Any suggestions on how to "onion skinning"?

[Ken Ray]

I'm working on a program with my son that does simple card-based 
animation, but one of the things he asked how to do in Rev stumped me, 
and that is doing an "onion skin" - showing a "grayed out" version of 
the previous card's objects on the current card. 

I know how to do *some* of it (I can take actual Rev objects from the 
previous card, copy them, change their lines/fills/blends/etc. to work 
and then group them and put them behind the current set of objects on 
the current card), but what stumps me is how to handle imported/created 
images. 

At best, what I'd *like* to do is to basically convert images to 
grayscale, and at worst I'd settle for turning all non-white pixels a 
single color of gray. I know I can walk through the imageData and 
convert things pixel by pixel, but I fear that would be really slow on 
larger (say 800x600) images (although I haven't tried it yet).

Any suggestions?

Ken Ray
Sons of Thunder Software, Inc.
Email: kray at sonsothunder.com
Web Site: http://www.sonsothunder.com/