[MPlayer-users] cropping

[frytek]

can anybody explain to me what are the rules of cropping? 

i am ripping VHS. on the bottom of the screen there's always a kind of a 
distorted strap of picture. i would like to crop it. as it is not black or 
fixed, i can't rely on cropdetect. 

i tried to crop it using a preview. the file i made can be played and looks 
fine, but instead it can't be edited in avidemux. is there anything i should 
know about? do i have too  keep certain proportions?

best regards,