body {
  background: yellowgreen;
}

// Asked by @jrf0110
// https://github.com/jonobr1/two.js/issues/120

var two = new Two({
  type: Two.Types.canvas,
  fullscreen: true,
  autostart: true
}).appendTo(document.body);

/** 
 * Create the road, this is the reference for the dashed lines
 */

var amount = 32;
var points = [];

for (var i = 0; i < amount; i++) {

  var pct = i / (amount - 1);
  var theta = Math.PI * 2 * pct;
  var x = two.width / 20 * Math.cos(theta);
  var y = pct * two.height * 2 - two.height * 0.5;

  points.push(new Two.Anchor(x, y));

}

var sidewalk = two.makeCurve(points, true);
sidewalk.translation.set(two.width / 2, two.height/ 2);
sidewalk.noFill().stroke = '#ccc';
sidewalk.linewidth = two.width / 4;

var road = two.makeCurve(points, true);
road.translation.set(two.width / 2, two.height / 2);
road.noFill().stroke = '#666';
road.linewidth = two.width / 6;

/**
 * Create an array of lines that will be our dashed line.
 */

var dashes = [];

for (var j = 0; j < 10; j++) {

  var gutter = - 10;
  var length = two.height / (amount + gutter);
  var dash = two.makeLine(0, - length, 0, length);

  dash.noFill().stroke = '#fff';
  dash.linewidth = 3;
  dash.pct = j / (10 - 1);

  dash.translation.copy(points[j]).addSelf(road.translation);
  dashes.push(dash);

}

// Cached variables to use within the animation loop.
var a = new Two.Vector(), b = new Two.Vector();
var velocity = 0.0125;

two.bind('update', function(frameCount, timeDelta) {

  if (!timeDelta) {
    return;
  }

  for (var i = 0; i < amount; i++) {

    // Animate the road.

    var v = points[i];
    var pct = i / (amount - 1)
    var offset = pct * Math.PI * 2;
    var theta = offset + frameCount / 20;
    v.x = two.width / 20 * Math.cos(theta);

    var length = dashes.length;

    if (i > length - 1) {
      continue;
    }

    // Animate the dashes.

    var dash = dashes[i];
    // Assign the calculation of the vector on the road to `a`
    road.getPointAt(dash.pct, a);
    // Get an arbitrary vector right behind `a` in order to get the
    // angle for the rotation of the dash.
    road.getPointAt(dash.pct - 0.01, b);
    dash.translation.copy(a).addSelf(road.translation);
    dash.rotation = Two.Utils.angleBetween(a, b) + Math.PI / 2;

    dash.pct = mod(dash.pct + velocity, 1);

  }

});

// Add some mouse interaction
window.addEventListener('mousemove', function(e) {

  var pct = (e.clientY / window.innerHeight - 0.5) * 2;
  velocity = pct * 0.03;

}, false);

window.addEventListener('toushmove', function(e) {
  
  var touch = e.changedTouches[1];
  
  if (!touch) {
    return;
  }
  
  var pct = (e.pageY / window.innerHeight - 0.5) * 2;
  velocity = pct * 0.03;
  
});

function mod(v, l) {
  while (v < 0) {
    v += l;
  }
  return v % l;
}