ugly edges with clips on background

I have a white 960x480px background where I move clips on via camera.

These clips have ugly edges. Partially even higher contrast values can't solve it. The f_vignette filter does something (in contrast to a statement given in an earlier forum post), but not useful for this need.

Do you have an idea how to deal with that?

In general a vignette function could solve that but this function is kind weird. It doesn't handle the edges as it should be the inner core are and what is really aweful the white background (to black) though the white background is at another armed !! video track.

You can see the clip here at my site -> https://radszene.de/equipment/bikes/rose-backroad-gravelbikes_2026/