I want to remove a portion of the video between say coordinates (x,y) and (m,n), where (x,y) are the upper left coordinates while (m,n) are the lower right ones. Is there a ffmpeg command I could use?
You can use the drawbox filter to umm. draw a box on top of the video.
Basic syntax is
ffmpeg -i in.mp4 -vf "drawbox=X:Y:M-X:N-Y:c=black:t=fill" -c:a copy out.mp4
where X, Y are replaced with top-left co-ordinates of box.
N-Y are width and height of the box. The value
fill for thickness will make a solid box. This value was very recently changed, so if you have an older version of ffmpeg, use