Author Topic: Direct Copy output converts constant video frame rates to variable frame rate.  (Read 417 times)

tomfank

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I've just become aware of a glitch when loading an mp4 file with a constant video frame rate and doing a direct copy (video) output after trimming the file.  The output file shows a variable video frame rate (as measured by MediaInfo utility) with a significant span between the slowest and fastest frame (e.g. as much as ~50 fps with the native clip having a 60 fps constant frame rate).  The overall average variable frame rate is very close to the original constant frame rate, however (e.g. like 59.570 fps vs a 60 fps native video frame rate).

This happens with the two Mp4 variant muxers, but NOT with the .MKV output file format.  If the native file requires re-encoding when output after editing, the mp4 muxers correctly retain the original video's constant frame rate. This has been in prior WIndows versions for some time, but I just became aware of it.

Is this a bug that can be fixed?

eumagga0x2a

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Probably this, so you might get a single frame or a couple of frames with half the duration depending on editing steps. Basically completely harmless as long as the frame rate denominator doesn't get too high (and <= 120 should be okay with most hardware decoders and absolutely fine with any software player), but still worth being fixed.

Unfortunately, a proper fix would require a transition from timestamps in ┬Ás into timestamps in a timescale (as a rational num/den) as well as big changes to the strategy of stream copy. This means a lot of work.

eumagga0x2a

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The situation should be better now, the other part of the equation, aimed at preserving the video stream timebase is in the works.