How to double every frame (and the length of the video itself) in an mp4 video (timelapse)? The video has length of t and is 30fps. I want it to have a length of 2*t.
I just want each frame to be shown a little longer. According to VLC the codec is H264-MPEG4 AVC.
Is there some simple command with ffmpeg?
setpts filter can be used to retime frames. Basic usage is

ffmpeg -i in -vf setpts=PTS*2 out.mp4


PTS*2 sets output timestamp of each frame to 2x its input value.