Re: Chapter marks not where specified.

Hi Scott,
> 
> Chapter (really, any) boundary must be on a VOBU boundary.  VOBU
> boundary should be at an I frame.  There shouldn't be more than one I
> frame per VOBU, if so, your multiplexer sucks (use mplex from
> mjpegtools -- the one I always tested/developed with).

I think you mean one gop per frame, but, on the basis of what I've found and 
read, that is not a requirement: it is simply typically the case.  A VOBU must 
contain between 0.4 and 1 second of playback.  In one case, I've seen ffmpeg 
produce 5 I-frames in 0.32 second, and there's no indication that it thereby 
violates the DVD standard.

> 
> Other "shoulds" include 2 B frames per P frame, minimum GOP length of
> (I think) 9 frames, and max of (I think) 18 frames -- though that
> might be NTSC dependent.

"should" and "must" are not at all the same thing.  My point is that dvdauthor 
allows the specification of chapter marker times, but evidently does not 
always apply them.  It may be that dvdauthor cannot place the markers as 
specified if they do not align with VOBU boundaries in the input file, in 
which case, it should, at the very least, warn that it has not been able to 
comply with its instructions.  On the other hand it may be that dvdauthor is 
intended to be capable of adjusting VOBU boundaries when necessary, so as to 
align with chapter markers, but fails, for reasons at present unknown, to do 
so, in which case it should be fixed.

dvdauthor is an excellent tool that I've been using for years, but it seems 
there's a disconnect between an advertised feature and the ability to deliver 
it.

> 
> (having the right number of B frames helps with hardware decoders that
> won't FF/REW well if you only have I/P frames)

True, but it doesn't help me put chapter marks where I want :-)

Tom.




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