<html>
<head>
  <title>Canvas 3D</title>
</head>
<body>
  <div style="position:relative; padding: 25px">
    <canvas id='canvas1' style="position:absolute;"></canvas>
  </div>
</body>
<script type="text/javascript">
  var canvas = document.getElementById('canvas1');
  var ctx = canvas.getContext('webgl');

  // Some devices, e.g. krane, have a non-zero rotation in landscape mode. We
  // precompensate this via the rotateZ CSS transform. See crbug.com/1046109.
  const angle = screen.orientation.angle % 360;
  canvas.style.transform = `rotateZ(${angle}deg)`;

  // Make the canvas large but still falling inside the viewport; |height|
  // also has to account for the Shelf (taskbar) at the bottom.
  const integerWidth = Math.min(500, Math.floor(window.innerWidth * 0.9));
  const integerHeight = Math.min(300, Math.floor(window.innerHeight * 0.9));

  // We need subpixel accuracy for non-integer aspect ratios crbug.com/1042110.
  const dpr = window.devicePixelRatio || 1;
  canvas.style.border = `${1 / dpr}px solid black`;
  if (angle % 180 == 90) {
    canvas.width = integerHeight;
    canvas.height = integerWidth;
    canvas.style.width = `${integerHeight / dpr}px`;
    canvas.style.height = `${integerWidth / dpr}px`;

    // On krane, the canvas needs to be shifted "rightwards" when the screen is
    // rotated 90 or 270 degrees, to bring it in view, see crbug.com/1046445/
    const offset = ((integerWidth / dpr) - (integerHeight / dpr)) / 2;
    canvas.style.left = `${offset}px`;
    //canvas.style.top = `-${offset}px`;
  } else {
    canvas.width = integerWidth;
    canvas.height = integerHeight;
    canvas.style.width = `${integerWidth / dpr}px`;
    canvas.style.height = `${integerHeight / dpr}px`;
  }

  var draw_passes_count = 0;
  function draw_pass() {
    // Consider a seeded random number generator if there are reproducibility
    // problems.
    ctx.clearColor(0, Math.random(), 0, 1.0);
    ctx.clear(ctx.COLOR_BUFFER_BIT);
    draw_passes_count++;
  }
  setInterval(draw_pass, 1000);

  function get_draw_passes_count() {
    return draw_passes_count;
  }

</script>
</html>