Kostya's Boring Codec World

Since I have nothing better to do, I decided to look at the formats used in various Tex Murphy games.

So here they are (the game from this millennium does not count):

• Mean Streets—it uses raw images in ILBM-inspired format (so that even some chunk names are still the same) grouped with e.g. palette files and packed in ZIP using ancient compression methods (shrink and reduce, I wonder if anything but ancient PKZIP.EXE can unpack them all);
• Martian Memorandum—the only VID file present looks like mostly raw image data interleaved with some metadata to tell which regions to update (though judging from ScummVM sources it may also employ RLE for sprites);
• Under a Killing Moon—this one has new format starting with 32-bit size followed by signature “PTF” and after 64-byte header I see the suspiciously familiar chunk structure: 32-bit size, 16-bit signature 0xF1FA that contain some sub-chunks of its own… I refer you to The Multimedia Mike, he should be able to recognize the format;
• The Pandora Directive—this one uses format that starts with “H2O” and seems to be Huffman-compressed RLE;
• Tex Murphy: Overseer—this one decided not to be creative and used Smacker files (along with some FLICs).

So, despite its nine-year history (again, only the games from last millennium) those games have changed the format used for video segments yet they have not moved further than RLE.

As most of you don’t know and don’t care, modern advanced video codecs may use the special prediction mode called “chroma from luma” where, as it’s obvious from the name, the chroma components are reconstructed from the luma using some coefficients. And what do you know, I’ve found a codec that used this approach back in 1997.

So there’s a French company called Kalisto Entertainment and back in the day it developed a codec for the cutscenes in some of its games (at least Dark Earth and Nightmare Creatures). 15-bit RGB video is split into three components and each is coded separately using simple LZ77-like method (i.e. it’s either RLE mode, or copy with an offset from the current or previous frame). The twist comes from the fact that those components are split into tiles (usually 20×20 ones) and each tile has coding mode and two sets of scale/offset coefficients, so for each tile one of RGB components is selected as the base one and two others are coded as the differences from the scaled (and offset) base value.

So one component plane contains the base components for each tile (which may be different for each tile) and the other two contain the differences for the predicted non-base components (which, at least in theory, should be mostly zeroes and thus better compressible). So when some people wonder if it’s time for video codecs to perform optimal component decorrelation on per-frame basis, here’s a practical codec from the last century that did it per-tile.

Since OSQ format requires obtaining a copy of some expensive software of unknown old version, I’ll leave it to somebody else. Meanwhile I’ve looked closer at the AVC format mentioned in the previous post and its relatives.

For those of you who don’t recognize the name immediately, Digital Pictures is the company responsible for some FMV-based action games, including the infamous Night Trap. As I rediscovered previously, the AVC files they use are really SGA files with compression method 0x81 but what about the other formats?

About half of the games I could look at contain an archive occupying most of the CD space, inside that archive are the same AVC files. Other half of the games usually has one or two AVC files with a company logo and one megamovie in various formats. And after some research it turned out to be the same 0x81 compression format but with audio data and varying headers.

And since nobody bothered to document it for The Wiki, I’ll explain it here.
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Since I have nothing better to do as usual, I decided to look at some game formats.

For instance, there’s a game called The Fuel Run promoting a product from a Swiss company supporting russian war crimes. This game has animations in VDO format. VDO file header starts with the string “Artform” and it employs RLE compression, each line of the frame prefixed with its size. What’s funny is that its RLE uses only bottom 6 bits for the run length and top bit is completely unused. For obvious reasons the format is not worthy documenting further.

Or there’s a game called Double Switch, this one has AVC videos. It uses RGB555 palette and 8×8 tiles that depending on opcode may be either coded with one of 25 predefined patterns and 1-7 colours or split into tiles of the smaller size. And only afterwards I decided to look into The Wiki and it seems to match SGA format description (except that this particular format variant is not documented). I don’t know if I should bother writing a decoder for it but with the lack of PC codecs to RE I might try my hoof at console ones instead.

So I decided to look at yet another game at archive.org with an FMV tag. This time it was The Lawnmower Man. It had NXL format that should be a rather unremarkable container with raw video and audio except for one fact: it is EGA full-motion video (320×200, 16 colours, custom palette per each frame). It is the first time I see video being stored in planar form (i.e. for each line first the lowest bits of each colour index value are stored, then bits 1, bits 2 and finally bits 3). Now consider that the format is big-endian (even if DOS version came out reportedly earlier than Mac or SEGA CD versions) and it feels positively weird.

Some of the details I bothered to document are at the usual place (somebody should keep The Wiki alive beside Mike after all).