protected void handle_keyboard (int key, int state, int x, int y) {
    switch (key) {
    case 'r':                   // fall through
    case 'R': change_diffuse (0); break;
    case 'g':                   // fall through
    case 'G': change_diffuse (1);  break;
    case 'b':                   // fall through
    case 'B': change_diffuse (2); break;
    }
  }