Kostya's Boring Codec World

I know that MPEG-4 ASP is a better-known candidate for this role, but Indeo 3 is a strong contender too.

Back in the day it was ubiquitous, patented, and as I re-implemented a decoder for it I discovered another fun similarity with MP3: checksums.

Each Indeo 3 frame has a checksum value embedded in it, calculated as XOR of all pairs of pixels. I had an intent to use it to validate the decoder output but after playing a bit I’ve given up. Some files agree on checksums, others disagree while the output from the reference decoder is exactly the same, in yet another files checksums are correct but byte-swapped and one file has only zeroes for checksums. This is exactly like MP3, and like there Indeo 3 decoders ignore that field.

Also I’ve encountered other fun quirks. For example, one Indeo file is 160×120 but its frame header claims it’s 160×240 (but you still have to decode it as 160×120). You’d think it’s the rule but I know some VMD files from Urban Runner game where the first or last frame are double the size. Another file errors out on the first frame because of the inappropriate opcode encountered (essentially “skip to the line 2” right after “skip to the line 2”) but it turns out that VfW decoder does not check that case and simply uses that opcode as a codebook index.

At least my new decoder should be good enough to iron out the obvious bugs from the encoder and after that I shall forget about that codec again for a long time.

This entry was posted on Tuesday, February 14th, 2023 at 1:26 pm and is filed under Various Video Codecs. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.