Archive for January, 2018

ClearVideo: Some Progress!

Sunday, January 21st, 2018

I don’t know whether it’s Sweden in general or just proper Swedish Trocadero but I’ve managed to clarify some things in ClearVideo codec.

One of the main problems is that binary specifications are full of cruft: thunks for (almost) every function in newer versions (it’s annoying) and generic containers with all stuff included (so you have lists with elements that have actual payload which are different kinds of classes—it was so annoying that I’ve managed to figure it all out just this week). Anyway, complaining about obscure and annoying binary specifications is fun but it does not give any gain, so let’s move to the actual new and clarified old information. Plus it has several different ways of coding information depending on various flags in extradata.

The codec has two modes: intra frames coded a la JPEG and inter frames that are coded with fractal transforms (and nothing else). Fractal frame is split into tiles of predefined size (that information is stored in extradata) and those tiles may be split into smaller blocks recursively. The information for one block is plane number, flags (most likely to show whether it should be split further), bias value (that should be added to the transformed block) and motion vector (a byte per component). The information is coded with static codebooks and it depends on the coding version and context (it’s one set for version 1, another for version 2 and completely different single codebook for version 6). Codebooks are stored in the resources of decoder wrapper, the same as with DCT coefficients tables.

Now, the extradata. After the copywrong string it actually has the information used in the decoding: picture size (again), flags, version, tile sizes and such. Fun thing is that this information is stored in 32-bit little-endian words for AVI but it uses big-endian words for RealMedia and probably MOV.

And the tables. There are two tables: CVLHUFF (single codebook definition) and HUFF (many codebooks). Both have similar format: first you have byte array for code lengths, then you have 16-bit array of actual codewords (or you can reconstruct them from code lengths the usual way—the shortest code is all zeroes and after that they increase) and finally you have 16-bit array of symbols (just bytes for case of 0x53 chunks in HUFF). The multiple codebook definition has 8-byte header and then codebook chunks in form [id byte][32-bit length in symbols][actual data], there are only 4 possible ID bytes (0xFF—empty table, 0x53—single byte for symbol, the rest is as described above). Those IDs correspond to the tables used to code 16-bit bias value, motion values (as a pair of bytes with possible escape value) and 8-bit flags value.

So, overall structure is more or less clear, underlying details can be verified with some debugging, and I hope to make ClearVideo decoder for NihAV this year. RMHD is still waiting 😉

Posted in Various Video Codecs | 2 Comments »

Popular Swedish Bus Routes

Tuesday, January 9th, 2018

Sweden has a lot of local bus routes and every region (or län) has its own most popular bus route:

  • for Stockholm and Örebro län it’s “Ej i trafik” (something like “not participating in public transit service”, “trafik” in Swedish often means both [car] traffic and public transport service);
  • for Södermanland it’s “Är ej i trafik” (“Is not in service”);
  • in Östergötland it’s “Tyvärr, ej i tjänst” (“Sorry, not in service”).

The joke is that while there are many numbered bus routes (hundreds in Stockholm län), the regulations make bus drivers rest after completing a route so quite often a bus arrives to the end station, unloads all passengers, changes its route number to the one above and goes away; then, obviously, another bus (or the same one after the driver has rested) comes to pick passengers. Since I almost never travel by bus in Germany (we have trams here after all), most of my bus trips happened in Ukraine and Sweden—and those countries differ in approach to drivers indeed.

Another interesting thing is the variety of buses: in Stockholm län you have buses going on trunk lines—quite often those are articulated buses and they’re always painted blue—and ordinary buses (always red); some buses are double-deckers, like on bus route 676 (Stockholm-Norrtälje) and some coaches are double-deckers too (I still fondly remember travelling on top floor of one from Luleå to Sundsvall—no fond memories about Ukrainian bus trips though). And in Norrland they still have skvaders (aka buses with additional cargo departments). Also buses in Stockholm län quite often have USB chargers for every seat and even WiFi—everything for passenger comfort.

It’s quite interesting that some bus routes are operated by two buses: for example, if I want to get from Bromma to Portugal (a place on Adelsö island near Stockholm) I’d take bus 312 which goes to Sjöangen, there I’d step out, get into new bus 312 waiting there while the previous bus goes to the rest. Also it’d travel on a ferry which I also like for some reason.

So there’s something interesting about Swedish buses after all. But railways are still much better (more comfort, higher speeds, less problems from car traffic etc etc) and definitely more awesome (I’ve witnessed rail bus pushing a fallen fir from the tracks less than a week ago—try finding an ordinary bus doing that). But it’s still nice to know that Sweden has good things beside people, trains, food, drinks and nature.

P.S. This seems to have gone a bit further than just describing how popular bus routes differ in various Swedish regions. Hopefully my upcoming NowABitClearerVideo post would go the same way.

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