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