Re: Chapter marks not where specified.

On Tue, 03 Apr 2012 23:06:23 +0100, Tom Brock wrote:

>> What tools exist to analyse the structure of an mpeg2 file?
>> 
>> Well, mpeg2desc will show you the VOBU structure, at least. I guess
>> you want something which shows that as well as the GOPs.
> 
> Exactly.  I'm fairly sure what's going on in ffmpeg, given both the
> name of the option, '-force_key_frames', and the apparently truncated
> GOPs.  Something that shows the GOP/VOBU structure with and without
> forced key frames would be a smoking gun.

Interesting. There is already code in mpeg2desc to show info about
GOPs and related stuff, but unfortunately it’s looking in the wrong
place, so it will never display anything...

As for what FFmpeg is up to, the source file libavformat/mpegenc.c is
quite instructive: look at all the parts conditional on the “is_dvd”
flag. The frequency of output of NAV PACKs (marking the start of a
VOBU) is limited by the “vobu_start_pts” field to never occur more often
than every 0.4 of a second.

The occurrence of I-frames would be triggered by 2 things that I can
think of: the -force_key_frames option that you mentioned, and the
motion estimator. A quick look at the man page shows a bewildering
choice of motion-estimation algorithms and parameters for them that can
be tweaked—particularly suggestive ones are those with “threshold” in
their name. Have you tried playing with these?

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