First of all, here’s some information for the context: MVI codecs rely on out-of-band flags to signal what capabilities and subsampling they use (the fact that they decided to store those flags in FOURCC is a different annoyance); and despite the potential variety, only couple of flags are used for each codec. For instance, of all MVI1 files I saw only one flag has been in use (golden frame—and it’s only in one game). MVI2 has two distinct sets of flag combinations, 0x804 and 0x200. The former means bog standard MVI coding (with one chroma sample set for 4×4 block) plus one extension feature, the latter means MVI2 version 2 (if that makes any sense) where they decided to make subsampling and features selectable per frame (as well as adding more of them) and moved them to the frame header while at it.
So far I concentrated my efforts on format 0x804 to see what the feature it is. It turned out to be low-resolution deltas, just like Truemotion 2. In this mode every odd pixel is coded as previous pixel plus half of luma delta for the next pixel. I still have to make the reconstruction run properly, but that’s nothing a lot of debugging can’t fix.
This should allow me to decode even some of MovieCD samples (including the one hidden in samples.mplayerhq.hu/drivers32 for some reason) and I’ve seen quite recognizable frames already.
It’s hard to tell what features the other flavour uses but it’s reasonable to assume that it uses lowres coding as well. Hopefully I’ll get to it soon.