[FFmpeg-devel] [PATCH] avformat/movenc: Support for variable timescale in mov containers

[Kevin Wheatley]

On Tue, Nov 5, 2019 at 4:52 PM Michael Niedermayer
<michael at niedermayer.cc> wrote:
> Assuming this doesnt violate any specifications and assuming that
> it works with all players.
> Then it would make sense to select this value automatically based
> on the stream timestamps or timebases.
> maybe there could be still a -mov_timescale but with an option for
> "auto" to autoselect a small one which allows accurate representation
> of most streams

Michael,

thanks for the feedback, I did consider this, but I am not that
familiar with the code base of FFmpeg to know what the 'right' data
fields to use are and at what point during the initialisation process
of the movenc code it is valid to use them.

There certainly seams to be a number of points at which the code tries
to compute a lowest common multiple in the general FFmpeg code and a
number of fields representing timebases, frame rates etc.

If somebody can help point me at what I should use, then I'm happy to
add something to automatically determine something.

In terms of the specification, the Apple documentation certainly
recommends the use of 600 on p221 of
https://developer.apple.com/standards/qtff-2001.pdf for most integer
frame rates, but as the following section in that document says this
does not work for 23.976 or 29.97.

For newer frame rates like 48 and any other number of 'new' content
types, those number fail so whilst  I'd suggest using the Apple number
for the well used rates, I guess a computation would be needed for
other things.

Finally what about the default behaviour, would it be the old use a
fixed setting or would we change the default to be automatic with the
option of explicitly passing in 1000 to mimic the previous behaviour
if required (plus any updates to fate to account for the change).

Thanks

Kevin