<!DOCTYPE html>
<html>
<body>
<canvas id='output' width='150' height='150'></canvas>
<script id='myWorker' type='text/worker'>
self.onmessage = function(e) {
    var aCanvas = new OffscreenCanvas(150, 150);
    var ctx = aCanvas.getContext('2d');
   
    var myPattern = ctx.createPattern(e.data, 'repeat');
    ctx.fillStyle = myPattern;
    ctx.fillRect(0, 0, aCanvas.width, aCanvas.height);

    var image = aCanvas.transferToImageBitmap();
    self.postMessage(image, [image]);
};
</script>
<script>
if (window.testRunner) {
  testRunner.waitUntilDone();
}

var patternCanvas = document.createElement('canvas');
patternCanvas.width = 30;
patternCanvas.height = 30;
var patternCtx = patternCanvas.getContext('2d');        
patternCtx.fillStyle = '#f00';                                                     
patternCtx.fillRect(0, 0, 15, 15);                                              
patternCtx.fillStyle = '#0f0';                                                     
patternCtx.fillRect(15, 0, 15, 15);                                             
patternCtx.fillStyle = '#00f';
patternCtx.fillRect(0, 15, 15, 15);
patternCtx.fillStyle = "#ff0";
patternCtx.fillRect(15, 15, 15, 15);
createImageBitmap(patternCanvas).then(consumeImageBitmap);

function consumeImageBitmap(patternImage) {
    var blob = new Blob([document.getElementById('myWorker').textContent]);
    var worker = new Worker(URL.createObjectURL(blob));
    worker.addEventListener('message', msg => {
      var outputCtx = document.getElementById('output').getContext('bitmaprenderer');
      outputCtx.transferFromImageBitmap(msg.data);
      if (window.testRunner) {
        testRunner.notifyDone();
      }
    });
    worker.postMessage(patternImage, [patternImage]);
}
</script>
</body>