Since I still have nothing better to do, I decided to take a look at some old codec. Apparently I tried looking at it before and abandoned it because Ghidra cannot disassemble its code properly let alone decompile. I think this is a recurring theme with the old 16-bit code, especially the one reading data using non-standard segments.
So I located Sourcer, the best disassembler of the era (that seems to be abandonware nowadays but I cannot swear on that) and used it to disassemble the binary, referring to Ghidra database to locate the functions I should care about. It is not that much fun to translate assembly by hand but at least there was not that much of it.
The codec itself turned out to be a moderately complex DPCM codec compressing 7-bit YUV 4:1:1 data using per-frame codebook and not so trivial delta compression. Codebooks contain pair of delta values calculated depending on number of bits per delta. The data is coded per plane with prediction running continuously for all pixels in the plane:
// before decoding data
(delta0, delta1) = get_code();
pprev = 64;
prev = 64 + delta0;
pdelta = delta1;
for each pixel pair {
(delta0, delta1) = get_code();
delta = ((prev + delta0 - pprev) >> 3) + pdelta;
pix0 = clip_uint8((prev + delta) * 2);
pix1 = clip_uint8((prev - delta) * 2);
pprev = prev;
prev += delta0;
pdelta = delta1;
}
Normally such codecs would not bother to generate a codebook for the specific delta size or use something more complex than pix = prev + delta; so this was a rather interesting codec to look at. Hopefully there will be more of interesting formats to study even if sometimes I get the feeling that all undiscovered formats are either trivial or rip-offs of some standard.