Scanning docs for bitsavers

Eric Smith spacewar at
Mon Dec 2 14:57:52 CST 2019

On Tue, Nov 26, 2019 at 8:51 PM Jay Jaeger via cctalk <cctalk at>

> When I corresponded with Al Kossow about format several years ago, he
> indicated that CCITT Group 4 lossless compression was their standard.

There are newer bilevel encodings that are somewhat more efficient than G4
(ITU-T T.6), such as JBIG (T.82) and JBIG2 (T.88), but they are not as
widely supported, and AFAIK JBIG2 is still patent encumbered. As a result,
G4 is still arguably the best bilevel encoding for general-purpose use. PDF
has natively supported G4 for ages, though it gained JBIG and JBIG2 support
in more recent versions.

Back in 2001, support for G4 encoding in open source software was really
awful; where it existed at all, it was horribly slow. There was no good
reason for G4 encoding to be slow, which was part of my motivation in
writing my own G4 encoder for tumble (an image-to-PDF utility). However, G4
support is generally much better now.

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