[FFmpeg-user] Selecting/dropping frames without re-encoding

[Erik Dobberkau]

> Hello,
>
> I have a camera recording which is an MXF file and the codec is DNxHD
> I suppose it is a single-frame codec so literally it is a container of
> images which are independently stored in the file.
> Recordings are taken at 50 fps progressive.


Hi Michail,

this is a TV standard, 720p50. Nothing exotic.


> Now what I want is to extract only half of the frames, so that it becomes
> 25 fps


Why would you want to do that?
You're getting standard-incompliant video. In other words: This is wrong.


> Note that I need to do it _without re-encoding_ for two reasons:
> 1. Conversion must be as fast as possible, files can be really huge
> 2. No changes to original information should occur
>
> Theoretically this is an elementary operation, it should just take
> every second frame from source and write to output
> leaving everything else as is.
> I have tried several options whith "-r ..." and added "-c:v copy"
> These commands I have tried:
>
> ffmpeg -i Capture0006V_720p.mxf -r 25 -c:v copy out1.mxf
> ffmpeg -r 50 -i Capture0006V_720p.mxf -r 25 -c:v copy out1.mxf
> ffmpeg -r 2 -i Capture0006V_720p.mxf -r 1 -c:v copy out1.mxf
>
> But they all produce again 50 fps video, so I suppose stream copy just
> ignores all "-r" options so I don't know what can I do to achieve
> what I want. I have searched a lot, so I've registered here
> hoping for solution.
>
> Is it possible that ffmpeg cannot do this without re-encoding?
> If no, how can one do this?
> Thanks in advance.
>

Stream Copy does that... it copies streams.
Dropping every second frame will destroy motion smoothness thus ruin your
video. But you can do that with another filter.

Erik