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| 54 | <H1><A NAME="SECTION00221200000000000000000"> | |
| 55 | 20.12 <tt class="module">jpeg</tt> -- | |
| 56 | Read and write JPEG files</A> | |
| 57 | </H1> | |
| 58 | ||
| 59 | <P> | |
| 60 | <A NAME="module-jpeg"></A> | |
| 61 | <p class="availability">Availability: <span | |
| 62 | class="platform">IRIX</span>.</p> | |
| 63 | ||
| 64 | <P> | |
| 65 | The module <tt class="module">jpeg</tt> provides access to the jpeg compressor and | |
| 66 | decompressor written by the Independent JPEG Group | |
| 67 | <a id='l2h-5218' xml:id='l2h-5218'></a>(IJG). JPEG is a standard for | |
| 68 | compressing pictures; it is defined in ISO 10918. For details on JPEG | |
| 69 | or the Independent JPEG Group software refer to the JPEG standard or | |
| 70 | the documentation provided with the software. | |
| 71 | ||
| 72 | <P> | |
| 73 | A portable interface to JPEG image files is available with the Python | |
| 74 | Imaging Library (PIL) by Fredrik Lundh. Information on PIL is | |
| 75 | available at <a class="url" href="http://www.pythonware.com/products/pil/">http://www.pythonware.com/products/pil/</a>. | |
| 76 | <a id='l2h-5219' xml:id='l2h-5219'></a> | |
| 77 | ||
| 78 | <P> | |
| 79 | The <tt class="module">jpeg</tt> module defines an exception and some functions. | |
| 80 | ||
| 81 | <P> | |
| 82 | <dl><dt><b><span class="typelabel">exception</span> <tt id='l2h-5214' xml:id='l2h-5214' class="exception">error</tt></b></dt> | |
| 83 | <dd> | |
| 84 | Exception raised by <tt class="function">compress()</tt> and <tt class="function">decompress()</tt> | |
| 85 | in case of errors. | |
| 86 | </dd></dl> | |
| 87 | ||
| 88 | <P> | |
| 89 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 90 | <td><nobr><b><tt id='l2h-5215' xml:id='l2h-5215' class="function">compress</tt></b>(</nobr></td> | |
| 91 | <td><var>data, w, h, b</var>)</td></tr></table></dt> | |
| 92 | <dd> | |
| 93 | Treat data as a pixmap of width <var>w</var> and height <var>h</var>, with | |
| 94 | <var>b</var> bytes per pixel. The data is in SGI GL order, so the first | |
| 95 | pixel is in the lower-left corner. This means that <tt class="function">gl.lrectread()</tt> | |
| 96 | return data can immediately be passed to <tt class="function">compress()</tt>. | |
| 97 | Currently only 1 byte and 4 byte pixels are allowed, the former being | |
| 98 | treated as greyscale and the latter as RGB color. | |
| 99 | <tt class="function">compress()</tt> returns a string that contains the compressed | |
| 100 | picture, in JFIF<a id='l2h-5220' xml:id='l2h-5220'></a> format. | |
| 101 | </dl> | |
| 102 | ||
| 103 | <P> | |
| 104 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 105 | <td><nobr><b><tt id='l2h-5216' xml:id='l2h-5216' class="function">decompress</tt></b>(</nobr></td> | |
| 106 | <td><var>data</var>)</td></tr></table></dt> | |
| 107 | <dd> | |
| 108 | Data is a string containing a picture in JFIF<a id='l2h-5221' xml:id='l2h-5221'></a> format. It | |
| 109 | returns a tuple <code>(<var>data</var>, <var>width</var>, <var>height</var>, | |
| 110 | <var>bytesperpixel</var>)</code>. Again, the data is suitable to pass to | |
| 111 | <tt class="function">gl.lrectwrite()</tt>. | |
| 112 | </dl> | |
| 113 | ||
| 114 | <P> | |
| 115 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 116 | <td><nobr><b><tt id='l2h-5217' xml:id='l2h-5217' class="function">setoption</tt></b>(</nobr></td> | |
| 117 | <td><var>name, value</var>)</td></tr></table></dt> | |
| 118 | <dd> | |
| 119 | Set various options. Subsequent <tt class="function">compress()</tt> and | |
| 120 | <tt class="function">decompress()</tt> calls will use these options. The following | |
| 121 | options are available: | |
| 122 | ||
| 123 | <P> | |
| 124 | <div class="center"><table class="realtable"> | |
| 125 | <thead> | |
| 126 | <tr> | |
| 127 | <th class="left" >Option</th> | |
| 128 | <th class="left" >Effect</th> | |
| 129 | </tr> | |
| 130 | </thead> | |
| 131 | <tbody> | |
| 132 | <tr><td class="left" valign="baseline"><code>'forcegray'</code></td> | |
| 133 | <td class="left" >Force output to be grayscale, even if input is RGB.</td></tr> | |
| 134 | <tr><td class="left" valign="baseline"><code>'quality'</code></td> | |
| 135 | <td class="left" >Set the quality of the compressed image to a value between | |
| 136 | <code>0</code> and <code>100</code> (default is <code>75</code>). This only affects | |
| 137 | compression.</td></tr> | |
| 138 | <tr><td class="left" valign="baseline"><code>'optimize'</code></td> | |
| 139 | <td class="left" >Perform Huffman table optimization. Takes longer, but results in | |
| 140 | smaller compressed image. This only affects compression.</td></tr> | |
| 141 | <tr><td class="left" valign="baseline"><code>'smooth'</code></td> | |
| 142 | <td class="left" >Perform inter-block smoothing on uncompressed image. Only useful | |
| 143 | for low-quality images. This only affects decompression.</td></tr></tbody> | |
| 144 | </table></div> | |
| 145 | </dl> | |
| 146 | ||
| 147 | <P> | |
| 148 | <div class="seealso"> | |
| 149 | <p class="heading">See Also:</p> | |
| 150 | ||
| 151 | <dl compact="compact" class="seetitle"> | |
| 152 | <dt><em class="citetitle" | |
| 153 | >JPEG Still Image Data Compression Standard</em></dt> | |
| 154 | <dd>The | |
| 155 | canonical reference for the JPEG image format, by | |
| 156 | Pennebaker and Mitchell.</dd> | |
| 157 | </dl> | |
| 158 | ||
| 159 | <P> | |
| 160 | <dl compact="compact" class="seetitle"> | |
| 161 | <dt><em class="citetitle"><a href="http://www.w3.org/Graphics/JPEG/itu-t81.pdf" | |
| 162 | >Information | |
| 163 | Technology - Digital Compression and Coding of | |
| 164 | Continuous-tone Still Images - Requirements and | |
| 165 | Guidelines</a></em></dt> | |
| 166 | <dd>The ISO standard for JPEG is also published as | |
| 167 | ITU T.81. This is available online in PDF form.</dd> | |
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