Easy wayis with _NEWIMAGE and _PUTIMAGE.
Say my main screen is 1024x720 in size.
I want to copy from (200,200)-(400,400)
That’s a 200x200 area of the screen, so I’d:
my_copy = _NEWIMAGE(200, 200, 32)
_PUTIMAGE (0,0), 0, my_copy, (200,200)-(400,400)
Now I have a new image with the partial part which I wanted to do whatever with.