protected void handle_keyboard (int key, int state, int x, int y) {
    switch (key) {
    case 's':
      gl.tex_parameteri (GL.TEXTURE_2D, GL.TEXTURE_WRAP_S, GL.CLAMP);
      break;

    case 'S':
      gl.tex_parameteri (GL.TEXTURE_2D, GL.TEXTURE_WRAP_S, GL.REPEAT);      
      break;

    case 't':
      gl.tex_parameteri (GL.TEXTURE_2D, GL.TEXTURE_WRAP_T, GL.CLAMP);
      break;

    case 'T':
      gl.tex_parameteri (GL.TEXTURE_2D, GL.TEXTURE_WRAP_T, GL.REPEAT);
      break;
      
    default: return;
    }

    mark_window_dirty ();
  }