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81 lines
3.1 KiB
TeX
81 lines
3.1 KiB
TeX
\section{\module{jpeg} ---
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Read and write JPEG files}
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\declaremodule{builtin}{jpeg}
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\platform{IRIX}
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\modulesynopsis{Read and write image files in compressed JPEG format.}
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The module \module{jpeg} provides access to the jpeg compressor and
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decompressor written by the Independent JPEG Group
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\index{Independent JPEG Group}(IJG). JPEG is a standard for
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compressing pictures; it is defined in ISO 10918. For details on JPEG
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or the Independent JPEG Group software refer to the JPEG standard or
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the documentation provided with the software.
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A portable interface to JPEG image files is available with the Python
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Imaging Library (PIL) by Fredrik Lundh. Information on PIL is
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available at \url{http://www.pythonware.com/products/pil/}.
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\index{Python Imaging Library}
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\index{PIL (the Python Imaging Library)}
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\index{Lundh, Fredrik}
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The \module{jpeg} module defines an exception and some functions.
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\begin{excdesc}{error}
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Exception raised by \function{compress()} and \function{decompress()}
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in case of errors.
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\end{excdesc}
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\begin{funcdesc}{compress}{data, w, h, b}
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Treat data as a pixmap of width \var{w} and height \var{h}, with
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\var{b} bytes per pixel. The data is in SGI GL order, so the first
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pixel is in the lower-left corner. This means that \function{gl.lrectread()}
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return data can immediately be passed to \function{compress()}.
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Currently only 1 byte and 4 byte pixels are allowed, the former being
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treated as greyscale and the latter as RGB color.
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\function{compress()} returns a string that contains the compressed
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picture, in JFIF\index{JFIF} format.
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\end{funcdesc}
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\begin{funcdesc}{decompress}{data}
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Data is a string containing a picture in JFIF\index{JFIF} format. It
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returns a tuple \code{(\var{data}, \var{width}, \var{height},
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\var{bytesperpixel})}. Again, the data is suitable to pass to
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\function{gl.lrectwrite()}.
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\end{funcdesc}
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\begin{funcdesc}{setoption}{name, value}
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Set various options. Subsequent \function{compress()} and
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\function{decompress()} calls will use these options. The following
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options are available:
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\begin{tableii}{l|p{3in}}{code}{Option}{Effect}
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\lineii{'forcegray'}{%
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Force output to be grayscale, even if input is RGB.}
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\lineii{'quality'}{%
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Set the quality of the compressed image to a value between
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\code{0} and \code{100} (default is \code{75}). This only affects
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compression.}
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\lineii{'optimize'}{%
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Perform Huffman table optimization. Takes longer, but results in
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smaller compressed image. This only affects compression.}
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\lineii{'smooth'}{%
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Perform inter-block smoothing on uncompressed image. Only useful
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for low-quality images. This only affects decompression.}
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\end{tableii}
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\end{funcdesc}
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\begin{seealso}
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\seetitle{JPEG Still Image Data Compression Standard}{The
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canonical reference for the JPEG image format, by
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Pennebaker and Mitchell.}
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\seetitle[http://www.w3.org/Graphics/JPEG/itu-t81.pdf]{Information
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Technology - Digital Compression and Coding of
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Continuous-tone Still Images - Requirements and
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Guidelines}{The ISO standard for JPEG is also published as
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ITU T.81. This is available online in PDF form.}
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\end{seealso}
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