1994-01-02 09:22:07 +08:00
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\section{Built-in Module \sectcode{jpeg}}
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\label{module-jpeg}
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\bimodindex{jpeg}
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1995-03-13 18:03:32 +08:00
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The module \code{jpeg} provides access to the jpeg compressor and
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decompressor written by the Independent JPEG Group. JPEG is a (draft?)\
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standard for compressing pictures. For details on jpeg or the
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1994-08-08 20:30:22 +08:00
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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 \code{jpeg} module defines these functions:
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\renewcommand{\indexsubitem}{(in module jpeg)}
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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 \var{b} bytes per
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pixel. The data is in SGI GL order, so the first pixel is in the
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lower-left corner. This means that \code{lrectread} return data can
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immediately be passed to compress. Currently only 1 byte and 4 byte
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pixels are allowed, the former being treated as greyscale and the
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latter as RGB color. Compress returns a string that contains the
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compressed picture, in 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 format. It returns a
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tuple
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\code{(\var{data}, \var{width}, \var{height}, \var{bytesperpixel})}.
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Again, the data is suitable to pass to \code{lrectwrite}.
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\end{funcdesc}
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\begin{funcdesc}{setoption}{name\, value}
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Set various options. Subsequent compress and decompress calls
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will use these options. The following options are available:
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\begin{description}
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\item[\code{'forcegray' }]
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Force output to be grayscale, even if input is RGB.
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\item[\code{'quality' }]
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Set the quality of the compressed image to a
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value between \code{0} and \code{100} (default is \code{75}). Compress only.
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\item[\code{'optimize' }]
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Perform Huffman table optimization. Takes longer, but results in
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smaller compressed image. Compress only.
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\item[\code{'smooth' }]
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Perform inter-block smoothing on uncompressed image. Only useful for
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low-quality images. Decompress only.
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\end{description}
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\end{funcdesc}
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1995-03-13 18:03:32 +08:00
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Compress and uncompress raise the error \code{jpeg.error} in case of errors.
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