Create a sliding and bouncing overlay

In this example, the foreground video slides over the background one and bounces on the four edges. The output audio stream selected is the background one.

It can be used "as-is" under Windows. Unix users should replace ending ^ by \. The command line below has been splitted to make it more understandable.

ffmpeg -i "background.mp4" -i "foreground.mp4" -filter_complex ^
overlay='^
if(eq(mod(floor(st(8,t*320)/(W-w)),2),0),^
mod(ld(8),W-w),^
W-w-mod(ld(8),W-w))^
:^
if(eq(mod(floor(st(9,t*140)/(H-h)),2),0),^
mod(ld(9),H-h),^
H-h-mod(ld(9),H-h))^
:^
eval=frame^
:^
repeatlast=1^
' ^
"OUTPUT.mp4"

This example works properly with a 1080p (16/9) background and a 360p (16/9) foreground but can be modified to work with any scale.

The horizontal speed (t*320) and the vertical one (t*140) can be tuned. They are stored in internal variables 8 and 9 to avoid duplicates.

Pay attention to the input durations and use either repeatlast or shortest options of the overlay filter.

Have in mind that frames are taken from each input video in timestamp order, hence, if their initial timestamps differ, it is a good idea to pass the two inputs through a setpts=PTS-STARTPTS filter to have them begin in the same zero timestamp.