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Add documentation for the help() built-in; contributed by Ka-Ping Yee. This is part of SF patch #494622.
author Fred Drake <fdrake@acm.org>
date Tue, 18 Dec 2001 16:31:08 +0000
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1 \section{\module{jpeg} ---
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2 Read and write JPEG files}
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3
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4 \declaremodule{builtin}{jpeg}
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5 \platform{IRIX}
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6 \modulesynopsis{Read and write image files in compressed JPEG format.}
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7
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9 The module \module{jpeg} provides access to the jpeg compressor and
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10 decompressor written by the Independent JPEG Group
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11 \index{Independent JPEG Group}(IJG). JPEG is a standard for
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12 compressing pictures; it is defined in ISO 10918. For details on JPEG
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13 or the Independent JPEG Group software refer to the JPEG standard or
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14 the documentation provided with the software.
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15
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16 A portable interface to JPEG image files is available with the Python
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17 Imaging Library (PIL) by Fredrik Lundh. Information on PIL is
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18 available at \url{http://www.pythonware.com/products/pil/}.
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19 \index{Python Imaging Library}
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20 \index{PIL (the Python Imaging Library)}
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21 \index{Lundh, Fredrik}
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23 The \module{jpeg} module defines an exception and some functions.
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24
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25 \begin{excdesc}{error}
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26 Exception raised by \function{compress()} and \function{decompress()}
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27 in case of errors.
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28 \end{excdesc}
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29
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30 \begin{funcdesc}{compress}{data, w, h, b}
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31 Treat data as a pixmap of width \var{w} and height \var{h}, with
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32 \var{b} bytes per pixel. The data is in SGI GL order, so the first
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33 pixel is in the lower-left corner. This means that \function{gl.lrectread()}
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34 return data can immediately be passed to \function{compress()}.
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35 Currently only 1 byte and 4 byte pixels are allowed, the former being
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36 treated as greyscale and the latter as RGB color.
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37 \function{compress()} returns a string that contains the compressed
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38 picture, in JFIF\index{JFIF} format.
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39 \end{funcdesc}
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40
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41 \begin{funcdesc}{decompress}{data}
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42 Data is a string containing a picture in JFIF\index{JFIF} format. It
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43 returns a tuple \code{(\var{data}, \var{width}, \var{height},
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44 \var{bytesperpixel})}. Again, the data is suitable to pass to
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45 \function{gl.lrectwrite()}.
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46 \end{funcdesc}
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47
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48 \begin{funcdesc}{setoption}{name, value}
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49 Set various options. Subsequent \function{compress()} and
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50 \function{decompress()} calls will use these options. The following
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51 options are available:
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52
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53 \begin{tableii}{l|p{3in}}{code}{Option}{Effect}
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54 \lineii{'forcegray'}{%
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55 Force output to be grayscale, even if input is RGB.}
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56 \lineii{'quality'}{%
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57 Set the quality of the compressed image to a value between
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58 \code{0} and \code{100} (default is \code{75}). This only affects
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59 compression.}
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60 \lineii{'optimize'}{%
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61 Perform Huffman table optimization. Takes longer, but results in
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62 smaller compressed image. This only affects compression.}
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63 \lineii{'smooth'}{%
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64 Perform inter-block smoothing on uncompressed image. Only useful
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65 for low-quality images. This only affects decompression.}
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66 \end{tableii}
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67 \end{funcdesc}
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68
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69
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70 \begin{seealso}
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71 \seetitle{JPEG Still Image Data Compression Standard}{The
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72 canonical reference for the JPEG image format, by
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73 Pennebaker and Mitchell.}
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74
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75 \seetitle[http://www.w3.org/Graphics/JPEG/itu-t81.pdf]{Information
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76 Technology - Digital Compression and Coding of
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77 Continuous-tone Still Images - Requirements and
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78 Guidelines}{The ISO standard for JPEG is also published as
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79 ITU T.81. This is available online in PDF form.}
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80 \end{seealso}