code>, set by

<a class="citerefentry" href="glPixelStorei"><span class="citerefentry"><span class="refentrytitle">glPixelStorei</span></span></a>,

affects the way that data is written into memory.

See <a class="citerefentry" href="glPixelStorei"><span class="citerefentry"><span class="refentrytitle">glPixelStorei</span></span></a> for a description.

</p></div><div class="refsect1"><a id="notes"></a><h2>Notes</h2><p>

If the currently bound framebuffer is not the default framebuffer object, color

components are read from the color image attached to the

<code class="constant">GL_COLOR_ATTACHMENT0</code> attachment point.

</p><p>

Only two <em class="parameter"><code>format</code></em>/<em class="parameter"><code>type</code></em> parameter pairs are

accepted. <code class="constant">GL_RGBA</code>/<code class="constant">GL_UNSIGNED_BYTE</code> is

always accepted, and the other acceptable pair can be discovered by querying

<code class="constant">GL_IMPLEMENTATION_COLOR_READ_FORMAT</code> and

<code class="constant">GL_IMPLEMENTATION_COLOR_READ_TYPE</code>.

</p><p>

Values for pixels that lie outside the window

connected to the current GL context are undefined.

</p><p>

If an error is generated,

no change is made to the contents of <em class="parameter"><code>data</code></em>.

</p></div><div class="refsect1"><a id="errors"></a><h2>Errors</h2><p>

<code class="constant">GL_INVALID_ENUM</code> is generated if <em class="parameter"><code>format</code></em> or <em class="parameter"><code>type</code></em> is not an

accepted value.

</p><p>

<code class="constant">GL_INVALID_VALUE</code> is generated if either <em class="parameter"><code>width</code></em> or <em class="parameter"><code>height</code></em> is negative.

</p><p>

<code class="constant">GL_INVALID_OPERATION</code> is generated if <em class="parameter"><code>type</code></em> is

<code class="constant">GL_UNSIGNED_SHORT_5_6_5</code>

and <em class="parameter"><code>format</code></em> is not <code class="constant">GL_RGB</code>.

</p><p>

<code class="constant">GL_INVALID_OPERATION</code> is generated if <em class="parameter"><code>type</code></em> is

<code class="constant">GL_UNSIGNED_SHORT_4_4_4_4</code> or

<code class="constant">GL_UNSIGNED_SHORT_5_5_5_1</code>

and <em class="parameter"><code>format</code></em> is not <code class="constant">GL_RGBA</code>.

</p><p>

<code class="constant">GL_INVALID_OPERATION</code> is generated if <em class="parameter"><code>format</code></em>

and <em class="parameter"><code>type</code></em> are neither <code class="constant">GL_RGBA</code> and

<code class="constant">GL_UNSIGNED_BYTE</code>, respectively, nor the format/type pair

returned by querying <code class="constant">GL_IMPLEMENTATION_COLOR_READ_FORMAT</code>

and <code class="constant">GL_IMPLEMENTATION_COLOR_READ_TYPE</code>.

</p><p>

<code class="constant">GL_INVALID_FRAMEBUFFER_OPERATION</code> is generated if

the currently bound framebuffer is not framebuffer complete (i.e. the

return value from <a class="citerefentry" href="glCheckFramebufferStatus"><span class="citerefentry"><span class="refentrytitle">glCheckFramebufferStatus</span></span></a>

is not <code class="constant">GL_FRAMEBUFFER_COMPLETE</code>).

</p></div><div class="refsect1"><a id="associatedgets"></a><h2>Associated Gets</h2><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument

<code class="constant">GL_IMPLEMENTATION_COLOR_READ_FORMAT</code>

or <code class="constant">GL_IMPLEMENTATION_COLOR_READ_TYPE</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_PACK_ALIGNMENT</code>

</p></div>

{$pipelinestall}{$examples}

<div class="refsect1"><a id="seealso"></a><h2>See Also</h2><p>

<a class="citerefentry" href="glCheckFramebufferStatus"><span class="citerefentry"><span class="refentrytitle">glCheckFramebufferStatus</span></span></a>,

<a class="citerefentry" href="glPixelStorei"><span class="citerefentry"><span class="refentrytitle">glPixelStorei</span></span></a>

</p></div><div class="refsect1"><div id="Copyright"><h2>Copyright</h2><p>

Copyright © 1991-2006

Silicon Graphics, Inc. This document is licensed under the SGI

Free Software B License. For details, see

<a class="ulink" href="https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/" target="_top">https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/</a>.

</p></div></div></div>