[x264-devel] Re: AW: Re: Error Concealment

[Ryan Dalzell]

On Wednesday 17 January 2007 12:38, List, Peter wrote:
> > -----Ursprüngliche Nachricht-----
> > Von: x264-devel-bounce at videolan.org [mailto:x264-devel-
> > bounce at videolan.org] Im Auftrag von Ryan Dalzell
> > Gesendet: Mittwoch, 17. Januar 2007 13:28
> > An: x264-devel at videolan.org
> >
> > Betreff: [x264-devel] Re: Error Concealment
> >
> > On Wednesday 17 January 2007 09:53, Awadh Bihari wrote:
> > > Please let me know how to detect the slice loss at the decoder side.
> > > It can only be detected if active sps/pps is not equal to sps/pps ?
> > Due to arbitrary slice order and the fact that slice headers do not
> > tell
> > you how many macroblocks are in the slice (and assuming you are
> > receiving
> > the video data over a network), I believe that the only way is to
> > actually
> > decode each slice and count the number of macroblocks that it
> > contributes
> > to a picture. Then, after a suitable period of time to allow late
> > packets
> > to arrive, if you still haven't decoded all the macroblocks of a
> > picture
> > then you have lost some slices.
>
> I dont see why you need to count anything. The slice always tells you
> where it's locates in the frame, so you just dump the macroblocks to
> that place. If you have arbitrary slice order, things get more difficult,
> but in principle it should be the same. You should be able to detect
> when the next frame starts (by POC and/or frame_number). If that happens
> you can finalize the previous frame, and that's it. Am I missing
> something
> here?

Yes, I think you are. Let's say that your receive six slices for a picture 
before a slice with a different POC arrives. Is this all the slices for 
that picture, or are there supposed to be seven and one got lost? You can't 
tell, short of decoding all the slices and counting the number of 
macroblocks provided.

This gets even worse when slices can be out of order across picture 
boundaries. I have searched high and low the spec and I cannot find a 
statement that precludes this from happening. (Of course, due to the lack 
of plain English in the spec it could be there.) Also this could happen 
when packets become reordered due to network jitter or taking different 
routes. Then you don't even know that you have the last slice you are going 
to get when the POC changes. So you have to wait a while to see if there 
are more packets to arrive containing slices for the current picture.

IMO, it's pretty horrible and ASO is a good example of a little extra 
flexibility with not much real value but with a high cost in real world 
systems.

Cheers,
-Ryan

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