[FFmpeg-user] Slide show with vfr

[Wolfgang Hugemann]

> Express duration in seconds
> Repeat the last image as per 
> https://trac.ffmpeg.org/wiki/Slideshow#Concatdemuxer

Thanks for this hint. This really works and -vsync vfr seems to be
necessary for mp4. (For mkv it works without that.)

It works as well if you provide the durations in milliseconds, but there
seems to be a problem with short durations, which are not reproduced
exactly. So one should rather use durations larger than one second, as
in the cited example.

I now successfully produced a vfr mp4 with ffmpeg. Indeed, some
applications as VLC do not handle the timestamps correctly, but AVidemux
and ffplay do. Plus the vfr is preserved when the slide show is
processed with ffmpeg (using -vsync vfr).

Possibly, the relatively short durations in my original video are the
reason for the problems with the original video, as there seem to be
rounding errors.

Wolfgang Hugemann