Archive for July, 2023

Why I work on NihAV

Sunday, July 30th, 2023

I started NihAV as a more or less toy project to play with different concepts and try new stuff like finding out how vector quantisation works or attempting to write an encoder. Having enough experience with libavcodec and libavformat, I did not want to touch them again (and still don’t) and there was a hope that rust-av will provide a viable albeit limited alternative for multimedia playback (it still hasn’t). In theory I’ve achieved my original goals—NihAV supports decoding a lot of exotic formats (some of which are not handled by any other open-source project), it even has some encoders and its own transcoder tool and there’s even two players (one for audio files, another one can also play videos). So I could relax and do something else entirely but yet I’m working on adding new features to NihAV that take a lot of effort and do not bring me joy. Why?


NihAV: updated for Rust 1.69

Thursday, July 27th, 2023

Since I had nothing better to do I decided to optimise my H.264 decoder a bit more, and that required a rather recent version of rustc that supports sym construct in asm!{} (so I can reference data tables in the inline assembly). Why this specific version though? I picked whatever was both recent enough to support the aforementioned feature (and older version had multiple micro version releases which hints on some problems with them) and not too recent either (again, I’m no beta tester of the compiler and I don’t need other shiny features).

And while at it I decided to make the code a bit more up to date. cargo-clippy is still annoying with its default warning about all-caps names and some lints that changed names and their suppressors no longer work. Getting rid of some leftover hints for the old versions of the compiler (like explicit drop()s for the objects borrowing code and some type hints) was nice though. Inline assembly is still only halfway done, especially considering that using const in it won’t be possible in stable for a long time and sym sucks compared to GCC inline assembly (it provides just a symbol name and you should magically know for yourself how the target platform works in order to make it possible to load it correctly; on AMD64 it’s rather simple but on aarch64 and on 32-bit ARMs that depends on target OS and PIC mode). Who would’ve thought that assembly may be platform-dependent! Looks like the current solution to that problem is to expose current configuration to the user so it’s up to you to check all environment variables and write the appropriate code. And of course even that solution will be available some time in the future since the developers haven’t thought about it at all.

Anyway, now my H.264 decoder features some more assembly optimisations and decodes video even faster than before. Though I fear it still takes too much CPU for the comfortable playback of my typical content so I’ll have to dabble in the hardware video acceleration. NihAV is a learning project after all.

Simple things

Thursday, July 20th, 2023

Simple things are usually the hardest to accept and follow. Here I’ll list the things considering the current situation and we’ll see how those will be understood by the world:

  1. There’s currently a world war going on. The definition of world war concerns mostly not the number of countries involved but rather that the goals of war (or its consequences) affect the world order in general. In this case if russia wins it means that the old system built on respecting country sovereignty and resolving international conflicts in peaceful manner via Useless Nations (formerly United Nations) does not work and any country with the nukes can do whatever it likes. And when the temporarily existing despicable mistake known as russia will lose the war, this may lead to its dissolution as well as making entities like UN and NATO reform or perish. And it’ll impact the future of China too;
  2. russia has demonstrated that it is a terrorist state countless times (trying to disrupt the world order by force is the very definition of international terrorism), but in addition to that it demonstrates that it’s not above the economic blackmailing as well. Just look at the recent development of the grain deal—it did not merely stop participating in it until its simple demand of fulfilling its countless demands is met but also started missile strikes at Ukrainian ports (again) and threatening to start a war with the countries that will keep participating in the grain deal without russia. And of course spewing obvious lies instead of saying directly that it’s racketeering;
  3. People who commit such crimes are either arrested and isolated in prisons or executed, so they can bring no harm to the society. Armed people (especially if they’re shooting during the arrest) are often shot on spot to eliminate the immediate danger (that’s not the best outcome but it’s an acceptable one). Countries should have the same treatment, out of self-preservation if not anything else (and stop point at nukes, russia demonstrated that it poses more nuclear threat when nothing is done about it);
  4. Speaking of isolation, it should be maintained airtight instead of trying to earn money while hoping that whatever russia does with your resources won’t be used against you later. I’m not so sure about the business risks of (usually French) companies that still have their russian subsidiaries operating as usual but if they suffer from the reputational losses in Europe and their businesses would be confiscated in russia, that would be a completely foreseen outcome. Also considering the current isolation and slow implosion of russian economy, it’s hard to tell what good the income earned there can bring (as you can’t transfer those money from russia and there’s risk of losing them entirely);
  5. When NATO talks about eliminating corruption as one of the demands for the candidates, it should serve an example and do something about the glaring example of Hungary. EU should take note as well.

Again, those are very simple things to understand but apparently not for the countries or large businesses. For now though, I find it ironic that I could travel with less restrictions and was significantly less ashamed of my country (and even its government) when I had Ukrainian citizenship than now when I’m a German citizen.

TM1 encoder: probably done

Wednesday, July 19th, 2023

After some trial I decided to release what I’ve done and probably not return to it ever again.

Currently my encoder can encode 15-bit TrueMotion 1 format with different block sizes. It’s probably not very adjustable but there’s not that much to adjust really. I’ll talk why I gave up on 24-bit mode (again!) below, for the other options here’s a condensed version: it does not matter. I’ve tried encoding files with an alternative delta set and it resulted in significantly worse picture quality (but at least encoded frames were usually larger as well); as I mentioned in the previous post, only the first codebook makes sense for 15-bit data (as other two codebooks waste space on coding delta value 7 which is not used in 15-bit mode). Inter mode uses simple skip block as I didn’t bother to think about the possible threshold but it works good enough anyway. In theory I could calculate gradients to determine what sub-block sizes to use for each frame (as I did in Indeo 3 encoder) but again, I decided not to bother.

Now, here are the reasons why 24-bit mode is much harder. For 15-bit mode you can easily calculate deltas for each (decorrelated) component independently rather easily—and the coding method allows selecting deltas in fine-grained way too. In 24-bit mode you have chroma delta pair that updates red and blue components and luma delta pair that updates red component with one value and green and blue components together with another value. In theory decorrelating just green and blue components should help but there we hit another issue: the amount of possible deltas is good enough to represent different delta values occurring during the prediction stage. Essentially you can’t process each component independently and should rather apply deltas as 32-bit values to the 32-bit pixel value, then unpack it and see that the individual components aren’t far enough from the desired ones. It is not that hard to implement but it essentially means writing a second TrueMotion 1 encoder that processes 24-bit data in an entirely different way. Considering its limited use and the fact that it shrinks down horizontal resolution in two times—the coduck (that’s their very original name for it) always processes blocks of two 32-bit words but now those are two 24-bit pixels instead of four 15-bit ones. In either case, even if I see how it should be solved I’m not going to actually do it.

I need to find myself a better task to undertake.

Restarting the work on TM1 encoder

Saturday, July 15th, 2023

Back in February I wrote about my failed attempt to write TrueMotion 1 encoder. And since I was bored and really had nothing better to do, I tried my hoof at it again.

Last time it was 24-bit encoding, now I tried to approach 15-bit encoding instead and got some results. I guess the moral of the story is that you should not overthink it and use the simplest approach to coding.

Looking at NUVision

Friday, July 7th, 2023

Since I still have nothing better to do, I decided to look at some obscure video codec I had laying around for really long time. And it turned out to be simple yet rather original.

Unlike many other codecs, this one codes YUV 4:2:2 (at least it looks like that) line per line in chunks of 24, 16 or 8 elements (essentially 24-pixel chunks plus shorter tail and the line width being a multiple of eight). Each chunk can be coded using one of four modes (leave it as is, decode and apply delta, copy chunk from the previous line with or without delta). Delta values are coded as chunk quantiser, delta values (-q, 0, q, escape) plus escape values. And since those mode/delta values can fit into two bits, they’re packed together into 16-bit words.

If you look at it, there’s nothing really inventive: short slices are present in many codecs, lossy delta coding is common too. But together they create a combination that I’ve not seen anywhere else. And that’s why looking at older codecs is pleasing: beside seeing rip-offs of the standard codecs sometimes you also encounter such original ideas as well.