When you refactor code (even if your own one) any other activity looks better. So I decided to look at VivoActive Video instead of refactoring H.263-based decoders in NihAV.
In case you don’t know, Vivo was a company that created own formats (container and video, no idea about audio) that seems to that old that its beard rivals the beard of its users. Also it’s some MPlayer-related joke but I never got it.
Anyway, it’s two H.263-based video codecs, one being vanilla H.263+ decoder will all exciting stuff like PB-frames (but no B-frames) and another one is an upgrade over it that’s still H.263+ but with different coding scheme.
Actually, how the codec handles coding is the only interesting thing there. First, codebooks. They are stored in semi-readable way: first entry may be an optional FLC marker, last entry is always End marker, the rest of entries are human-readable codes (e.g. 00 1101 11 — the codebook parser actually parses those ones and zeroes and skips white spaces) with some binary data (the number of trailing bits, symbol start value, something else too). The way how bitstream is handled reminds me of VPx somewhat: you have a set of 49 codebooks, you start decoding tokens from certain codebook and then if needed you switch to secondary codebook. In result you get a stream of tokens that may need to be parsed further (skip syncword prevention codes that decode to 0xB3, validate the decoded block—mind you, escape values are handled as normal codes there too, assign codes to proper fields etc etc). In result while it’s easy to figure out which part is H.263 picture/GOB/MB header decoding because of the familiar structure and get_bits() calls, Vivo v2 decoding looks like “decode a stream of tokens, save first ones to certain fields in context, interpret the rest of them depending on them”. For example, macroblock decoding starts with tokens for MB type, CBP and quantiser, those may be followed up by 1 or 4 motion vector deltas and then you have block coefficients (and don’t forget to skip stuffing codes when you get them).
Overall, not a very interesting codec with some crazy internal design (another fun fact: it has another set of codebooks in slightly different format but they seem to be completely unused). I’m not sure if it’s worth implementing but it was interesting to look at.