predicate_orient2d

float  predicate_orient2d(vector2 a, vector2 b, vector2 c)

Given 2 points a and b in the plane, return a positive value if c is on the left of the segment (a,b), a negative value if it is on the right of the segment, and zero if a, b and c are colinear.

More precisely, this function computes the determinant of the matrix:

[a_x a_y 1; b_x b_y 1; c_x c_y 1]

…with a guaranteed correct sign.