This is really interesting! There seems to be some additional technical information here: https://boards.openpandora.org/topic/18485-free-lossless-ima... - for interlacing it uses a generalization of PNG's Adam7; unlike PNG, the geometry of the 2D interlacing is exploited heavily to get better pixel estimation, which means the overhead of interlacing is small (vs simple scanline encoding, which has the benefit of loc…
Great, thanks. It seems there is no technical description available anywhere beyond what you quoted. They haven't written it up, and I couldn't even find comments in the source providing details. That's too bad, but he (I assume that's Jon Sneyers) does say he hopes to write it up later.
Also, comments in that thread on speed [1]:
In terms of encode/decode speed: both are slow and not very optimized
at the moment (no assembler code etc, just C++ code). A median file
took 3 seconds to encode (1 second for a p25 file, 6 seconds for a p75
file), which is slower than most other algorithms: WebP took slightly
less than a second for a median file (0.5s for p25, 2s for p75), PNG
and JPEG2000 took about half a second. It's not that bad though: BPG
took 9 seconds on a median file (2.5s for p25, 25s for p75), and
brute-force pngcrushing took something like 15 seconds on a median
file (6s for p25, over 30s for p75), so at least it's already better
than that.
Decode speed to restore the full lossless image and write it as a png
is not so good: about 0.75s for a median file, 0.25s for a p25 file,
1.5s for a p75 file. That's roughly 3 to 5 times slower than the other
algorithms. However, decoding a partial (lossy) file is much faster
than decoding everything, so in a progressive decoding scenario, the
difference would not be huge.