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HTML

              
                <div id="bank-note"></div>
              
            
!

CSS

              
                #bank-note {
  width: 912px;
  height: 389px;
  background: #ebeec7;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  margin: 20px;
}

#bank-note::before {
  content: "";
  display: block;
  position: absolute;
  width: 1px;
  height: 1px;
  color: #998;
}
              
            
!

JS

              
                /* 
The following functions are used to calculate the difference between two colors.
They are original from https://stackoverflow.com/a/52453462/3695983
*/
function deltaE(rgbA, rgbB) {
  let labA = rgb2lab(rgbA);
  let labB = rgb2lab(rgbB);
  let deltaL = labA[0] - labB[0];
  let deltaA = labA[1] - labB[1];
  let deltaB = labA[2] - labB[2];
  let c1 = Math.sqrt(labA[1] * labA[1] + labA[2] * labA[2]);
  let c2 = Math.sqrt(labB[1] * labB[1] + labB[2] * labB[2]);
  let deltaC = c1 - c2;
  let deltaH = deltaA * deltaA + deltaB * deltaB - deltaC * deltaC;
  deltaH = deltaH < 0 ? 0 : Math.sqrt(deltaH);
  let sc = 1.0 + 0.045 * c1;
  let sh = 1.0 + 0.015 * c1;
  let deltaLKlsl = deltaL / (1.0);
  let deltaCkcsc = deltaC / (sc);
  let deltaHkhsh = deltaH / (sh);
  let i = deltaLKlsl * deltaLKlsl + deltaCkcsc * deltaCkcsc + deltaHkhsh * deltaHkhsh;
  return i < 0 ? 0 : Math.sqrt(i);
}

function rgb2lab(rgb){
  let r = rgb[0] / 255, g = rgb[1] / 255, b = rgb[2] / 255, x, y, z;
  r = (r > 0.04045) ? Math.pow((r + 0.055) / 1.055, 2.4) : r / 12.92;
  g = (g > 0.04045) ? Math.pow((g + 0.055) / 1.055, 2.4) : g / 12.92;
  b = (b > 0.04045) ? Math.pow((b + 0.055) / 1.055, 2.4) : b / 12.92;
  x = (r * 0.4124 + g * 0.3576 + b * 0.1805) / 0.95047;
  y = (r * 0.2126 + g * 0.7152 + b * 0.0722) / 1.00000;
  z = (r * 0.0193 + g * 0.1192 + b * 0.9505) / 1.08883;
  x = (x > 0.008856) ? Math.pow(x, 1/3) : (7.787 * x) + 16/116;
  y = (y > 0.008856) ? Math.pow(y, 1/3) : (7.787 * y) + 16/116;
  z = (z > 0.008856) ? Math.pow(z, 1/3) : (7.787 * z) + 16/116;
  return [(116 * y) - 16, 500 * (x - y), 200 * (y - z)]
}

/*
Function that converts RGB to shortened HEX (#rgb instead of #rrggbb).
Initially based on CSS Tricks: https://css-tricks.com/converting-color-spaces-in-javascript/
*/
function simplifiedRGBToHex(color) {
  const hex = [
    Math.round(color[0]/16).toString(16),
    Math.round(color[1]/16).toString(16),
    Math.round(color[2]/16).toString(16)
  ];
  return "#" + hex.join("");
}

/*
The code
*/

// Create a canvas to set and read the image from
const canvas = document.createElement("canvas");
canvas.width = 912;
canvas.height = 389;
const context = canvas.getContext('2d');

// Load the image (original from Wikimedia: https://upload.wikimedia.org/wikipedia/commons/2/23/US_one_dollar_bill%2C_obverse%2C_series_2009.jpg)
base_image = new Image();
base_image.crossOrigin = "Anonymous";
base_image.src = 'https://s3-us-west-2.amazonaws.com/s.cdpn.io/1506195/US_one_dollar_bill%2C_obverse%2C_series_2009.jpg';

// When the image is loaded
base_image.onload = function(){
  
  const width = 912;
  const height = 389;
  
  // draw it into the canvas
  context.drawImage(base_image, 0, 0, 1216, 519, 0, 0 , width, height);
  
  // High values = less colors/quality and smaller size; low values = more colors/quality and higher sizes
  const minDiff = 20;
  let shadows = [];
  let count = 0;
  
  // traverse the whole image pixel by pixel
  for (let x = 0; x < width; x++) { 
    for (let y = 0; y < height; y++) { 
      // get the color of that particular pixel and compare to the background green
      const color = context.getImageData(x, y, 1, 1).data;
      const delta = deltaE(color, [235, 238, 199]);
      
      // if the difference is big enough, then include it to the box-shadow
      if (delta > minDiff) {
        // convert the color to a shortened HEX (lose quality, but smaller size)
        const newColor = simplifiedRGBToHex(color);
        // in a previous iteration I gound that #998 is the most common color, so I used it as the main color to cut the size a little
        shadows.push(`${x}px ${y}px${newColor === "#998" ? "" : newColor}`);
        count++;
      }
    }
  }

  // console.log(shadows.join(","));
  console.log("Number of color points: ", count);
  const styles = document.createElement("style");
  styles.innerHTML = `#bank-note::before { box-shadow: ${shadows.join(",")}; }`;
  document.querySelector("body").appendChild(styles);
  
  // delete styles;
  // delete canvas;
  
}



              
            
!
999px

Console