61 lines
2.3 KiB
TeX
61 lines
2.3 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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\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}%
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. JPEG is a (draft?)
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standard for compressing pictures. For details on JPEG or the
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Independent JPEG Group software refer to the JPEG standard or the
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documentation provided with the software.
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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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