PImage img;
PGraphics mask;
void setup() {
  size(1090, 755);
  img = loadImage("https://www.ibiblio.org/wm/paint/auth/kandinsky/kandinsky.comp-8.jpg");
  mask = createGraphics(width,height);
}

void draw() {
  if (mousePressed) {
    mask.beginDraw();
    mask.noStroke();
    mask.fill(0);
    mask.ellipse(mouseX, mouseY, 50, 50);
    mask.endDraw();
  }
  background(255);
  loadPixels();
  mask.beginDraw();
  for(int i = 0; i < pixels.length; i++) {
    int y = i / width;
    int x = i % width;
    color c = img.get(x,y);
    if(mask.get(x,y) == color(0)) {
      pixels[i] = c;
    }
  }
  mask.endDraw();
  updatePixels();
}