Kostya's Boring Codec World

This British company had developed quite an amount of good games itself and published even more. As I mentioned in one of my previous posts, I’d like to take a look at some of those (after both seeing their games mentioned in dexvert unsupported formats and realizing that I’ve unknowingly added support for some other games to na_game_tool already). Though I’ve REd the most important format of them all, BMV, long time ago from ScummVM code (their code is still slightly prettied direct conversion of assembly into C++).

Alien Breed

Since this game comes from a wormy developer, its ANM files for the PC version in reality are mere AVIs using KMVC. They don’t play with my old decoder so I’ll have to look what’s different in that version.

Chronicles of the Sword

Fun thing is that most of the game resource files have .PC extension regardless of the type and content. But here I’ll talk about SEQ/*.PC of course.

Those files expose rudimentary chunk structure i.e. they have three short chunks with some basic metadata and frame sizes in the beginning but that’s all. The rest seems to be frame data starting with frame dimensions and image data size (if VGA palette follows it seems to be unaccounted for).

I’d like to take a closer look at it but it’s too annoying: the executable uses CauseWay DOS Extender with packed 32-bit part so I either need to learn how to dump it or RE the extender code to see how it unpacks the code (the trick I used for the next game didn’t work). In either case I’ll postpone it until I’m sufficiently bored and really out of ideas what to do.

Microcosm

This games uses the simplest yet still rather weird codec: scalable RLE.

What do I mean by that? It employs several methods of compression, all are rather simple variations on RLE, except that some have “masked update” opcode beside the usual run/copy/skip. That opcode is a bit mask telling which ones of the following 7 or 15 pixels should be updated with new values or left intact.

But you probably wondered more about the “scalable” part of the description. Well, that format actually codes four 80×200 images per frame (each one may use its own method) and then interleaves the columns. And if you’re not satisfied with the spatial scalability only, it has temporal scalability as well: every even frame is coded independently (i.e. frame 2 updates frame 0, frame 4 updates frame 2 while frame 3 updates frame 1 and so on).

Also it stores palette in BRG order for some reason, which I don’t remember seeing in any other codec—unlike 2-pixel RLE (IIRC I was surprised to see such approach in some ARMovie codecs but it turned out to be not that rare after all).

This format was surprisingly hard to reverse engineer not merely because of its five different coding methods but also because its binary specification is somewhat inaccessible. The executable uses Phar Lap DOS Extender with compressed extended part. I did not know how to approach it until eventually I managed to use dosbox-x debugging facilities and dump what turned out to be the unpacked 32-bit part, which I could later load in Ghidra as raw 32-bit binary and find proper decompression routines. No such luck with the previous game because it seems to load the code into XMS and I don’t know if it’s possible to dump that part of memory at all…

In either case this codec will be supported in na_game_tool 0.2.0 and I can finally start forgetting about it (but not before documenting it in The Wiki).

Novastorm

This game uses a rather curious approach to the video coding—it treats all blocks as being coded with 16 colours plus mask and during video decoding it updates current colours (often by reusing them from some previous block) and mask. And then the draw routine uses those per-block colours and mask to reconstruct it.

Of course you can have fewer colours but it’s an interesting approach. “You want fill block? Just replicate this colour sixteen times. You want two-colour block? Just copy/read two colour values and (optionally) update mask using just 1 bit per cell. And yes, some opcodes tell you to read the full mask for the block while others tell you to read first a mask that tells you for which block rows the mask is actually transmitted.

What about inter coding? The skip mode is simple: you have a fixed-size array of flags telling you which block is coded and for which operations should be skipped.

So it’s a bit hairy to implement and I left that for later.

WipEout

This one is somewhat funny—it uses IF chunked format (i.e. just two bytes per chunk name and 16-bit chunk size) and seems to pack frame data with an LZ77 algorithm before actual video decompression kicks in. The scheme seems to operate on nibbles as some of the opcodes are “fill 32-bit word of output with 0x44444444”, “XOR 32-bit word of output with 0x00F00000”, “replace top/bottom three nibbles of 32-bit word with 0x777” and so on. Also there’s a post-decoding image permutation step (and in case of one mode, it seems to use raw image that gets pre-permuted just to permute it back to the original state in the end; I haven’t looked too closely but it looks suspicious).

Another curious thing is that it has a table of 32-bit values transmitted at the beginning of each frame with several opcodes to copy that value to the output.

Overall, it’s an interesting codec and I’d like to have decoder for it implemented for the upcoming na_game_tool 0.2.0 release but it’s too hairy for that. So it also goes to my “what to do when I’m bored and can’t think of anything better to do” list.

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As you can see, I have not done much but even a cursory glance at those formats show the variety of approaches to video coding that you cannot see in the modern codecs. And that’s why I like looking at old formats.

P.S. With the intended amount of decoders implemented I hope to finally release na_game_tool this weekend. There should be a follow-up post about it soon (and then probably just occasional rants for long time).

This entry was posted on Thursday, September 12th, 2024 at 1:12 pm and is filed under Game Video. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.