Archive for May, 2024

All of legendary animation formats

Friday, May 31st, 2024

Since I’m still not in the mood to do something serious, I decided to play some adventure games from Legend Entertainment (you know, parser-based, mostly with EGA graphics). And some of their VGA games like Companions of Xanth or Eric the Unready contain full-screen cutscenes in unknown format. The earlier released Gateway II used the standard FLIC for many of those, SVGA games switched to .Q format—but nothing obvious for these ones. Blackstone Chronicles used QuickTime and thus is not interesting.

So I decided to look what ScummVM source code has to say about it (unrelated fun fact: it’s the only open-source project I donated some money to). Of course there’s a fork with some halfway done support of later Legend Entertainment adventure games, including its picture format support (no such luck for EGA-only games, it seems).

Apparently .PIC files are more a collection of sprites and even full backgrounds. Depending on the game one file may contain all room backgrounds, or just single area backgrounds plus related animations (e.g. flowing water or burning torches), or it may be character sprites, or—as one should expect—a cutscene.

Frames in .PIC may be full frames or delta frames. In the later case they only update a part of the screen (either by replacing an area or XORing it). The more interesting thing is how frame data is compressed. The reference code is reverse-engineered and not so informative, so it took some time to understand it. First apparently there are tables and code used to generate new tables, which turned out to be exactly what I suspected them to be—Huffman codebooks. After a bit more messing with the algorithm, it turned out to be yet another LZ77-based compressor with static codebooks and rather complicated coding that reminds me of deflate… Of course I got suspicious at this stage (it looked a bit more complex than in-house developed compression schemes for game engines usually are) and indeed, it turned out to be Pkware Data Compression Library. And I could’ve found that out simply by looking into strings in one of the overlay files. Oh well…

At least it’s yet another puzzle with formats solved. Also it’s the second time I recently encounter animation format using DCL for compression (previously it was Gold Disk animation). Which makes me wonder what other common LZ77 flavours were used in the animation and video formats. deflate (along with newer contenders like LZO, FLZ and such) is very common in screen-recording codecs. LZS was used in Sierra games .RBT, I vaguely remember RNC ProPack being used by some video format. Nightlong on Amiga used PowerPacker. Did anything use LZX (it came from Amiga before being bought by M$ so maybe it had a chance there)? LZRW? Homebrew schemes are dime a dozen (like all those LZSS variations), I wonder more about the (de facto) standard compression libraries being used in video compression. Anyway, I’ll document them as I encounter them 😉

Duck Control 1

Saturday, May 18th, 2024

Back in the day there was no Bob but he had toys nevertheless. And of those toys for Bob was Star Control II. It is not a game for me to play (since my reaction time is not good enough for space battles), its 3DO version mentions “TrueMotion “S” Video Compression by The Duck Corporation” in the credits. Now that’s more my thing! I looked at it once, found it to be a lot like TrueMotion 1 but split into different files and looked no further.

But recently I got a curious request: apparently some people want to reuse the player code out of the game for playing videos on the console. And since the console is too old to support even micro-transactions (despite coming from the EA founder) let alone FullHD AV1, they look for something more fitting, which is mostly TrueMotion and Cinepak. As I remember, this format offers a bit more flexibility than the usual TM1 in some aspects (since the tables are transmitted in the video data instead of being stored in the decoder), so writing an encoder for it may be more interesting than it was for plain TM1.

Anyway, here I’d like to talk about this technology and how it differs from the conventional TrueMotion 1.

Looking at random PAF

Thursday, May 16th, 2024

So apparently there’s RIFF-inspired PAF format used in Bi-Fi racing game for DOS (essentially a promotional game for a local brand for sausage-in-a-roll snack).

It’s nothing to write a Multimedia Wiki article about but it’s somewhat interesting. Apparently it has one single CODE chunk that contains commands for updating frame—for all frames. It is pairs of (skip,copy) bytes that tell how many quads of bytes should be skipped on output or updated from the delta frames.

Delta frames are RLE-packed data in DLTA chunks, first 32 bits tell the unpacked data size, then it’s traditional RLE (top bit clear—copy data, top bit set—repeat next byte low seven bits times).

Apparently those files were intended to be used as an animated TV screen overlay on the jukebox background (maybe also for an intro but the CD-rip of the game didn’t have it). So on one hand it’s a mediocre format, on the other it’s somewhat interesting anyway.

Next I should explore the usage of a Duck codec on “iamaduck” console…

Oh No! More Amiga Formats

Sunday, May 12th, 2024

I keep looking at those, no significant progress yet but here’s what I have got so far:

  • ClariSSA—I’ve managed to locate two routines in ssa.library that look like video decompression routines (one is more advanced version of the other SSA with over a hundred of opcodes, another one is a simple RLE) but I still don’t know how they relate to e.g. BEST or COST chunks inside IFF. Update: apparently it separates data like opcodes or plane values into those chunks, I’ll write in more details about it later;
  • DeluxeVideo—haven’t looked at it that closely but it resembles more of an animation system (e.g. Macromedia Flash) than compressed video frames. Update: that’s because it is, the file essentially contains references to the actual sprites and backgrounds plus the commands and effects to apply on them;
  • TheDirector Film—the player for the only known sample uses some LZ-based cruncher but I’m too lazy to spend time on reconstructing the unpacked executable (or mess with UAE let alone unpacking and powering on my expensive door-stopper with AmigaOS 4.1);
  • Magic Lantern Diff—nothing much to say, looks doable. Update: it turned out to be RLE-based and I’m not sure but it might be coding data vertically like Zoetrope did;
  • NTitler—this is essentially a binary script for the program, referring to external resources for everything and such. Not something particularly interesting or fun to look at.

And there you are. After I do what I can with these formats (though as you can see for two of them it’s “not worth looking further” resolution already) I’ll move to something non-Amiga entirely.

Looking at Adorage SSA

Wednesday, May 8th, 2024

So I’m still looking at the list of unsupported video formats from dexvert in hope something curious comes by.

First, out of curiosity, I looked at the unsupported Delphine Software CIN files. Apparently they merely don’t have audio streams and contain garbage in an audio part of the header (plus they have 1-2 video frames). Ignoring the audio header makes them decode. Next.

Second, EA MAD file—apparently it comes from a game rip with audio and video files being dummied out so it’s easier to share. Nothing to look here in any sense. Next.

So, one of SSA formats (no, not that one SSA animation format for Amiga, another one). It turned out to be rather complicated RLE for bit-plane coding. I.e. frames are coded as several bit-planes with the codec commands essentially being something like “skip to this offset in the current bit-plane, then update following 16-bit values with these new values, then skip a bit more and update some more values then skip…”. There are different opcodes for run/copy of certain sizes (or ranges) plus some additional operations for repeating a pair of values (and I thought LinePack was rather unique at that).

I actually ended up hacking a some decoder. It does not seem to handle 8-bit palettes well and there are still some glitches here (just look at the letter ‘D’ in the following image) but in works in principle.

From technical point of view it was not that hard. Even if Ghidra failed to decompile decoder function properly (mostly because of the opcode jumptable) it was nothing hard, even if I don’t know M68k assembly, expressions like move.l (A0)+,(A1)+ are rather intuitive so I could figure out which piece of code, say, copied 18 bytes and which one replicated the same 16-bit value to those 18 bytes.

I’ll try to find something else to look next, there’s still half a dozen of potentially interesting formats in the list.

ARMovie: trying codec wrappers

Tuesday, May 7th, 2024

I’ve managed to locate two ARMovie samples with video format 600 and 602 (which is M$ Video 1 and Cinepak correspondingly), so I got curious if I can decode them. The problem is that ARMovie stores video data clumped together in a large(r) chunks so you need to parse frame somehow in order to determine its length.

Luckily in this case the wrapped codec data has 16-byte header (first 32-bit word is either frame size or a special code for e.g. palette or a skipped frame, followed by some unknown value and real codec width and height) so packetising data was not that hard. The only problem was that Video 1 stream was 156×128 while the container declared it as 160×128 but after editing the header it worked fine.

Supporting such wrappers actually poses more of a question of design for NihAV—how to link those rather container-specific packetisers to a demuxer. Making demuxer parse all possible formats is rather stupid, my current solution of simply registering global packetiser and hoping there’s no need for another is a bit hacky. I should probably make it namespaced so that the code first probes e.g. “armovie/cinepak” packetiser before “cinepak” one but it’s an additional effort for no apparent gain. Speaking of which, I should probably change code to feed the stream provided by the packetiser to a decoder (instead of always using the one from demuxer) but since I’m lazy I’m not going to do that either.

Anyway, I’m not going to spend more time on ARMovie unless new samples for the formats I don’t support show up (beside newer Eidos Escape codecs which are supported elsewhere already). There are other formats left to look at. For example, I’ve made a good progress with Adorage animation format.

Looking at Ace Film

Friday, May 3rd, 2024

This is an old animation format on Acorn computers made by Ace Computing. It exists in several flavours.

There is bare Ace film that has barely any header (mostly just data size, title, some offsets) and chunks which start and end with the chunk size (so that you can seek backwards in the file). Then there’s chunked Ace film where all that data is wrapped into ACEF chunk with some other chunks following it (like RATE, FULL or PALE—the last one is for palette).

Presumably at offset 32 of raw data there is a compression method: 0, 1 or 2 (last one added in EuclidX). 0 means uncompressed data, 1 means LZW and 2 means unpacked data XORed with the previous frame.

Let’s take a closer look at method 1. It looks like a more or less conventional LZW. If the second byte of the stream is zero, the stream contains 16-bit little-endian LZW indices, otherwise it is variable-length LZW with initial index length being 9 bits (and value equal to 256) and growing up (the decoders don’t check upper limit but I suspect it should be around 16 bits). It also uses value 256 to signal the stream end and there are no other reserved values.

Apparently for convenience it decodes indices, stores them in the dictionary (it’s the property of LZW that each new index involves adding a new dictionary entry) and then decodes them backwards. After all, LZW dictionary entries are essentially stored in reverse so either you have to decode an entry, reverse and output it—or you can decode reverse string and output it at the end. This decoder chose the latter implementation (not that you can do it in any other way).

Unfortunately I’ve not managed to reconstruct data properly because of the implementation subtle details but I’m pretty sure anybody knowing how LZW works and willing to spend a bit of time fiddling with the format can come with a working decoder. As for me, it’s not interesting enough to spend more time on it. There are still several more ARMovie codecs to support and other formats to look at.