ew Uint32Array(resultBuffer.getMappedRange());

  const to3 = v => v.toString().padStart(3);

  const sum = [0, 0, 0, 0];
  for (let i = 0; i < 256; ++i) {
    const off = i * 4;
    console.log(to3(i), to3(result[off]), to3(result[off + 1]), to3(result[off + 2]), to3(result[off + 3]));
    for (let j = 0; j < 4; ++j) {
      sum[j] += result[off + j];
    }
  }
  console.log('sum:', sum);
}

function showImageBitmap(imageBitmap) {
  const canvas = document.createElement('canvas');
  canvas.width = imageBit