Implicit line equation
The implicit equation of the line through
p1 =[x1, y1], and p2 =[x2, y2] is
(n,q-p1) = 0, with n = [y2,-y1, x1,-x2]
We need to test on which side of the line is the point
To do this, we need to determine the sign of
Note that multiplication by two makes
everything integer again!
Key idea: compute this quantity incrementally.