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.
