Lines in homogeneous coordinates

lines.py

# homogenous line from two points (this isn't on the internet...)
line = [0,0,0]
line[0] = (f1[1] - f2[1]) * (f1[1] * f2[0] - f1[0] * f2[1])
line[1] = (f1[0] - f2[0]) * (f1[0] * f2[1] - f1[1] * f2[0])
line[2] = (f1[0] * f2[1] - f1[1] * f2[0]) * (f1[1] * f2[0] - f1[0] * f2[1])
line = np.array(line)

# line intersection (https://en.wikipedia.org/wiki/Line%E2%80%93line_intersection#Using_homogeneous_coordinates)
p = np.array([l1[1] * l2[2] - l2[1] * l1[2], l2[0] * l1[2] - l1[0] * l2[2], l1[0] * l2[1] - l2[0] * l1[1]])
p /= p[2]