Step Two: Find p for a Positional Light Source

Using this α we find

p	=	s + α(l − s)
	=	(1 − α)s + α l
	=	nTl nTl − nTs s − nTs nTl − nTs l

Notice that the denominators in the last line of this equation are scalar values. It is convenient to set β = n^Tl−n^Ts and multiply both sides by β to obtain