Archive for the ‘VC-1’ Category

VC-1 interlaced: sine-wave expectations

Sunday, August 8th, 2010

Indeed, my expectations on completing at least field-interlaced VC-1 support can be represented as (possibly modulated) sine wave.

So here’s a short log of my mood during my attempts on implementing it:

  • initial state — no feelings at all
  • discovered old unfinished patch — mood goes up
  • tried to make it decode more than one field — fail, mood goes down
  • found out that first frame is actually composed of I and P field — mood goes up
  • looked at decoded picture — mood goes down
  • “that P-frame structure differs a bit but that’s all” — mood goes up
  • read about actual motion compensation routine for interframes and related bitreading —can you guess the consequences?

Some may argue this may be better represented with triangle or sawtooth wave though.

Seriously, now I understand why some people consider interlaced video to be evil. First of all, it’s an artefact of bandlimited era. Thus it adds unnecessary complexity to modern progressive systems.

I’m pretty sure there are people who will cry when they hear about interlaced coding and coded field order. There may be people who wince at word “telecine”. There may be H.264 interlaced modes (yes, several of them, MBAFF seems to be most popular) decoder implementers. Probably I’ll join one of those groups too.

Seriously, I consider adding interlaced mode (at least to some codecs) an offence against humanity.

I don’t see why interlaced decoding must differ from progressive one that much. Okay, we have two fields and we may need to select better reference for one of them. No problem. Select two references for motion vector prediction (which is described as several pages of blah-blah-code, yes, that comprehensible)? Gentlemen, include me out!

To make things worse they decided to complicate motion vector encoding along with prediction. Honestly, one should suspect that field MVs should be smaller due to fields having half of original picture height; in reality there is an additional group of bits read to augment motion vector. Why?

And a bit of icing. FFmpeg seems not to be adapted well for interlaced decoding. For instance, who knew that you should use picture->linesize[0] instead of mpegenccontext->linesize because the former will be used in calculating offsets for current block data and if you set mpegenccontext->picture_structure to say PIC_TOP_FIELD it will modify something for you? Not to mention how to deal with motion block data needed for reference (I honestly have no idea how well it will work).

Thus, I invite somebody able and fearless to finish this task. I don’t have any samples to interest me (for the reference, in the best times my DVD collection was around two or three discs, guess the number of Blu-Rays I own) and I found better ways to spend my time (and probably more interesting codecs as well).

P.S. Moving VDPAU chunks to dedicated AVHWAccel is also needed and is trivial even for somebody without deep FFmpeg knowledge.

Some notes on old WMV3

Friday, January 1st, 2010

Vad kan man göra på nyårsafton om han dryck inte alkohol och han är FFmpeg developer också? (Translation: what can man do on evening before New Year if he does not drink alcohol and he is FFmpeg developer?).

Naturally, I spent it hacking at some codec. One of the most annoying issues with it (at least for some people like me) is that it does not support some features, like interlaced VC-1 and decodes only I-frames in old version of WMV3. So I’ve tried to fix the latter.

There is a flag called RES_RTM which tells whether it’s final bitstream format (a.k.a. “release to manufacturer”, hence the name) or not. I’ve tried tracing its effect through binary decoder and it turned out that it only alters transform pattern decoding.

Here’s a brief outline of transform concept: WMV2, WMV3 and VC-1 may partition P or B blocks into 4×8, 8×4 or 4×4 subblocks; each subblock, obviously, may be coded or not, so decoder also needs to extract subblock coding pattern unless transform type specifies it (like “8×4 transform, left half coded”).

It turns out that older version uses different condition for subblock coding pattern reading (for 8×4 and 4×8 transforms only), without checking whether this is transform specified only for current block or for whole macroblock (i.e. group of 6 blocks). Quick hacking has not made FFmpeg decode those old file correctly though, so there is still some work left to figure out correct condition.

A Round Date

Thursday, August 14th, 2008

Today is the day when my first contribution to FFmpeg went to CVS.

You know that story – a curious programmer once decided to look how one of video codecs works, found out that was deflated MS RLE data expanded for bitdepths > 8bpp, wrote a decoder for it, adapted for ffmpeg, sent that decoder to one person and guess what that person did? Added necessary ties and committed it to FFmpeg CVS.

Well, since then I contribute to FFmpeg a bit of different stuff time from time. Plans for this year include AAC encoder, finishing RV3/4 decoder, pushing LucasArts SMUSH decoder to SVN, making Mike finish Xan4 decoder and some random stuff I’m not aware of now.

VC-1 test source

Friday, May 23rd, 2008

To my great surprise there are people working with reference decoder and asking for files with some features and digestable by it. Well, here is an ultimate answer – RCV muxer for FFmpeg (hey, writing a muxer and patching FFmpeg build system were two things I haven’t done yet).

You can get FFmpeg sources, apply this patch, compile and try ffmpeg -i file.wmv -vcodec copy out.rcv. Good luck!

BIG FAT WARNING. It is not guaranteed to work with reference decoder on all files. Try files with smaller dimensions.
Or hack reference decoder to ignore image dimensions (ffplay would play those files correctly while reference decoder complains on something about “image size is too big for this level”).

X8intra is there!

Saturday, November 10th, 2007

Now we have X8intra frames support in ffmpeg! Mike has already expressed his joy in his blog. I think anime fans who tried to play WMV3 in AVI would also be glad.

Why this has not been done earlier? Well, sheer disgust. The person who invented this scheme should be either fired or promoted to M$ CTO. Here is the list of reasons:

  • It is used for some keyframes coding so it cannot be skipped
  • Design is totally unlike anything standard
  • You should perform bitexact decoding. If your DCT produces slightly different results or you forgot about loop filtering then you won’t be able to decode picture properly. Hey, that’s utter crap by _any_ standard
  • It has made it to WMV3 too.

To put it mildly, X8Intra is an illegal offspring of JPEG and some early H.264 draft. It is mainly Huffman-coded 8×8 DCT-transformed blocks with spatial prediction and loop filtering. It does not have macroblocks like decent codecs. Spatial prediction has some directions and relies on previously decoded blocks and bits read depending on that. I have not seen anything with such a bad bitstream-transform dependency (in other codecs you can decode coefficients first and then perform image reconstructions but not here). X8Intra excrementum est(pardon my Latin).

Still I very grateful to the person named “someone” who did this. Otherwise I should clean this cesspool.


Sunday, April 29th, 2007

Looks like the best way to collect a lot of samples is to become a developer and maintain demuxer/decoder. You may receive a lot of samples that do not decode with it 😉

I’d like to thank three major contributors (from Dania, Netherlands and Japan) of VC-1 in HD-DVD/Blu-Ray samples. It is because of them VC-1 video playback is less buggy than it was before (I know it still is but at least it’s improving).

VC-1: Some Bugs Squashed

Sunday, March 25th, 2007

This and last weekend I fixed some bugs in my decoder so now watching HD-DVD/BluRay movies will be a bit more pleasant ;-). But for the whole watching and listening experience you should wait for E-AC3 decoder (maybe it will be implemented during Google Summer of Code).

As for me, my systems neither are fast enough to decode it in realtime (1.42Ghz PPC G4 and PII 266Mhz) nor can even display full picture (max. resolution is 1280×1024 but I’m fine with it). So you may conclude I’m not interested in watching HD and you will be right. But I care for decoder and will fix bugs in it as I do already.

VC-1: Complex Profile is supported a bit

Wednesday, February 28th, 2007

I’ve just added some support for WMV3 Complex Profile (aka old Advanced Profile).
I don’t know why most samples (users complaining about) I’ve met are anime. It’s a good chance they will work now.

And here is updated list of CP features explained:

  • RES_X8 – additional bit at the end of I-frame header pointing if this is normal I-frame or this is special coded frame (I’m sure that’s exactly WMV2/8 J-frame with different header)
  • RES_FASTTX – somehow interferes with P-frames but bitstream is not changed
  • RES_RTM – old P-frame format (header should be the same but some additions on macroblock level). This is also quite common in old WMV3 files. Not RE’d yet

VC-1: Blu-Ray is now supported too

Saturday, February 10th, 2007

Now FFmpeg and MPlayer support VC-1 in MPEG2-TS streams which can be found on Blu-Ray discs.

Thanks to Martin for providing sample and Nico Sabbi for implementing handling of this in both FFmpeg and MPlayer.

HD-DVD decoding

Wednesday, February 7th, 2007

For those whom it may interest.
What are the things needed to play HD-DVD movies in open-source multimedia player?

  • Decrypted movie (use this tool)
  • Demuxer support
  • Video decoder
  • Audio decoder

EVO demuxer is really just slightly modified MPEG stream demuxer. FFmpeg and MPlayer fresh SVN versions support it (MPlayer was there first). I don’t know about other multimedia players but that’s quite easy to implement.

Video decoding: there are three possible choices – MPEG-2, H.264 and VC-1. First two should work fine, the last one should work but with some bugs (I will fix them eventually).

Audio decoding: is also tricky – standard AC3 audio will work, DTS is supported via libdca and E-AC3 is still to be figured out.

Conclusion – try current SVN of MPlayer and see if it works to you. And if you care about Blu-Ray support (and it does not work) – please provide a sample.