So I found another pair of rather peculiar game video formats. What’s curious is that while MVI1 and MVI2 are rather different internally, they both are used in Battle Race demo, and that’s not saying anything about their design (yet).
Both MVI1 and MVI2 are codecs used in some old DOS games developed by Ikarion Software. The only common thing in their structure is that is starts with magic containing “MVI” followed by 16-bit version and a table of frame sizes. And that’s where the differences begin. In MVI1 frame sizes are coded as radix-7 (or MIDI encoding, also known as LEB128 nowadays); in MVI2 they didn’t bother with it and left them as 32-bit numbers. In MVI1 frame consists of several optional parts, in MVI2 frame structure is fixed. And there’s another small difference: MVI1 has LZ77-compressed paletted images while MVI2 has vector-quantised 5-bit YUV420.
Of course formats employing LZ77 are dime a dozen but MVI1 still manages to stand out: first, it employs radix-7 encoded offsets, and it also de-interleaves palette by components before compressing (i.e. first all red components are sent, then all green components and finally all blue components). Optionally it can employ motion compensation on 10×10 tiles (which is uncommon already) with motion vectors for each tile being compressed with RLE and also in de-interleaved form (i.e. 768 16-bit offsets are split into high and low bytes and those are compressed one after another). I can’t think of many formats back then that thought about de-interleaving data in order to increase compression.
MVI2 splits frames into 64×64 tiles that are composed of 4×4 blocks selected from a circular buffer of 4096 entries. So frame decoding consists of reading new 4×4 blocks for the codebook and then decoding 12-bit block indices for 64×64 luma plane and two 32×32 chroma planes (all they use the same codebook, which is rather unusual for such codecs). Of course such approach is not particularly new (and reminds me more of 16-bit consoles) but I still can’t remember another VQ-based codec with exactly the same design.
P.S. And there’s also MVI used in Archimedean Dynasty—yet another MVI format from yet another German company. I’ve not looked at it yet (beside a cursory glance in the hex viewer) but it looks different from those two. Let’s see what surprises it has…