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In 1992, a joint ISO/CCITT committee known as JPEG (Joint Photographic
Experts Group) established and published the first draft international
standard (DIS) for compressing continuous-tone still images, both
grayscale and color. JPEG has defined four mode of operations,
summarized them as follows.
(1) Sequential encoding: each image component is encoded in a single
left-to-right, top-to-bottom scan;
(2) Progressive encoding: the image is encoded in multiple scans for
applications in which transmission time is long, and the viewer
prefers to watch the image build up in multiple coarse-to-clear
passes;
(3) Lossless encoding: the images is encoded to guarantee exact recovery
of every source image sample value (even though the result is low
compression compared to the lossy modes);
(4) Hierarchical encoding: the image is encoded at multiple resolutions
so that lower-resolution versions may be accessed without first having
to decompress the image at its full resolution.
Our lossless JPEG encoding program has an automatic prediction
selection value (PSV) selector which selects the best PSV among a
user provided or default set of PSVs. This selector guarantees the
best compression ratio for lossless JPEG.
The encoding program "pnmtoljpg" compresses a Portable Pixmap or
Portable Graymap image to a lossles JPEG (ljpg) image using the
JPEG standard (DIS) lossless mode of operation. The decoding program
"ljpgtopnm" decodes a ljpg image to either a Portable Pixmap or
Portable Graymap image depending on the number of color components
stored in the ljpg image file.
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