Allow setting ticks_per_frame

[gsakkis]

CodecContext.ticks_per_frame is a read-only property, hardcoded to 1 when adding a new stream:

>>> output = av.open("out.mp4", 'w')
>>> stream = output.add_stream('h264', 24)
>>> stream.ticks_per_frame
1
>>> stream.ticks_per_frame = 2
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
...
AttributeError: attribute 'ticks_per_frame' of 'av.codec.context.CodecContext' objects is not writable

It should be settable, most notably H.264/MPEG-2 set it to 2.

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[jlaine]

We seem to have two contradictory requests here. PR #790 advocates changing the way this member is set, but not exposing it.

@rawler would making ticks_per_frame writable solve your issue too?

[rawler]

Writeable would solve, but it would require a user of PyAV to be intimately familiar with these settings or one might get surprising results. libvpx-vp9, for example, seem to use this value in the bandwidth-calculations (how much time has passed between frames), so not explicitly setting ticks_per_frame might break bandwidth-control quite badly. (IIRC, this is how we discovered the need to set it)

[jlaine]

I'd argue that setting ticks_per_frame automatically qualifies the user as an "advanced" user. I've never once done this myself :)

Another thing that worries me is that I can't find any code in the FFmpeg CLI that sets "ticks_per_frame" outside the codecs and avformat_transfer_internal_stream_timing_info. Try grepping for "ticks_per_frame = ".

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.