Re: [mpeg2-dev] Fwd: segfault on bad pictures / low link power

[Keith W. <ke...@MI...>]

Hi İşbaran,

That video doesn't crash mpeg2dec or libmpeg2. It does have a heck of a 
lot of corruption! And it is unusual in that it's encoded using field 
pictures instead of frame pictures.

As a practical matter, you need to reconsider how you're sending this 
stuff if you want to get usable video. You need forward error correction, 
retransmissions (like what TCP would give you), more link power per bit, 
or something.

On the theory for the crash, my hunch is that VLC is not engineered to be 
robust against streams that use field pictures and don't observe the 
required field sequence. (E.g., you have to follow a top-field picture 
with a bottom-field picture, and this stream has so much invalid data that 
it doesn't always do that.)

Why are you using field pictures anyway? This is a pretty nasty stream and 
it is going to touch a lot of code paths that free software is not well 
tested on.

Anyway, as a formal matter, mpeg2dec seems to be handling this properly 
(and not crashing). If you want to stop VLC from crashing, my guess is you 
need to switch to frame pictures. (You can still encode interlaced video 
this way, of course!) And if you actually want usable video, you need some 
of the practical improvements above...

Happy hunting,
Keith

On Tue, 22 Sep 2009, İşbaran Akçayır wrote:

> 2009/9/8 Keith Winstein <ke...@mi...>
>       Hello İşbaran,
> 
> 
> Helloes again,
>
>       If you feed libmpeg2 bad data (e.g., if you're dropping
>       packets and not replacing them), you will certainly see green
>       blotches -- that's generally what you see when there's
>       missing intraframe data.
>
>       (There are more sophisticated ways of doing error concealment
>       in the presence of unreliable transport, but this is the
>       basic behavior.)
>
>       You shouldn't be able to make libmpeg2 segfault, though -- it
>       should simply detect the invalid elementary stream. Can you
>       capture the exact data you're feeding into it (after the UDP
>       reception) and find a replicable segfault? Then we can debug
>       it. Otherwise, no offense, it's probably a bug in VLC. :-)
> 
> 
> It's been a while, sorry, i didnt have time for video stuff, but now i
> captured some video that causes crash with vlc, splitted it so you can
> instantly see the crash :)
> 
> Can you please check if you can reproduce this crash, and comment on what
> is causing this ?
> 
> http://www.ish.kodzilla.org/minimum-crash.mpeg
>
>       Best,
>       Keith
> 
> 
> Thank you, best regards
> işbaran
> 
> ps: dammit i first forgot to add video, than to "reply all" :S
> 
>