<html>
<head>
  <title>Canvas 2D</title>
</head>
<body>
  <canvas id='canvas1' style="border: 1px solid black;"></canvas>
</body>
<script type="text/javascript">
  var canvas = document.getElementById('canvas1');
  // Make the canvas very large but still falling inside the viewport; |height|
  // also has to account for the Shelf (taskbar) at the bottom.
  const dpr = window.devicePixelRatio || 1;
  const width = (window.innerWidth / dpr) * 0.9 / dpr;
  const height = (window.innerHeight / dpr) * 0.9 / dpr;
  canvas.width = width;
  canvas.height = height;

  var ctx = canvas.getContext('2d');
  var draw_passes_count = 0;

  function draw_pass() {
    ctx.beginPath();
    ctx.lineWidth = '5';
    // Consider a seeded random number generator if there are reproducibility
    // problems.
    ctx.strokeStyle = 'rgb(' + 0 + ',' + Math.random() * 255 + ',0)';
    ctx.moveTo(Math.random() * width, Math.random() * height);
    ctx.lineTo(Math.random() * width, Math.random() * height);
    ctx.stroke(); // Draw it
    draw_passes_count++;
  }
  setInterval(draw_pass, 1000);

  function get_draw_passes_count() {
    return draw_passes_count;
  }

</script>
</html>