offscreencanvas.html

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<title>Offscreen Canvas</title>
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<body>

<h2>Offscreen canvas demo</h2>

<p><b>What</b>: <a href="https://www.chromestatus.com/feature/5681560598609920" target="_blank">OffscreenCanvas</a>
allows <code>&lt;canvas></code> rendering contexts (2D and WebGL) to be used
in workers. This can increase parallelism and better performance on multi-core
systems.</p>

<p><b>Support</b>: Available in Chrome 69. FF is in development.</p>

<div class="support">Your browser does not support OffscreenCanvas.</div>

<p><b>Demo</b>: The canvas animation is fully driven inside of the worker. <code>Worker.requestAnimationFrame()</code>
is used to drive the animation withi the worker and pixels are manipulated / drawn to the canvas
using <code>OffscreenCanvas</code>. The worker introduces jank to the
animation every so often to simulate a CPU-intensie operation. However, the main thread
says responsive!</p>

<p>
  <b>Instructions</b>: Select an image below. Click <button>this button</button> during jank. It stays responsive.</li>
</p>

<p><input type="file"></p>

<canvas></canvas>

<script type="script/worker" id="workerCode">
let angle = 0;
let ctx = null;
let backgroundBitmap = null;

function drawCircle() {
  const radius = ctx.canvas.height * Math.abs(Math.cos(angle));
  const w = ctx.canvas.width;
  const h = ctx.canvas.height;

  ctx.save();
  ctx.globalAlpha = 0.5;
  ctx.fillStyle = '#fff';
  ctx.beginPath();
  ctx.arc(w / 2, h / 2, radius, 0, Math.PI * 2, true);
  ctx.fill();
  ctx.restore();

  angle += Math.PI / 64;
}

function drawBackground() {
  ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  ctx.drawImage(backgroundBitmap, 0, 0);
}

function fibonacci(num) {
  if (num <= 1) {
    return 1;
  }
  return fibonacci(num - 1) + fibonacci(num - 2);
}

function animate() {
  // Periodic jank!
  if (counter++ >= 150) {
    fibonacci(40);
    counter = 0;
  }

  drawBackground();
  drawCircle();


  //ctx.commit(); // Don't need this since our animation is powered by rAF, but we could shipt a frame explicitly.
  self.requestAnimationFrame(animate);
}

let counter =  0;

self.onmessage = function(e) {
  switch (e.data.msg) {
    case 'start':
      backgroundBitmap = e.data.bitmap;
      const canvas = e.data.canvas;
      ctx = canvas.getContext('2d');
      animate();
      break;
  }
};
</script>
<script>
(async() => {

/**
 * Note: You can also create an OSC in a worker and postMessage transferrable
 * bitmaps back to the main page:
 *
 * // worker.js
 * self.onmessage = function(e) {
 *   const canvas = new OffscreenCanvas(e.data.width, e.data.height);
 *   ctx = canvas.getContext('2d');
 *   // draw fancy things ...
 *   const bitmap = ctx.canvas.transferToImageBitmap();
 *   postMessage({bitmap}, [bitmap]);
 * };
 *
 * // index.html
 * const canvas = document.createElement('canvas');
 * canvas.width = 400;
 * canvas.height = 400;
 *
 * const worker = new Worker('worker.js');
 * worker.onmessage = function(e) {
 *   const ctx = canvas.getContext('bitmaprenderer');
 *   ctx.transferFromImageBitmap(e.data.bitmap);
 * };
 *
 * worker.postMessage({width: canvas.width, height: canvas.height});
 */

// Feature detect.
document.querySelector('.support').classList.toggle(
      'notsupported', !('OffscreenCanvas' in window));

const canvas = document.querySelector('canvas');
const workerCode = document.querySelector('#workerCode').textContent;
const blob = new Blob([workerCode], {type: 'text/javascript'});
const url = URL.createObjectURL(blob);

const worker = new Worker(url);
worker.onmessage = e => {
  switch (e.data.msg) {
    case 'render':
      const ctx = canvas.getContext('bitmaprenderer');
      ctx.transferFromImageBitmap(e.data.bitmap);
      // ctx.canvas.toBlob(blob => console.log(blob), 'image/jpeg');
      break;
  }
};

const input = document.querySelector('input[type="file"]');
input.addEventListener('change', async e => {
  const image = e.target.files[0];

  const bitmap = await createImageBitmap(image);
  canvas.width = bitmap.width;
  canvas.height = bitmap.height;
  canvas.style.width = canvas.width / devicePixelRatio;
  canvas.style.height = canvas.height / devicePixelRatio;
  const offscreen = canvas.transferControlToOffscreen();
  worker.postMessage({msg: 'start', canvas: offscreen, bitmap}, [offscreen, bitmap]);
});

URL.revokeObjectURL(url); // cleanup

})();

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