public FImage2D invert() {
    FImage2D r = new FImage2D( xsize, ysize );
    int i = 0, j = 0 ;
    
    while(i < xsize && j < ysize){
	 r.set(i,j,255 - get(i,j)); 
    i++; j++;
    // Enumerate all (i,j) in the 'r' image
    // and set r(i,j) = 255 - image(i,j)
    // here should be your code
    }
    
    
    return r;
    
  }