[FFmpeg-user] Why are half the frames "duplicate" ?

[Moritz Barsnick]

On Mon, Oct 01, 2018 at 21:26:02 -0400, sean darcy wrote:
> > That does not sound correct, "-re" allows FFmpeg to encode in
> > real-time (instead of faster), it should not change the output frame
> > rate. Does it?
> 
>  From 12 years ago:
> 
> https://lists.libav.org/pipermail/ffmpeg-user/2006-February/002349.html
> 
> make an decoding/encoding with
> framerate " -re" (instead of "-r pal" or -r ntsc" or "-r 25" or "-r
> 30000:1001", ...) the "-re" parameter for framerate will just 
> decode/encode your input without forcing framerate (read your input at 
> its own framerate and write it "the same")
> You'll will see (even if your input was a ntsc-film with pulldown)
> that your output will be 24000:1001 too.

Ah, well, that was already incorrect 12 years ago. "-re" does not
change how ffmpeg maps the inputs frame (or field) rate to its output's
frame rate. It changes how fast the input is processed:

https://ffmpeg.org/ffmpeg.html#Advanced-options

"This option will slow down the reading of the input(s) to the native
frame rate of the input(s). It is useful for real-time output (e.g.
live streaming)."

Cheers,
Moritz