[vlc-devel] decoding behavior of VLC when splicing streams.

[hornsby]

Hi,
I would like to work on the decoder behaviour of VLC to enable it to handle 
well splicing of streams.
I developed a software that enable to splice of MPEG2 ts streams in real time.
The challenge here was to be able in real time to virtually reconstruct the 
display order out of the sending o2.12. Gig at the university (event starrder 
without buffering.
I came up with different conclusion on decoder behaviour.

1 - in case the stream structure is made of I and P frames things are very 
easy as the I frames are sent before P frames. So display order is the same 
as the sending order.

2- in case the stream consist of I, P and also B frames  things get more 
complicated.
Consider the following stream (M=3, N=12):

<--- P0 B1 B2 I3 B4 B5 P6 B7 B7 P9 B10 B11 P12 B13 B14 I15<---
             |-------------------------display order--------------|
         0              I3 B1 B2 P6 B4 B5 P9 B7 B8 P12 B10 B11 P15 B13 B24  
                       |--Encoding Order(GOP=picture 1 to 12)----|

the GOP contains pictures 1 to 12.
B-pictures 1 and 2 are encoded after I-picture 3, using P-picture 0 and 
I-picture 3 as reference. Note that B-pictures 13 and 14 are part of the next 
GOP because they are encoded after I-picture 15.


In case of a big time gap (or full GOP missing) VLC restart decoding at the 
first next I-picture (which is correct) but VLC seems to want to display the 
2 B-pictures (in the example, B1 and B2) based on the last P-pictures stored 
in VLC memory. The resulting display is a mix of pictures and squares 
appearing. VLC should indeed restart decoding at the first next I-picture but 
it also should drop the next B-pictures following the first I-picture (only 
if Time_reference of the B-pictures < Time_reference of I-picture)
Problems also appears in case of fields pictures. 

You can easily recreate the behaviour without splicing tool, only with 2 VLCs.
Setup one VLC as a server and another as Client.
From the server send a MPEG2 ts stream in multicast to the client. Then stop 
the server, wait a while and restart it. The client sometimes display huge 
artifacts and sometimes doesn't even restart.


I developed that behaviour handling in my splicing tool(dropping B-pictures 
after first next I-picture) and now VLC reacts perfectly without any 
artifacts.Now I would like to implement this on the decoder side because it 
would be easier. where could I look at ??

Thanks very much if you read that far.

Adrian



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