async function loadImage(src) {
  const img = new Image();
  await new Promise((resolve, reject) => {
    img.onload = resolve;
    img.onerror = () => reject(new Error('image load failed: ' + src));
    img.src = src;
  });
  return img;
}

function samplePixel(img, x, y) {
  const canvas = document.createElement('canvas');
  canvas.width = img.naturalWidth;
  canvas.height = img.naturalHeight;
  const ctx = canvas.getContext('2d');
  ctx.drawImage(img, 0, 0);
  return ctx.getImageData(x, y, 1, 1).data;
}