Re: Chapter marks not where specified.

On Sun, 01 Apr 2012 10:38:08 +0100, Tom Brock wrote:

> 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.

> Nevertheless, the truncated VOBUs properly declare the number of
> frames they contain ...

I don’t know of any such field containing a number of frames.

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