PImage orig, filtered;
int w, h;

void setup() {
  size(1024, 768);
  orig = loadImage("https://upload.wikimedia.org/wikipedia/commons/thumb/e/ed/SLU_portals.jpg/1024px-SLU_portals.jpg");
  w = orig.width;
  h = orig.height;

  filtered = createImage(w, h, RGB);

  for (int x=0; x < w-1; x++) {
    for (int y=0; y < h-1; y++) {
      color a = orig.get(x,y);
      color b = orig.get(x+1,y);
      color c = orig.get(x,y+1);
      float rv = 2*red(a) - red(b) - red(c);
      float gv = 2*green(a) - green(b) - green(c);
      float bv = 2*blue(a) - blue(b) - blue(c); 
      filtered.set(x,y,color(rv,gv,bv));
    }
  }

  image(filtered,0,0);
  save("edgeDiag.jpg");
}