[mpeg2] Re: KFir MPEG2 editing

[Kees Cook <kees@outflux.net>]

GOPchop will do this for you.  I designed it using kfir streams, so I 
think most of the bugs are worked out.  Check:

gopchop.sourceforge.net


I haven't found a single decoder that actually pays attention to the 
"broken_link" flag.  I find this _highly_ irritating.  Since GOPs are 
optional headers, I assume that these decoders are just cutting corners 
and ignoring the GOPs.  To solve this problem in GOPchop, I would do two 
things: the "correct" thing, which is setting the "broken_link" flag, and 
then I actively remove all the B-frames from the video stream that follow 
the first I-frame.  This appears to totally do the trick.  I also set the 
"closed" bit, since now the GOP is closed.

Can you share your code with us?  In my most recent CVS, I wrote "mpegcat" 
which will spit out all kinds of information about an MPEG2-PS file, 
reporting locations, sizes, and flags of all the headers.

On Wed, Feb 06, 2002 at 02:12:21PM +0000, MAL wrote:
> This is a last call for aid...
> I am trying to cut sections from an MPEG2 generated by a KFir.
> By my understanding, the KFir uses bi-directional frame coding (IBP), as 
> opposed to progressive frame coding, which makes cutting the files at 
> clean boundaries very hard.
> I need to be able to cut a file into sections, (for burning to cd 
> progressively), and those sections need to be sequential without loss.
> 
> I have written a program to almost completely dissect an MPEG2, 
> (everything bar decoding the actual pictures), and output a section, but 
> regardless of whether I set the group of pictures' header's 
> 'broken_link' flag, I still get corrupt starts to the beginning of my 
> sections.  The closed_gop flag within the mpeg2 is set, meaning the fkir 
> doesn't encode cleanly cuttable group_of_pictures.
> 
> Does anyone know of a way I can "fix" the mpeg2 to have cleanly cuttable 
> boundaries, or will I need to re-encode the whole file?
> 
> _Any_ help on this subject would be great, I refuse to believe there is 
> no way to do it :)
> 
> Also, if anyone knows whether you can get the KFir to encode 
> differently, shout me pls.
> 
> All the best,
> 
> MAL
> Komcept Solutions Ltd.
> United Kingdom
> 

-- 
Kees Cook                                            @outflux.net