private void draw_bottom () {
    if (!show_bottom) return;

    gl.push_matrix ();

    gl.scalef (zoom, zoom, zoom);
    gl.rotatef (z_angle, 0.0f, 0.0f, 1.0f);
    gl.color3f (0.0f, 0.0f, 1.0f);

    switch (draw_state) {
    case SOLID: gl.begin (GL.POLYGON); break;
    case LINE: gl.begin (GL.LINE_LOOP); break;
    case POINT: gl.begin (GL.POINTS); break;
    }

    gl.vertex3fv (POINT0);
    gl.vertex3fv (POINT1);
    gl.vertex3fv (POINT2);
    gl.end ();
    
    draw_vertex ();
    gl.pop_matrix ();
  }