void setup() {
  PImage img = loadImage("prinzipal.jpg");
  size(img.width, img.height);
  
  img.loadPixels();
  for (int i=0; i<img.pixels.length; i++) {
    color c = img.pixels[i];
    img.pixels[i] = color(255-red(c), 255-green(c), 255-blue(c));
  }
  img.updatePixels();

  // Draw modified images
  image(img, 0, 0);
}