[FFmpeg-user] Random repetitions in output

[Moritz Barsnick]

On Sun, Nov 05, 2017 at 21:14:26 +0100, Mikael Persson wrote:
> Regarding: -vsync vfr
> From the doc: Frames are passed through with their timestamp or dropped so
> as to prevent 2 frames from having the same timestamp.
> 
> This would ensure no repetitions, but does not guarantee that no frames are
> lost. I want both to be ensured.

Did you read the "-vsync" docs? Value "passthrough" is described like
"vfr", without the dropping:

    ‘0, passthrough’
        Each frame is passed with its timestamp from the demuxer to the muxer.

> Further does -vsync have any effect at all if no output framerate is
> specified?

Yes, as there often needs to be an output framerate. How ffmpeg derives
it, I would have to dig through the sources to understand. It seems the
image2 muxer uses the framerate reported by the demuxer (but as cfr).

> I cannot believe that there is no way to tell ffmpeg to extract all frames
> without repetition or dropping, shouldnt that be the default, and the
> behaviour I see a bug?

It believe it is the default for most cases, but the image2 muxer (and
others perhaps - mp4?) forces cfr, which implies a change in the flow
or frames. (If it's not documented well, then perhaps it should be.)

Moritz