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