Kostya's Boring Codec World

I have not written anything about NihAV for the last two months. If you thought it’s because nothing was done you’d be mostly correct (but not in thinking about the project at all—this is pointless). And yet there has been some progress recently (but before that I spent a whole month not doing anything NihAV-related).

First thing, I’ve finally split NihAV into smaller crates. This was done in order to reduce compilation and testing time for separate components. It started to feel a lot like FFmpeg and Libav times except that I don’t have to rerun configure and recompile everything in case I want to add a new decoder. Now codecs reside in standalone crates that contain just relevant decoders and demuxers so “write code—fail to compile—fix—run tests” cycle goes faster now.

And here’s the new structure:

• nihav-core — the core. It contains the definitions for basic stuff like packets and video/audio buffers, I/O routines (byte- and bitreader), DSP and common video stuff for H.263-based decoders;
• nihav-commonfmt — for various common formats. Currently it has all leftover codecs I could not move elsewhere (and AVI demuxer). Maybe in the future it will grow too large so I’ll split out some groups like speech codecs into new crates;
• nihav-duck — for Duck game codecs up to VP7;
• nihav-game — for various game codecs and containers;
• nihav-indeo — for Indeo video and audio codecs;
• nihav-rad — for totally RAD formats like Smacker and Bink;
• nihav-realmedia — full RealMedia support (except for common decoders in nihav-commonfmt);
• and finally nihav-allstuff — the crate that binds them all so you don’t have to search for decoders in separate crates.

Of course there’s nihav-tool to perform the actual decoding and test whether the code works but it’s always been a standalone crate.

This split required some changes in the decoder and demuxer handling. Previously I could get away with one table where all decoders (or demuxers) were registered and then search for an appropriate decoder there. Now it is not possible since there are many crates with individual codecs enabled or disabled in configuration. In result I had to introduce a structure called RegisteredDecoders and have crate-specific function for registering all decoders featured in crate in it. And nihav_allstuff::nihav_register_all_codecs() simply invokes them all for convenience. It’s exactly the same with demuxers too.

N.B.: I should probably write a separate post on overall NihAV design, missing parts and excuses for certain decisions.

Second, as you could spot from the crate names above, instead of trying to make NihAV do something useful (you have rust-av for that though) I decided to focus more on supporting various game formats.

So far I want to support the following formats:

1. Sierra VMD (developed by Coktel Vision but I care more for Sierra games). Should be an easy one-shot;
2. Discworld II and Noir BMV. I have added support for DW2 BMV format already (and it took a day to reimplement). DW3 BMV format is almost the same, DW3 audio is known but video codec still requires some reverse engineering. Also fun fact—DW2 BMV video decoder is based on reverse engineered decoder from ScummVM where they did not understand how decoding lengths works and the code still looks like a beautified assembly. For libavcodec decoder I tried my best to simplify the code and give better names to the variables. And when implementing the decoder for nihav-game I found out that “read byte, advance pointer, output nibble and save another one for later” leads to the same results but with a much cleaner code;
3. RAD codecs. I’ve reimplemented Smacker and Bink formats support (the worst part was NIH-y implementations for DCT-II/DCT-III used in Bink audio decoder but that’s a tale for another time). Maybe now I’ll finally write a decoder for Bink2 video;
4. Duck codecs. Yes, they are game formats that also were tried as general codecs but with lesser success. There are many games using TrueMotion 1, there are many games using TrueMotion 2, there are some games using TrueMotion VP3 and VP6 in their cutscenes. And the only their codec that got widespread is VP6. I want to implement all the family of the codecs that Duck produced before dying ascending to the higher plane of existence. While most of them are supported there are still missing features like sprites in TrueMotion 1, interlaced mode in VP6 or TM2X entirely. It requires some REing of course but that’s the appeal.

After that it would be nice to work on actual player for the supported formats, which opens a new can of worms like proper frame reordering and format conversion, but I guess I’ll have to deal with that one day.

Anyway, back to doing nothing.

This entry was posted on Wednesday, February 6th, 2019 at 2:00 pm and is filed under NihAV. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.