Assuming p0 = (0, 0),
compute the signed area within (0,0), p1, p2, p1 + p2 = (x1 +
x2, y1 + y2).
This is the determinant of the matrix
Note if p1 = (4,0) and p2 = (2,4), we are computing the signed area
within (origin, p1, p2, p1 + p2) which is
= 16 - 0 = 16.
If we compute
the signed area is 0 - 16 =