Hi,

$subject allows crop detection even if the video is embedded in non-black areas.

Shares logic and purpose of lavfi/vf_cropdetect, though its edge detection is 8-bit formats only. Therefore this ends up in a separate filter.
It would also benew GPL code if it shares living in lavfi/vf_cropdetect.c - if we'd like to add this as LGPL for some reason it might go into its own file without sharing (useful) logic with the existing cropdetect.

Thanks,
Thilo