Kostya's Boring Codec World

OK, Now B-frames for simple and main profile work. Now there are only some exotic features left to implement as spatial downsampling and sync markers. And a lot of RE’ing to implement old WMV3 P-frames decoding and complex profile.

As for advanced profile, B-frames are still not implemented.

For those who interested, here is a very experimental patch to enable decoding of WVC1 codec in FFmpeg. I’m currently searching and downloading sample files but it is already tested a bit.

Instructions:

1. Get FFmpeg sources (r614x should work)
2. Download and apply patch (cd ffmpeg; patch -p0 < vc1-advanced.patch)
3. Run configure and make

As always, non-working samples are welcome.

If you look in VC-1 standard specification it’s hard to find header specification for Simple/Main Profile (at least I was unable to do so in comittee draft available – only some hints in bitstream container format). Was that intensional or not?

FFmpeg old and not working vc9.c contained header parsing code and reference decoder does this too. This header contains a lot of ‘reserved’ flags which hidden meaning become clearer if you meet with them:

• RES_RTM_FLAG – should be set to ‘1’ but all old WMV3 files have it set to zero. For now you can call it ‘P-frames will be decoded correctly’ flag as old WMV3 has I-frames decoding identical to standard
• PROFILE=2. This is not allowed profile but in vc9.c it was called “Complex Profile” and really is Advanced Profile for old WMV3. I don’t know why but one sample was decoded when I changed profile value to “Main”, other Complex Profile samples produced garbage (but they also had RES_X8 and RES_FASTTX flags set).

There is also new M$ codec – WVC1 which is not decoded by reference decoder.

So here is a small list of huge tasks:

1. Find out how to decode P-frames when RES_RTM_FLAG=0 thus enabling decoding of many old WMV3 videos
2. Find out how to parse WVC1 movies
3. Support Complex Profile (not really important)

Some notes on forward transform.

For 4×4 transform matrix is:

17 17 17 17
22 10 -10 -22
17 - 17 - 17 17
10 -22 22 -10

And suggested norm is (8/289 ; 8/292; 8/289; 8/292)
The forward transform for (A B C D ) will be:

A1 = (17 * A + 17 * B + 17 * C + 17 * D) * 8 / 289;
B1 = (22* A + 10 * B - 10 * C - 22 * D) * 8 / 292;
C1 = (17 * A - 17 * B - 17 * C + 17 * D) * 8 / 289;
D1 = (10 * A - 22 * B + 22 * C - 10 * D) * 8 / 292;

Of course, don’t forget to round result.
I tested it on Octave and it proved to be correct.

Optimization is also straightforward: compute (A+D) , (B+C), (B-C) and use their sums/differencies.