SOURCE

<script src="https://unpkg.com/p5@0.7.2/lib/p5.min.js"></script>
<script src="https://unpkg.com/p5@0.7.2/lib/addons/p5.dom.min.js"></script>
<script>

let img;

function preload() {
  img = loadImage('https://i.loli.net/2019/01/13/5c3ac84832d73.jpg');
}

function setup() {
  createCanvas(img.width, img.height);
  pixelDensity(1);
  background(0);
}

function draw() {
  loadPixels();
  img.loadPixels();

  for (let x = 0; x < width; x++) {
    for (let y = 0; y < height; y++) {
      let index = (x + y * width) * 4;
      if (x < mouseX && y < mouseY) {
        pixels[index + 0] = img.pixels[index + 0];
        pixels[index + 1] = random(255);
        pixels[index + 2] = img.pixels[index + 2]
        pixels[index + 3] = random(255);
      } else {
        pixels[index + 0] = img.pixels[index + 0];
        pixels[index + 1] = img.pixels[index + 1];
        pixels[index + 2] = img.pixels[index + 2];
        pixels[index + 3] = 255;
      }
    }
  }
  updatePixels();
}

</script>