Re: Chapter marks not where specified.

Hi Lawrence,

> 
> > It seems that, when ffmpeg is writing out a VOBU that contains a
> > forced key frame, it simply truncates that VOBU (the shortest I've
> > seen contains only 1 frame) and starts a new one with the forced key
> > frame.  This new frame does not simply contain the remainder of the
> > frames from the truncated VOBU, but a full gop (12 frames in my case).
> 
> I was going to point out that VOBUs have no actual length, they simply
> begin with a NAV PACK and implicitly continue until the start of the
> next VOBU.
> 
> > My years-old "Unofficial DVD Reference" says that if a VOBU contains any
> > stream data, then it must contains at least 400 ms.  1 frame hardly
> > qualifies on that score. Could it be the case that when dvdauthor
> > sees a VOBU, it assumes (as perhaps it should be able to do) that it
> > will contain at least 10 frames (of PAL) and so can "know" that the
> > VOBU contains a chapter-marker frame that is actually at the start of
> > the next VOBU.
> 
> I don’t recall any assumption in the dvdauthor code about the
> permissible length of a VOBU. It just goes by the NAV PACKs or (in
> their absence) the I-frames.
> 
> It seems to me the behaviour you're observing can be easily explained
> by VOBUs containing more than one GOP, due to an excess of I-frames.

That was where I'd got to a couple of days ago, since it would indeed explain 
everything, and it would all be ffmpeg's fault for not realizing that a seek 
point must be a VOBU start, which is why I wanted to see exactly what ffmpeg 
was generating.  It tuns out that I was, again, misinterpreting the avidemux 
index file: it is indeed reporting GOPS, not VOBUs. It actually says "Ngop" at 
the top, which might have been a clue, if I'd been properly awake.  All of 
which raises an obvious question.  What tools exist to analyse the structure 
of an mpeg2 file?

Tom.

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