var go = function(b, img_id, interval) {
  var i = 0;
  var img0 = document.getElementById(img_id[0]);
  var img1 = document.getElementById(img_id[1]);
  var img = [img0, img1];
  var activeImage = 1; // index of visible image
  var url = 'index.php?mode=zap';

  if (isNaN(interval)) {
    interval = 5000;
  }

  // Trigger the next image fetch after the current one has loaded.
  img[0].onload = img[1].onload = function() {
    var inactiveImage = activeImage === 0 ? 1 : 0;
    img[activeImage].style.display = 'none';
    img[inactiveImage].style.display = '';
    activeImage = inactiveImage;

    setTimeout(function() {
      img[inactiveImage].src = url + '&i=' + (i++);
    }, interval);
  };

  // When the backend signals completion it returns a small HTML page.
  // Show the ready link and stop fetching new images.
  img[0].onerror = img[1].onerror = function() {
    document.getElementById('ready').innerHTML = '<a href="/">Ready</a>';
  };

  // Kick off the first request.
  img[activeImage].src = url + '&i=' + (i++);
};