Re: Chapter marks not where specified.

On Fri, 06 Apr 2012 08:12:46 +0100, Tom Brock wrote:

> Is this sufficient to demonstrate to the ffmpeg (and probably
> mplayer) developers what I believe is the case: that the
> '-force_key_frames' option in ffmpeg, while doing exactly what its
> name says it does, does not thereby fulfil the requirement stated in
> the rationale for its inclusion?
> 
> That rationale was, as written by Nicolas George, who wrote the patch:
> 
> "I want to use it to ensure that there is a seek point exactly at a
> place I will probably want to jump precisely sometime, the major
> example would be the end of an opening and beginning of the program
> proper. The scene change detection system will often make it happen,
> but not always if there is a fade-in."

I don’t think MPEG itself defines any restrictions on how often
I-frames are supposed to occur. Players of MPEG files should, in
principle, be able to start playing from any I-frame, assuming they
have some way to find them. They can even start playing from a
non-I-frame, assuming the user is willing to put up with partial
garbage on-screen until the next I-frame.

However, DVD-Video imposes additional restrictions on top of MPEG.
Seeking is done in units of whole VOBUs, and larger groupings built on
top of them (cells, programs, chapters, PGCs). There is a requirement
in the spec that VOBUs cannot be less than 0.4 seconds in length. FFmpeg
enforces this requirement, no matter how many I-frames you tell it to
insert.

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