[FFmpeg-soc] RTP packetizers

[Luca Barbato]

On 08/11/2010 10:28 AM, Martin Storsjö wrote:
> On Wed, 11 Aug 2010, Luca Abeni wrote:
> 
>> On 10/08/10 10:39, Martin Storsjö wrote:
>>
>> [...]
>>>> But I see that the "VP8 in RTP" draft is still under development... It
>>>> would
>>>> be nice if someone with some knowledge of the VP8 bitstream and a good RTP
>>>> understanding tries to make it smarter than the theora draft (by
>>>> suggesting to
>>>> fragment frames in appropriate points, so that the effect of lost packets
>>>> can
>>>> be minimised).
>>>
>>> Well, the issue there seems to be that you can't decode anything from the
>>> rest of the GOP if you miss the first data partition of any frame
>> Well, I do not know theora and VP8, but for mpeg{1,2,4} and for H.264 I think
>> this is not true... If a frame is composed by multiple slices (or NALs, or
>> similar things), then missing one slice (or NAL, or similar) will not be
>> critical for the whole frame (and for the whole GOP) - assuming that some
>> error concealment techniques are used in the decoder. You will (of course) see
>> artefacts in the video, but it will be decoded. So, having a "smart
>> fragmentation" in RTP (that is, fragmenting frames according to slices, NALs,
>> etc...) will improve error resilience.
> 
> 
> To quote David Conrad from earlier in the discussion on this:
> 
>> VP8's probability contexts and segment map aren't reset between 
>> interframes, so if an entire frame is lost the rest of the GOP will 
>> likely decode to total junk.
>>
>> If you want to do better than the proposal, you can read the vp8 frame 
>> header and ignore the rfc if the first data partition is intact; that's 
>> the only guarantee that arithmetic probabilities and the segment map 
>> remain correct.
> 
> And my tests with it showed the same - if anything else than the first 
> packet of a frame is broken, the frame itself might not decode totally 
> correct, but following frames are ok at least. If the first packet of a 
> frame is broken/missing, the whole rest GOP just becomes garbage.
> 
> 
>> Now, if a VP8 frame can be composed by something similar to NALs, this feature
>> should be exploited in RTP fragmentation (so, the RTP muxer should split the
>> frame dividing it in these small units, as the H.264 and the MPEG packetisers
>> do, etc...). I do not know if VP8 is done like this or not, but fortunately we
>> have experts, here :)
>> And since the draft is still being modified, I think it is worth rising this
>> issue.
> 
> Don't know if there's something such for VP8. David, Ronald?
> 
> // Martin
> 

we should move this discussion to upstream I think =)

lu

-- 

Luca Barbato
Gentoo/linux
http://dev.gentoo.org/~lu_zero