Line Segment Intersection

getLineIntersection.coffee

# via http://stackoverflow.com/questions/563198/how-do-you-detect-where-two-line-segments-intersect
#     http://mathworld.wolfram.com/Line-LineIntersection.html
getLineIntersection = (x1, y1, x2, y2, x3, y3, x4, y4) ->
  s1_x = x2 - x1
  s1_y = y2 - y1
  s2_x = x4 - x3
  s2_y = y4 - y3

  s = (-s1_y * (x1 - x3) + s1_x * (y1 - y3)) / (-s2_x * s1_y + s1_x * s2_y)
  t = (s2_x * (y1 - y3) - s2_y * (x1 - x3)) / (-s2_x * s1_y + s1_x * s2_y)

  if (s >= 0 && s <= 1 && t >= 0 && t <= 1)
    intX = x1 + (t * s1_x)
    intY = y1 + (t * s1_y)
    [intX, intY]
  else
    null