Segments intersection

intersection.js

  function segmentIntersection(a, b) {
    var denom = (((b[1].y - b[0].y) * (a[1].x - a[0].x)) -
                 ((b[1].x - b[0].x) * (a[1].y - a[0].y)));
    if (denom === 0) { return null; }
    var ua = ((((b[1].x - b[0].x) * (a[0].y - b[0].y)) -
               ((b[1].y - b[0].y) * (a[0].x -b[0].x))) / denom);

    var ub = ((((a[1].x - a[0].x) * (a[0].y - b[0].y)) -
               ((a[1].y - a[0].y) * (a[0].x - b[0].x))) / denom);

    if ((ua < 0) || (ua > 1) || (ub < 0) || (ub > 1)) { return null; }

    return {
      x: a[0].x + (ua * (a[1].x - a[0].x)),
      y: a[0].y + (ua * (a[1].y - a[0].y))
    };
  }