clip.js

function clip (subjectPolygon, clipPolygon) {

  var cp1, cp2, s, e;
  var inside = function (p) {
    return (cp2[0]-cp1[0])*(p[1]-cp1[1]) > (cp2[1]-cp1[1])*(p[0]-cp1[0]);
  };
  var intersection = function () {
    var dc = [ cp1[0] - cp2[0], cp1[1] - cp2[1] ],
    dp = [ s[0] - e[0], s[1] - e[1] ],
    n1 = cp1[0] * cp2[1] - cp1[1] * cp2[0],
      n2 = s[0] * e[1] - s[1] * e[0], 
      n3 = 1.0 / (dc[0] * dp[1] - dc[1] * dp[0]);
    return [(n1*dp[0] - n2*dc[0]) * n3, (n1*dp[1] - n2*dc[1]) * n3];
  };
  var outputList = subjectPolygon;
  cp1 = clipPolygon[clipPolygon.length-1];
  for (j in clipPolygon) {
    var cp2 = clipPolygon[j];
    var inputList = outputList;
    outputList = [];
    s = inputList[inputList.length - 1]; //last on the input list
    for (i in inputList) {
      var e = inputList[i];
      if (inside(e)) {
        if (!inside(s)) {
          outputList.push(intersection());
        }
        outputList.push(e);
      }
      else if (inside(s)) {
        outputList.push(intersection());
      }
      s = e;
    }
    cp1 = cp2;
  }
  return outputList
}