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chromium/third_party/blink/web_tests/fast/canvas/OffscreenCanvas-2d-imageData.html 

<!DOCTYPE html>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<canvas style="background: red"></canvas>
<script>

test(function() {
  var canvas = document.querySelector('canvas');
  var offscreenCtx = (new OffscreenCanvas(canvas.width, canvas.height)).getContext('2d');

  var imageData = offscreenCtx.createImageData(canvas.width, canvas.height);
  var data = imageData.data;

  // Set all pixels to [0, 255, 0, 255]
  for (var i = 0; i < data.length; i += 4) {
    data[i + 1] = data[i + 3] = 255;
  }

  offscreenCtx.putImageData(imageData, 0, 0);

  assert_array_equals(offscreenCtx.getImageData(0, 0, 1, 1).data, [0, 255, 0, 255]);
}, "Verify that imageData is properly set");

</script>