function polygonCanSee(polygon, a,b) {
    var p, dist, l1=tmpLine1, l2=tmpLine2;

    if (isLeftOn(polygonAt(polygon, a + 1), polygonAt(polygon, a), polygonAt(polygon, b)) && isRightOn(polygonAt(polygon, a - 1), polygonAt(polygon, a), polygonAt(polygon, b))) {
        return false;
    }
    dist = sqdist(polygonAt(polygon, a), polygonAt(polygon, b));
    for (var i = 0; i !== polygon.length; ++i) { // for each edge
        if ((i + 1) % polygon.length === a || i === a){ // ignore incident edges
            continue;
        }
        if (isLeftOn(polygonAt(polygon, a), polygonAt(polygon, b), polygonAt(polygon, i + 1)) && isRightOn(polygonAt(polygon, a), polygonAt(polygon, b), polygonAt(polygon, i))) { // if diag intersects an edge
            l1[0] = polygonAt(polygon, a);
            l1[1] = polygonAt(polygon, b);
            l2[0] = polygonAt(polygon, i);
            l2[1] = polygonAt(polygon, i + 1);
            p = lineInt(l1,l2);
            if (sqdist(polygonAt(polygon, a), p) < dist) { // if edge is blocking visibility to b
                return false;
            }
        }
    }

    return true;
}
Check if two vertices in the polygon can see each other
@method canSee
@param  {Number} a Vertex index 1
@param  {Number} b Vertex index 2
@return {Boolean}