(colormap-difference colormap1 colormap2)
This procedure creates a new colormap containing the difference for each color channel of each pixel. The two suppied colormaps need to have the same dimensions, otherwise an error is signalled. The difference for one single channel is computed by subtracting the smaller value from the larger.
> (colormap-difference frame1 frame2) #<colormap orow:-1 ocol:-1 xres:-1 yres:-1 h:480 w:640> >