save contents of canvas to an image?

Hi,

Can anyone help me find a way to save the contents of a canvas to an image?

I've tried the 'postscript' method, but I'm having two problems with that:
- I need an exact geometrical representation of what was on the canvas 
(size of the image must be identical with that of the canvas, etc.)
- I haven't found a nice way to convert the postscript into some other 
image format (pnm, whatever).

Or does someone know of another way to write out the canvas as an image? 
So far as I can see the Image method is only for displaying images, not 
creating them?

Cheers,

Matt