Revisiting legendary Q format

Since I had nothing better to do I decided to look again at Q format and try to write a decoder for NihAV while at it.

It turns out there are three versions of the format are known: the one in Death Gate (version 3), the one in Shannara (version 4) and the one in Mission Control (not an adventure game this time; it’s version 5 obviously). Versions 4 and 5 differ only in minor details, the compression is the same. Version 3 uses the same principles but some coding details are different.

The main source of confusion was the fact that you have two context-dependent opcodes, namely 0xF9 and 0xFB. The first one either repeats the previous block several times or reuses motion vector from that block. The second one is even trickier. For version 5 frames and for version 4 with mode 7 signalled it signals a series of blocks with 3-16 colours in each. But for mode 6 it signals a series of blocks with the same type as the previous one but with some parameters changed. If it was preceded by a fill block, these blocks with have fill value. After a block with patterns you have a series of blocks with patterns reusing the same colours as the original block. For motion-compensated block you have the same kind of motion information transmitted.

But the weirdest thing IMO is the interlaced coding in version 5. For some reason (scalability? lower latency?) they decided to code frame in two part, so frame type 9 codes even rows of the frame and frame type 11 codes odd rows—and in this cases rows are four pixels high as it is one block height. That is definitely not something I was expecting.

All in all, the format turned out to be even weirder than I expected it to be.

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