Doc/libjpeg.tex

   1 \section{Built-in Module \sectcode{jpeg}}
   2 \bimodindex{jpeg}
   3
   4 The module \code{jpeg} provides access to the jpeg compressor and
   5 decompressor written by the Independent JPEG Group. JPEG is a (draft?)\
   6 standard for compressing pictures.  For details on jpeg or the
   7 Independent JPEG Group software refer to the JPEG standard or the
   8 documentation provided with the software.
   9
  10 The \code{jpeg} module defines these functions:
  11
  12 \renewcommand{\indexsubitem}{(in module jpeg)}
  13 \begin{funcdesc}{compress}{data\, w\, h\, b}
  14 Treat data as a pixmap of width \var{w} and height \var{h}, with \var{b} bytes per
  15 pixel.  The data is in SGI GL order, so the first pixel is in the
  16 lower-left corner. This means that \code{lrectread} return data can
  17 immediately be passed to compress.  Currently only 1 byte and 4 byte
  18 pixels are allowed, the former being treated as greyscale and the
  19 latter as RGB color.  Compress returns a string that contains the
  20 compressed picture, in JFIF format.
  21 \end{funcdesc}
  22
  23 \begin{funcdesc}{decompress}{data}
  24 Data is a string containing a picture in JFIF format. It returns a
  25 tuple
  26 \code{(\var{data}, \var{width}, \var{height}, \var{bytesperpixel})}.
  27 Again, the data is suitable to pass to \code{lrectwrite}.
  28 \end{funcdesc}
  29
  30 \begin{funcdesc}{setoption}{name\, value}
  31 Set various options.  Subsequent compress and decompress calls
  32 will use these options.  The following options are available:
  33 \begin{description}
  34 \item[\code{'forcegray' }]
  35 Force output to be grayscale, even if input is RGB.
  36
  37 \item[\code{'quality' }]
  38 Set the quality of the compressed image to a
  39 value between \code{0} and \code{100} (default is \code{75}).  Compress only.
  40
  41 \item[\code{'optimize' }]
  42 Perform Huffman table optimization.  Takes longer, but results in
  43 smaller compressed image.  Compress only.
  44
  45 \item[\code{'smooth' }]
  46 Perform inter-block smoothing on uncompressed image.  Only useful for
  47 low-quality images.  Decompress only.
  48 \end{description}
  49 \end{funcdesc}
  50
  51 Compress and uncompress raise the error \code{jpeg.error} in case of errors.