[MPlayer-users] seeking explanation of -1 and -2 in -vop scale

[Chris Phillips]

hi, can someone briefly state precisely what -2 and -1 actually DO mean in 
the -vop scale filter? i know they preserve aspect ratios (scale=640:-2) 
but their behaviour seems confusing? what is the difference? how does 
cropping an image before scaling (scale=640:-2,crop=720:436:4:740) affect 
this -2 or -1 status? 

Also is it better to crop then scale or scale then crop? for a perl script 
i'm writing i really want to scale afterwards, but have found that many 
times i've tried to scale a cropped image it has been scaled relative to 
the original dimensions and aspect information, not the new size.

any information greatly recieved

thanks

chris
xxxx