Duck Control 1

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.
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Looking at random PAF

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

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

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

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

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.

ARMovie: Moving Blocks done

April 30th, 2024

I’ve finally finished Moving Blocks HQ and REing Super Moving Blocks. The main changes from vanilla Moving Blocks is a larger MV table (vectors can now target as far as 8 pixels instead of previous 4 and the fact that now raw luma samples use delta coding and static Huffman codebook. The only difference between two newer variations is that Super Moving Blocks uses 6-bit luma samples (and larger codebook for handling twice as many possible delta values).

With this part done, all is left is adding support for various Escape codecs as well as raw audio and video formats. And ACEFilm of course, but that’s a separate format.

A quick look at Eidos Escape codecs

April 28th, 2024

As people remember, Eidos started as a company offering video editing software for Acorn RISC machines (do not confuse with Acorn RISC Machines company), but later it merged with a game publisher (more than once) and now it’s a Japanese game company. Yet the codecs they developed survived for some time in the games.

Since I was looking at ARMovie and found decompressors for Escape 100 and 102 codecs (essentially the earliest in the series) I looked at those as well and here’s a brief overview.

Both codecs are almost identical in design (only the luma code seems to differ), they operate on 160×128 frames in 5-bit YUV420 format, operating on 2×2 blocks. Chroma values are picked from the table of 256 combined U/V values. The codecs code data as a variable-length code for the number of blocks to skip (the same code seems to have survived until Escape 124) and the following block update mode (luma and/or chroma). As I said before, chroma values are taken from the table or left intact from the previous frame; luma values are coded two 5-bit values and 3-bit mask telling which value to use for the block elements (first element in the block is always the first value of course).

And that’s it. No other modes, the only frame header is 32-bit word with codec ID (later revisions decided to add frame size and even flags that affect decoding process). Nevertheless it’s rather interesting to encounter codecs that use simple two-colour vector quantisation (and fixed codebook for chroma pairs) without any additional image transformation tricks (e.g. motion compensation beside skipping, or even allowing raw blocks).

ARMovie: towards NihAV support

April 27th, 2024

Since I had nothing better to do I’ve documented various ARMovie codecs in The Wiki. Essentially what’s left is to document newer Moving Blocks variants (and implement a decoder for one with samples), early Eidos Escape codecs (no idea where to find samples for them though). Also there’s Iota Software codec 500 which has no samples but looks like it’s used primarily in ACE Film format so figuring out nuances of LZW compression there may help with yet another obscure format.

From the technical side the most annoying thing is that data is stored in multiple frames in a single chunk without any kind of size, separator or frame end marker being present. The original player simply expected decoders to report number of bytes consumed after decoding a frame. Which means packetisers that should parse the stream in the same way as decoders do and report the frame size—nothing too complex but still annoying (and I had to augment NAPacketiser API to take reported stream details into account, otherwise it has no way to know frame dimensions). The worst one was Moving Blocks parser. The main problem is that unlike most of the formats this one has frames not aligned to 16 bits—while video chunks are—so it has to deal with possible padding byte at the end of chunk.

Another annoying thing I haven’t really dealt with is detecting all those various sound formats from the format string.

I don’t know if I bother adding various raw video formats support but in either case I don’t regret looking at ARMovie. Even simple codecs turned out to be not so simple and sometimes with interesting peculiarities like run of colour pairs and even four-colour pattern painting in what should’ve been a simple RLE codec.

Acorn Moving Blocks variants

April 24th, 2024

Implementing decoders in NihAV goes slowly since ARMovie essentially stores track data all clumped together and requires a packetiser to split it into video frames. Of course it can always get worse: Actimagine VX stored audio data right after video frame data without storing video part size so you can decode audio only after you’ve decoded video part of the frame. Anyway, I’ve managed to hack a demuxer and even a working Moving Lines decoder. Before tidying it all up and implementing other decoders, I want to give an overview of other Acorn video codecs.

Surprisingly enough, Moving Blocks has some documentation (from the authoritative source nonetheless!). Of course it contains some minor mistakes, mostly some values in motion vectors table (some values have their minus sign forgotten and the order or some values for spacial copy vectors is wrong).

For those who’re simultaneously too lazy to read the description and curious enough to know how the codec functions, it’s simple: frames are split into 4×4 blocks, each block has a variable-length code for its coding mode—raw data (subsampled YUV apparently), motion compensated block (with a variable-length motion vector pointing to a previous frame or an already decoded part of the current frame) or split into 2×2 blocks with either raw or motion-compensated mode.

But that’s not all, the codec got development as Moving Blocks HQ. I have not fully analysed it yet but the main changes that now motion vector table is four times larger and includes extended range motion vectors, there’s no longer raw mode for 4×4 blocks (it got replaced with a dedicated skip mode) and raw 2×2 blocks use static Huffman coding (or at least it looks like that; also maybe it uses delta coding but I’m really unsure about that).

And of course they did not stop on that and created Super Moving Blocks. From what I can see it has a larger Huffman codebook plus slightly different modes (like skip modes for 2×2 blocks). I was unable to locate samples for it so it’s a theoretical exercise.

Apparently there was also yet another attempt called Moving Blocks Beta but since I could not find a decoder for it, we can only speculate what changes it had.

At least there will be something to document on The Wiki when I’m done with all this.

Update: now as I have a working decoder, I can say that I was wrong about some things in Moving Blocks. The specification is correct except for some motion values. As for the later revisions of the codec, I’ll postpone them as REing them will involve manual decompilation. Luckily for me there are more codecs to look at.