Even before I started to write my series of posts on FFhistory, I had another work in progress already which I’m now making public in order not to chicken out (as I did several times already). I’m talking about Indeo 3 encoder.
Why Indeo 3 of all possible things? It’s both not your conventional DCT-based codec and it’s widespread enough to be of some limited use for me (being present in AVI, MOV and VMD containers, only Cinepak is more ubiquitous). I’m not as good as Mike Melanson but I’m willing to try my hoof at it.
The funny thing is, there’s an opensource decoder for it and even a decent description in US patent 5,386,232 from 1995 (so it’s expired already and anybody can write an encoder for it). The problem is that those two sources don’t match between each other and somewhat disagree with the official binary specification (I’m pretty sure that both Indeo3 decoders were REd from XAnim module). And Ghidra does not like VfW binary (maybe it’ll like the version inside QT6 better) so I can’t easily refer to it either.
Anyway, I attempted and gave up writing an encoder for Indeo 3 several times because of its perceived combinatoric complexity. First you need to split frame recursively into blocks—how to select them? Then you need to select one of the coding modes (again, how?) and codebooks (same question). Trying to think of a reasonable way to implement it all made me shudder and give up until I finally read the format description and persisted enough to write at least something working (side note: I also have the same problem with TrueMotion 1 encoder which I also want to write one day, hopefully it’ll be easier now).
Also I tried to look into the encoder implementation and found it as a bunch of magic numbers at work. I’m not joking, during initialisation it seems to set several dozens of various integers and floats and use them for various coding decisions (at least what I could understand from it is that codebook selection is kinda tied to the internal quantiser parameter which is calculated depending on bitrate/quality—and various magic numbers).
So I want to document how this codec works, what differs in the different descriptions of it and how my encoder decides what to use in different situations. This should amount to another dozen of posts that nobody will read.