Kostya's Boring Codec World

Last time I forgot to mention another obscure Russian adventure game called either Adventure History or Sin(d)bad’s Eighth Adventure which has quite interesting selection of video in its resources.

So it has some game sprites in FLIC format (most of the sprites are in its own custom format), half of the game size is occupied by clips from Soviet movie (the one featuring the same hero) in AVI with MJPEG format, and finally logo and ending are in the custom HSA format.

HSA is video-only format that is very simple: 32 bits – video width, 32 bits – video height, then you have video frames data prefixed with 32-bit size (0 means end of file) and followed by 768 bytes of VGA palette. And yes, each video frame has palette following it. And of course each frame is compressed independently (using unmodified classic LZSS.C).

At least that’s more variety than you usually get in the games.

Today I’d like to sum up my experience looking at various game video formats, none really finished or worth documenting. In case you wonder I’ve not played (or plan to play) either of these games, I just found a place with some adventure games and looked at them for anything not known to me (and below is what’s left after discarding games using Cinepak, Smacker or Truemotion).

First of all, SIFF format used by Beam Software. It turns out that in their game The Dame Was Loaded has various resources in this container format but even larger ones look more like special game resources than videos (think about .RBT vs .VMD in SCI32 games). But there’s Alien Earth with a newer video compression and audio—both 16-bit now. Since I was unable to find the code that plays it, I’ve found the details by studying the files. The compression seems to remain largely the same except that pixels are 16-bit, pattern coding has changed (and the patterns are coded in the binary) and old “palette present” flag is reused to signal that fill values take one byte instead of two (for black values it’s enough).

Now let’s look at various Cyberflix games like Dust: A Tale of the Wired West or Titanic: Adventure out of Time. From what I could find the .mov files found there are similar in the structure with other kinds of resource files. And even if the game explicitly demands 256-colour video mode I could not see anything resembling a palette inside those files. My conclusion is that they’re not real video files either but rather some game scripts and graphics.

And let’s end it with The Vampire Diaries. This is another game with video files without palette. It seems to store just video frames either uncompressed or compressed with LZSS scheme. It’s not interesting enough to dig further.

I guess this is it and I should finally return to writing proper NihAV video player.

Since I still have nothing better to do I keep looking at various formats in adventure games. And it turned out Prisoner of Ice has cutscenes in MUX format (that’s a stupid name in my opinion but what you can do now). IIRC there’s a newer version of Time Game that uses Smacker, maybe this game got a new release too but my interest was not in playing cutscenes but rather see how they’re packed. And the scheme turned out to be rather interesting.

Originally The Wiki said about it that it’s similar to HNM version 1 from Cryo (subtype 173 after investigating more thoroughly) and that’s all. As it turned out the ideas are very similar but the implementation details are sufficiently different (were they all inspired by the same book or code?).

So it turned out you have a format with 8-bit PCM audio and video that optionally employs simple LZ77-based scheme and small-codebook RLE. The main problem during REing was the way the code was written: where it reads nibbles it conditionally jumps into the middle of NOP instruction (that does a different thing then, isn’t x86 fun?); in other place you have self-modifying code, functions modifying three or four registers for return value and opcode tables. Still after figuring out some of those I’ve managed to decode all cutscenes from Prisoner of Ice, Time Gate and half of those from Chaos Control. The rest of the files in that game use a completely different method that I’ve not seen elsewhere.

Overall, this was still an interesting experience.

Since I don’t like my work to go completely to waste I decided to document the formats on a separate page in this blog.

I’m not sure these formats are that fitting to be documented in The Wiki since they’re in an archive and not standalone. But since I’m too lazy to find a proper wiki for game formats and register there I dumped it here instead.

In my last post I said something about video compression but now I have much clearer picture in my head so I’d better document it a bit.

So it turns out bitmaps and videos share the same compression methods (just 1 and 3, 0 and 5 are video-only) and they’re even more interesting than at the first glance.

• Compression 0 is simple stream with 8- or 16-bit opcodes with follow-up pixel data. Top two bits mean operation – skip, error, copy, repeat; next bit signals whether actual operation length is 5 or 13 bits; next bits are obviously operation size.
• Compression 1 is tiling compression. Frame data is split into three chunks with first two starting with chunk size. First chunk is bit flags telling which 40×40 tiles in the frame are updated (this chunk can be ignored). Second chunk contains its own tile dimensions and tile metadata (one bit signals that tile is coded, 3 additional bits tell tile type). The last chunk contains tile contents in form of pixels and pattern data. Tile can be skipped, filled with single colour, filled with raw pixels, or it can be filled with 2-16 colours (with all intermediate values allowed) using 1-4 bits as index.
• Compression 3 is LZ77-like and works on two lines at once. It reads a code using static codebook (one for intra-frames, another one for inter-frames), for code 0 is for literal (reads and output two pixel values, one for each line), all other code values are treated as copy length which is followed by offset (and in case of inter frame also direction bit telling if data should be copied from start to end or from end to start).
• Compression 5 is virtually the same as compression 1 but tile type takes 4 bits now to fit operation 8 (copy tile from the same place in some previous frame).

Indeed, this game turned out to have very original formats.

In my recent post about video in Russian quests I said that the Russian adventure games I looked at used standard video formats (except for one game employing custom-modified FLIC). Well, now I can say there’s one game with an original format.

Escape from the Haunted House as its English version is known (unlike the original Russian title which translates to Spectre of the Old Park—because the goal of the game is to escape haunted house naturally) is a Myst or 7th Guest clone which means it should use animations for scene transitions and it has lots of them.

Almost all game resources are stored in a single 600 MB file in a peculiar format: unlike the usual fixed-size record pointing at actual resources, here you have 16-bit tag followed by the tag-speficic data (may be none, may be 1-128 bytes, may be some arbitrary size declared in fixed-size header). And 90% of the data are animations.

Animations by themselves are rather peculiar. First you have a header starting with " AN 0.90" (or some other floating point number), then you have a palette, chunk table and actual chunks. Audio is 5-bit DPCM (one bit for difference sign and four bits to update step index) which actually codes sample as x[i] = x[i - 1] * 2 - x[i - 2] + delta, I don’t remember seeing audio coding like this even if it’s rather simple.

Video coding employs RLE and static codebooks (depending on coding method) and some second pass to output image data in specific form. Nothing really complicated but quite original.

The problem with binary specification was the way it’s written (it’s fun to force Ghidra to decompile a function which it turns back into data because the code is self-modifying), particularly first I managed to locate a function to create those IMV data archives and only then its counterpart for loading data from archive. And after extracting data I found which type corresponds to animations and which functions handle it.

At least this all turned out to be even more original than I was asking for.

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.