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chromium/third_party/blink/web_tests/fast/canvas/bug1293471.html 

<!DOCTYPE html>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script>
test(function(t) {
  const canvas = document.createElement('canvas');
  canvas.width = canvas.height = 20;
  const context = canvas.getContext('2d');
  // This test verifies that pixel premultiplicatio/unpremul is done correctly
  // when putImageData covers the entire canvas.
  // This is a regression test for a bug that was caused by a fast path that
  // was short-circuiting alpha type conversions when the canvas was fully
  // overwritten.
  canvasData = context.createImageData(canvas.width, canvas.height);
  for(let i = 0; i < canvasData.data.length; i = i + 4) {
    canvasData.data[i] = 0;
    canvasData.data[i+1] = 0;
    canvasData.data[i+2] = 127;
    canvasData.data[i+3] = 127;
  }
  context.putImageData(canvasData, 0, 0);

  let pixel = context.getImageData(10,10,1,1).data
  const epsilon = 2;
  assert_array_approx_equals(pixel, [0, 0, 127, 127], epsilon, "ImageData pixel survived round trip");
})
</script>