Archive for April, 2025

A tale of three formats

Saturday, April 19th, 2025

Since I have nothing better to do, I keep looking at the odd formats here and there and occasionally do something about them. Here are three formats I took a look at recently and that have nothing in common beside being video codecs.

CGDI

This is a “capture” codec (aka Camcorder Video) if not for the fact that it records rather events than actual image data. I had a suspicion right from the start that it’s organised in the same way as WMF/EMF—opcodes with parameters that invoke GDI subsystem to draw actual image—and finally I’ve decided to check that.

Of course it turned out to be true. There are about 64 opcodes in total, some are for drawing things, some are for window management stuff, and some are for GDI state management (e.g. create or delete brush, pen, font and such).

Since implementing a decoder for it would mean replicating a good deal of Windows graphics subsystem (even if you can borrow code from Wine), I consider it completely impractical and merely improved codec documentation in The Wiki.

TCA

This is an animation format used on Acorn platform. Actually it has three layers of encapsulation: there’s raw TCA format that contains only the header and video frames, then there’s TCA wrapped in ACEF chunk with an optional SOUN chunk following it (no points for guessing what it contains), and finally there’s that format put inside ARMovie (as a single block).

I added its support to NihAV just for completeness sake. Of course not all of different flavours are supported (video is mostly just plain LZW but it has some alternative coding mode and an uncompressed alternative, audio is IMA ADPCM but sometimes it’s not without any reliable way to distinguish which is which). And looks like some animations may have variable frame rate (with DIR1 subchunk likely telling frame durations). All the details are there, in raw ARM binaries and semi-compiled BBC BASIC code, but I’m satisfied that it works at least for a couple of random plane samples I tried and have no desire to try supporting every known sample in existence.

Savage Warriors ANM

This one is a curious format. I’ve managed to locate decoding functions in one of the overlay files, it looked reasonable (LZ77 compression for intra frames and something a lot like FLI delta frame compression for the rest) but the decoder did not work properly. Curiously, demo version contains some of the same animations as the full game but in slightly different format (the initial magic is missing); after comparing them I found out that the release version uses a weird format with a 32-bit value inserted after each kilobyte of data. I ended up implementing my own buffered reader that loads those kilobyte blocks and skips those additional words for the release version.

Another thing is that LZ-compressed format had 17-byte header which the decoder skipped. Of course it made me suspect of being a third-party compression scheme, and after searching around it turned out to be Diet (you may remember it being used as an executable compressor but apparently it had other uses). It somewhat reminded me of MidiVid Lossless as it is yet another codec reusing third-party general compressor (with special preprocessing for executables, which was a dead giveaway).

In either case, both flavours of this ANM format are now supported by na_game_tool (and will be the part of the next release).

Posted in Game Video, Various Video Codecs | No Comments »

NihAV: even further encoder improvements

Wednesday, April 9th, 2025

Since the last time I wrote about it somebody actually decided to test my transcoder on a bunch of fringe formats. The main issue there was wrong audio output (since I didn’t bother to check the actual input format, it often passed e.g. floating-point audio as 16-bit PCM, resulting in complete garbage). Additionally I’ve fixed a stupid bug in Indeo IVF demuxer (so now both known samples are decoded fine) as well as improving support for some other corner cases.

And there was one fun failure only my transcoder could have: when video streams starts with skip frame(s) and ZMBV encoder was selected, it refused to encode. I’ve added a work-around so now it simply presumes that frame to be black and goes forth normally (if you encode into raw video, such initial frames would be skipped instead).

So whatever can be decoded seems to be decoded and re-encoded just fine (but there are probably more cases waiting for their turn to be discovered). And there are some more formats waiting for an open-source decoder (in case of Motion Pixels the wait is likely to be very long).

Posted in NihAV | 2 Comments »

na_game_tool: new season start

Sunday, April 6th, 2025

It’s been some time since na_game_tool release and I’ve finally started working on adding new formats for it again (for v0.4 the goal is to have a hundred of supported formats including a dozen of fresh ones, currently it has only about eighty). And since newly added formats are somewhat remarkable, I decided to talk about them.

Spidy Ani from Imperium Galactica. The name sounded vaguely familiar and indeed I’ve encountered it before. I did not bother to support old sprite format since they are just animations for which you need to know the background image and palette. Version 2 contains both palette and (optionally) audio, making its support much more reasonable. As for the compression, it actually keeps the old RLE method and introduces a new one (where low values mean copy length instead of a literal) plus it adds optional LZSS compression. The best part is that it’s not been reverse engineered by somebody else but it has been documented (in open-ig wiki) as well.

Shadowcaster cutscenes. This is a format used by a game published by Origin, it turned out to be RLE optionally further compressed by LZSS (I shan’t blame you if you see some similarities here). The main difference is that the format contains commands for e.g. playing an external XMI track (or stop playing it). And instead of one palette it can store several and switch between them arbitrarily.

Castles 2 (CD version) greyscale video. This one is very special: the game has colour cutscenes in Interplay MVE format (usually with castles) but beside that it apparently has some greyscale animations lifted from public domain films. The files have .m extension and start with “BYON PIZZA” string, definitely not hinting at Byon Garrabrant, who was the lead programmer of that game.

It was extremely hard to read the binary specification for it as the executable stored that code and data in executable overlay. That means that it was in a special part of executable that gets loaded on demand, thus lacking proper references to the data and even functions. Luckily it turned out that I didn’t need it at all.

One of the sequences turned out to be just one black (or white) frame. Seeing that the frame was exactly 1/4th of raw image and filled with 0x40, I decided to calculate a histogram of some other (larger) frame. It turned out that indeed if you output values in 0x00-0x3F range as is and repeat 0x40-0x7F values four times you get exactly full image. Then by looking at the reconstructed image I understood that it codes data in 2×2 blocks (and in inverse mode so 0x00 means white and 0x3F means black). After that figuring out that codes 0x80-0xFF in subsequent frames are used to signal skips was trivial.

That was fun. Hopefully I’ll encounter more interesting formats in the future.

P.S. I’ve also added PI-Video decoding support to NihAV. The codec was used at least in one multimedia encyclopaedia and did some non-conventional things with LZW coding. I’m still not sure that it was a good idea because codec developer may be russian but since I could not find any information about him (or the companies involved beside them being Western European), I’m giving it the benefit of the doubt.

Posted in Game Video, NihAV | 2 Comments »