public Point2D worldToScreen(ProjectionPointImpl w, Point2D p) {
    p.setLocation( pix_per_world*w.getX() + pix_x0,
                   -pix_per_world*w.getY() + pix_y0);
    return p;
  }
public Point2D worldToScreen(ProjectionPointImpl w, Point2D p) {
    p.setLocation( pix_per_world*w.getX() + pix_x0,
                   -pix_per_world*w.getY() + pix_y0);
    return p;
  }
public Point2D worldToScreen ( ProjectionPointImpl w , Point2D p ) { p . setLocation ( pix_per_world * w . getX ( ) + pix_x0 , - pix_per_world * w . getY ( ) + pix_y0 ) ; return p ; }
convert a world coordinate to a display point
convert a world coordinate to a display point