VP7 is such a nice codec that I decided to distract myself a little with something else. And that something else turned out to be MidiVid codec family. It turned out to be quite peculiar and somehow reminiscent of Duck codecs.
The family consists of three codecs:
- MidiVid — the original codec based on LZSS and vector quantisation;
- MidiVid Lossless — exactly what is says on a tin, based on LZSS and bunch of other technologies;
- MidiVid 3 — a codec based on simplified integer DCT and single codebook for all values.
I’ve actually added MidiVid decoder to NihAV because it’s simple (two hundred lines including boilerplate and tests) and way more fun than working on VP7 decoder. Now I’ll describe them and hopefully you’ll understand why it reminds me of Duck codecs despite not being similar in design.
MidiVid
This is a simple hold-and-modify video codec that had been used in some games back in PS2/Xbox era. The frame data can be stored either unpacked or packed with LZSS and it contains the following kinds of data: change mask for 8×8 blocks (in case of interframe—if it’s zero then leave block as is, otherwise decode new data for it), 4×4 block codebook data (up to 512 entries), high bits for 9-bit indices (if we have 257-512 various blocks) and 8-bit indexes for codebook.
The interesting part is that LZSS scheme looked very familiar and indeed it looks almost exactly like lzss.c from LZARI author (remember that? I still do), the only differences is that it does not use pre-filled window and flags are grouped into 16-bit word instead of single byte.
MidiVid Lossless
This one is a special best as it combines two completely different compression methods: the same LZSS as before and something used by BWT-based compressor (to the point that frame header contains FTWB or ZTWB IDs).
I’m positively convinced it was copied from some BTW-based compressor not just because of those IDs but also because it seems to employ the same methods as some old BTW-based compressor except for the Burrows–Wheeler transform itself (that would be too much for the old codecs): various data preprocessing methods (signalled by flags in the frame header), move-to-front coding (in its classical 1-2 coding form that does not update first two positions that much) plus coding coefficients in two groups: first just zero/one/large using order-3 adaptive model and then values larger than one using single order-1 adaptive model. What made it suspicious? Preprocessing methods.
MVLZ has different kinds of preprocessing methods: something looking like distance coding, static n-gram replacement, table prediction (i.e. when data is treated as series of n-bit numbers and the actual numbers are replaced with the difference between previous ones) and x86 call preprocessing (i.e. that trick when you change function call address from relative into absolute for better compression ratio and then undo it during decompression; known also as E8-preprocessing because x86 call opcode is E8 <32-bit offset> and it’s easy to just replace them instead of adding full disassembler to the archiver). I had my suspicions as n-gram replacement (that one is quite stupid for video codecs and it only replaces some values with some binary values that look more related to machine code than video) but the last item was a dead give-away. I’m pretty sure that somebody who knows open-source BWT compressors of late 1990s will probably recognize it even from this description but sadly I’ve not been following it that closely being more attracted to multimedia.
MidiVid 3
This codec is based on some static codebook for packing all values: block types, motion vectors and actual coefficients. Each block in macroblock can be coded with one of four modes: empty (fill with 0x80 in case of intra), DC only, just few coefficients DCT, and full DCT. As usual various kinds of data are grouped and coded as single array.
Motion compensation is full-pixel and unlike its predecessor it operates in YUV420 format.
This was an interesting detour but I have to return back to failing to start writing VP7 decoder.
P.S. I’ll try to document them with more details in the wiki soon.
P.P.S. This should’ve been a post about railways instead but I guess it will have to wait.