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<H1><A NAME="SECTION00221200000000000000000">
20.12 <tt class="module">jpeg</tt> --
Read and write JPEG files</A>
</H1>
<P>
<A NAME="module-jpeg"></A>
<p class="availability">Availability: <span
class="platform">IRIX</span>.</p>
<P>
The module <tt class="module">jpeg</tt> provides access to the jpeg compressor and
decompressor written by the Independent JPEG Group
<a id='l2h-5218' xml:id='l2h-5218'></a>(IJG). JPEG is a standard for
compressing pictures; it is defined in ISO 10918. For details on JPEG
or the Independent JPEG Group software refer to the JPEG standard or
the documentation provided with the software.
<P>
A portable interface to JPEG image files is available with the Python
Imaging Library (PIL) by Fredrik Lundh. Information on PIL is
available at <a class="url" href="http://www.pythonware.com/products/pil/">http://www.pythonware.com/products/pil/</a>.
<a id='l2h-5219' xml:id='l2h-5219'></a>
<P>
The <tt class="module">jpeg</tt> module defines an exception and some functions.
<P>
<dl><dt><b><span class="typelabel">exception</span> <tt id='l2h-5214' xml:id='l2h-5214' class="exception">error</tt></b></dt>
<dd>
Exception raised by <tt class="function">compress()</tt> and <tt class="function">decompress()</tt>
in case of errors.
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-5215' xml:id='l2h-5215' class="function">compress</tt></b>(</nobr></td>
<td><var>data, w, h, b</var>)</td></tr></table></dt>
<dd>
Treat data as a pixmap of width <var>w</var> and height <var>h</var>, with
<var>b</var> bytes per pixel. The data is in SGI GL order, so the first
pixel is in the lower-left corner. This means that <tt class="function">gl.lrectread()</tt>
return data can immediately be passed to <tt class="function">compress()</tt>.
Currently only 1 byte and 4 byte pixels are allowed, the former being
treated as greyscale and the latter as RGB color.
<tt class="function">compress()</tt> returns a string that contains the compressed
picture, in JFIF<a id='l2h-5220' xml:id='l2h-5220'></a> format.
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-5216' xml:id='l2h-5216' class="function">decompress</tt></b>(</nobr></td>
<td><var>data</var>)</td></tr></table></dt>
<dd>
Data is a string containing a picture in JFIF<a id='l2h-5221' xml:id='l2h-5221'></a> format. It
returns a tuple <code>(<var>data</var>, <var>width</var>, <var>height</var>,
<var>bytesperpixel</var>)</code>. Again, the data is suitable to pass to
<tt class="function">gl.lrectwrite()</tt>.
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-5217' xml:id='l2h-5217' class="function">setoption</tt></b>(</nobr></td>
<td><var>name, value</var>)</td></tr></table></dt>
<dd>
Set various options. Subsequent <tt class="function">compress()</tt> and
<tt class="function">decompress()</tt> calls will use these options. The following
options are available:
<P>
<div class="center"><table class="realtable">
<thead>
<tr>
<th class="left" >Option</th>
<th class="left" >Effect</th>
</tr>
</thead>
<tbody>
<tr><td class="left" valign="baseline"><code>'forcegray'</code></td>
<td class="left" >Force output to be grayscale, even if input is RGB.</td></tr>
<tr><td class="left" valign="baseline"><code>'quality'</code></td>
<td class="left" >Set the quality of the compressed image to a value between
<code>0</code> and <code>100</code> (default is <code>75</code>). This only affects
compression.</td></tr>
<tr><td class="left" valign="baseline"><code>'optimize'</code></td>
<td class="left" >Perform Huffman table optimization. Takes longer, but results in
smaller compressed image. This only affects compression.</td></tr>
<tr><td class="left" valign="baseline"><code>'smooth'</code></td>
<td class="left" >Perform inter-block smoothing on uncompressed image. Only useful
for low-quality images. This only affects decompression.</td></tr></tbody>
</table></div>
</dl>
<P>
<div class="seealso">
<p class="heading">See Also:</p>
<dl compact="compact" class="seetitle">
<dt><em class="citetitle"
>JPEG Still Image Data Compression Standard</em></dt>
<dd>The
canonical reference for the JPEG image format, by
Pennebaker and Mitchell.</dd>
</dl>
<P>
<dl compact="compact" class="seetitle">
<dt><em class="citetitle"><a href="http://www.w3.org/Graphics/JPEG/itu-t81.pdf"
>Information
Technology - Digital Compression and Coding of
Continuous-tone Still Images - Requirements and
Guidelines</a></em></dt>
<dd>The ISO standard for JPEG is also published as
ITU T.81. This is available online in PDF form.</dd>
</dl>
</div>
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