code: protected float gradient(int x, int y) {
  		float dx = getIntensity(x+1,y) - getIntensity(x-1,y);
  		float dy = getIntensity(x,y+1) - getIntensity(x,y-1);
  
  		return dx*dx + dy*dy;
  	}
docs: Computes the gradient at the specified pixel