(greymap-difference greymap1 greymap2)
This procedure creates a new greymap by computing the absolute difference for each each pixel. The two suppied greymaps need to have the same dimensions, otherwise an error is signalled.
The resulting greymap is returned.
> (greymap-difference f1 f2) #<greymap orow:-1 ocol:-1 xres:-1 yres:-1 w:640 h:480> >