pathlength.js

function getLength(points, steps) {
    
  steps = steps || 20;
  
  var length = 0;
  var last = points[0];
  var size = points.length;
  
  for (var i = 1; i < size; i += 3) {
    
    var p0 = last;
    var p1 = points[i];
    var p2 = points[i+1];
    var p3 = points[i+2];
        
    var x1 = p0.x;
    var y1 = p0.y;
    
    for (var t = 0; t <= 1; t += 1 / steps) {
      
      var x = interpolate(p0.x, p1.x, p2.x, p3.x, t);
      var y = interpolate(p0.y, p1.y, p2.y, p3.y, t);
      
      var dx = x1 - x;
      var dy = y1 - y;
      
      length += Math.sqrt(dx * dx + dy * dy);
            
      x1 = x;
      y1 = y;
    }
    
    last = p3;
  }
  
  function interpolate(a, b, c, d, t) {
    var t2 = t * t;
    var t3 = t2 * t;
    return a + (-a * 3 + t * (3 * a - a * t)) * t + (3 * b + t * (-6 * b + b * 3 * t)) * t + (c * 3 - c * 3 * t) * t2 + d * t3;
  }
    
  return length;
}