20.12 jpeg -- Read and write JPEG files

20.12 jpeg -- Read and write JPEG files

Availability: IRIX.

The module jpeg provides access to the jpeg compressor and decompressor written by the 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 http://www.pythonware.com/products/pil/.

The jpeg module defines an exception and some functions.

exception error

Exception raised by compress() and decompress() in case of errors.

compress(data, w, h, b)

Treat data as a pixmap of width w and height h, with b bytes per pixel. The data is in SGI GL order, so the first pixel is in the lower-left corner. This means that gl.lrectread() return data can immediately be passed to compress(). Currently only 1 byte and 4 byte pixels are allowed, the former being treated as greyscale and the latter as RGB color. compress() returns a string that contains the compressed picture, in JFIF format.

decompress(data)

Data is a string containing a picture in JFIF format. It returns a tuple

(data, width, height,
bytesperpixel)

. Again, the data is suitable to pass to gl.lrectwrite().

setoption(name, value)

Set various options. Subsequent compress() and decompress() calls will use these options. The following options are available:

Option Effect

'forcegray' Force output to be grayscale, even if input is RGB.
'quality' Set the quality of the compressed image to a value between 0 and 100 (default is 75). This only affects compression.
'optimize' Perform Huffman table optimization. Takes longer, but results in smaller compressed image. This only affects compression.
'smooth' Perform inter-block smoothing on uncompressed image. Only useful for low-quality images. This only affects decompression.

See Also:

JPEG Still Image Data Compression Standard: The canonical reference for the JPEG image format, by Pennebaker and Mitchell.

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.

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