This filter creates a type of motion trail. Here using a 10 second clip of a dancer, with slow movement on a dark backgronund. This is after testing many alternatives:
ffmpeg lagfun filter, very little information.
From Michael Koch's FFmpeg_Book.pdf, 2.78 Lagfun filter, page 129 (version August 30, 2022)
Using a command like this (on my Mac)
ffmpeg -i 20210313_01_p_10.mp4 -vf "format=gbrp,lagfun=decay=0.97:planes=1" planes=1_97.mp4
planes=1, 2, 3, 4, 5, 6 decay=0.97 (at end of video).
planes=1 decay=1.00 at 10 s. This can work with a linear movement across the screen, with a non-moving camera.
ffmpeg -i planes=7_97.mp4 -vf "format=yuv420p,scale='nhd'" s_planes=7_97.mp4
Allowed range for planes is from 0 to 15
planes=0 no effect
planes=7 white (here more like skin colour)
planes=8=0 no effect
If the "trails" of the frames are too distinct, the "tmix" filter can mix frames before the "lagfun" filter.
ffmpeg -i in.mp4 -vf tmix=frames=8:weights="1 1 1 1 1 1 1 1" out_tmix.mp4
Alexander Refsum Jensenius has worked more on this
tmix + lagfun planes=7 decay=0.97