{
	Compute difference between luminance of two images.
	Display towards red where first image is brighter,
	and towards blue where second image is brighter.

	29 August 1991
}
l1=li(1); l2=li(2);
avg=(l1+l2)/2;
dif=(l1-l2)/2;
ro=avg+dif;
go=avg-dif*dif/2;
bo=avg-dif;