Since I had been looking at video files in various games I’ve decided to look at AVX format in Jack Orlando, a Polish adventure game.
The format is not very complicated but somewhat WTFy. It combines audio track compressed with IMA ADPCM and RLE-compressed video. The format is organised into chunks (audio, video, palette update). Sound easy enough?
Well, here are WTFy bits:
- audio violates chunking structure: file starts with 32-bit word to signal this is video file, then you always have ~110kB of audio data (that starts with its own header) and only then you have video header and chunks;
- speaking of which, video frames are packed together with audio chunks so after reading/decoding one you have audio data without the usual chunk header (you can have standalone audio chunk of course);
- oh, and another thing—the video file header seems to list number of audio frames (which is usually several times larger than the number of video frames);
- another fun fact about video header: it may occur several times throughout the file;
- and another fun fact: while the format is paletted, palette can be either 24-bit (I’ve seen that only in intro) or 16-bit (all other files, which are predominantly short game over animations). And by 16-bit palette I mean 256-colour palette with actual colours represented in RGB565, I’ve never seen that before;
- as a cherry on top of that sometimes chunks start earlier or later than you’d expect from declared size.
And if you thought video compression would be sane, think again:
- actual data is compressed in two passes: first it groups data into coded segments in the form
(1 byte - number of skipped pixels, 1 byte - number of coded pixels, 0-254 bytes - actual pixels)and then all that data is RLE coded (0xFF– escape flag after which run value and run count bytes follow, otherwise byte value should be copied as is); - since this is too easy it also uses pixel value 0 as skip value;
- plus there’s a global offset in the beginning of the chunk telling from which pixel to start decoding;
- plus there are two video chunk types that seem to differ only in one header field being missing in one of them;
- and the actual video is 640×480 but it is coded as 1024×480 with right part of the frame being blank.
Some game formats are interesting but this one borders with being unbearably interesting.
Obscure games with even more obscure formats!
s/even more/equally/There’s a term “one-hit wonder” originally used for music bands known for just one song but it also applies to the games since there are many cases when some studio made a single game and sometimes those games use video format designed specifically for that game.
Even if it’s usually RLE/LZ77/VQ/quadtree, sometimes you get something more interesting.
I still can’t get VMIX decoder DC decoding correct, I feel rusty.
Change activity then and return to it later with clear head and maybe analyse that piece of code from scratch. That helped me in the past (not always though).