(graymap-low->greymap graymap)
A 8 bit greymap is returned with the same dimensions, each pixel containing only the 8 least significant bits of the corresponding 16 bit source pixel.
> (define a (graymap-create 640 480))
a
> (define b (graymap-low->greymap a))
b
> a
#<graymap(16) orow:-1 ocol:-1 xres:-1 yres:-1 w:640 h:480>
> b
#<greymap orow:-1 ocol:-1 xres:-1 yres:-1 w:640 h:480>
>