Archive for June, 2012

Some Notes on Un-RE’d Codecs

Saturday, June 23rd, 2012

If I haven’t REd a codec that doesn’t mean I haven’t looked at them at all.
So today I want to talk a bit about some un-REd codecs and what peculiarities they have.

Looks like that all interesting codecs can be divided into three groups: screen codecs, intermediate codecs and speech codecs.
Since I don’t understand the latter group I shan’t give details on it.

Screen codecs

We have lots of them and they can be divided into two categories: simple and monsters.
Simple codecs usually employ some standard data compression library (zlib, FastLZ, LZO or LZF) or Huffman coding with standard median prediction and interframe difference.
I.e. boring, let’s talk about monsters.

  • Windows Media Video 9 Screen (aka MSS2) — combines palettised regions coded like in its predecessor with WMV9-coded regions.
  • M$ Expressions Encoder Screen (aka Titanium Screen codec) — it uses variable-length codes and codes frames with one of two methods. One of them is DCT exactly the same as in M$ ATC Screen codec.
  • MSU Screen Lossless Codec — this one seems simply code R,G,B values with some arithmetic coder and lots of context modeling and prediction.
  • Go2Meeting codecs — a good demonstration of the fact that the best strategy against REing is employing shitty coding monkey.
    Version 4 of decoder was monolithic 8 MB .dll file, version 4 is 15 MB already, all in “fine” C++.
    There are two compression methods known.
    Version 2 employs some weird arithmetic coder substitution (suspiciously like ELS-coder by Wm.D. Withers).
    Version 3 employs libjpeg and zlib for coding image blocks somehow, frame data doesn’t look like it at all.

Intermediate codecs

Cineform — looks like they use Huffman coding and wavelets and it codes 10-bit video.

Fruit Intermediate Codec — looks a lot like its successor (ProRes) but with different bitstream format and fixed coding scheme instead of adaptive ones.

BitJazz SheerVideo — the main problem with it is that most of the codec code performs conversion between any of couple dozens of formats (8- and 10-bit YUV and RGB packed in any possible way). Actual decompression code gets lost somewhere.

Posted in Audio, Screen Codecs, Useless Rants | 2 Comments »

On reverse-engineering codecs

Saturday, June 9th, 2012

Sometimes I’m asked what codec I’ll do next. So here’s the answer: I don’t really know.

Since I’m no longer student I don’t have much time to dedicate to reverse-engineering.
Thus I work on different codecs time from time and select such codecs more or less in random.
When I see that it’s nearly completed I spend more time on it till I write a decoder.
That’s how I got RALF decoder, for example.
Or that’s how I have not got Discworld III BMV decoder: long time ago I’ve figured out container format and audio compression, a month ago I’ve advanced in video decoder; but it’s far from completion and I don’t know when I’ll work on it again.

Of course some factors affect my selection of codecs: if I have some interest in it, if it serves some theoretical purpose (e.g. I did Indeo Audio and RALF not because I needed them but to have respective families of codecs fully supported) or if somebody convinces me to do it (two words – GSoC 2007).

So if you ask for some decoder you might get it after a while (but no obligations unless you send me a box of Trocadero).
Don’t forget that samples should be present too (and decoder for the more complex formats).

Posted in Useless Rants | 3 Comments »

Some Notes on Indeo Audio (samples needed BTW)

Friday, June 1st, 2012

I’ve been working on this codec for a while and somewhat got it working.

Good news — it employs the same algorithms as its predecessor, except that it has stereo mode.

Bad news — it feeds slightly different values to those algorithms. So some tables used in calculations and number of free bits in the block (for allocation) differ. I’ve almost got it and hacked version of our IMC decoder outputs almost perfect sound. My suspicions are that it modifies original IMC tables for stereo mode case (since it codes audio in mid-side stereo mode it makes sense).

The problem is that there’s only one sample with this codec and it’s extremely short. So if someone has more files with Indeo Audio please provide them to us.

Posted in Audio | 3 Comments »