// onColor is called with null value when mouse moves outside of image surface
svoid imageSurfaceColorPicker(ImageSurface is, VF1<RGB> onColor) {
  if (is == null || onColor == null) ret;
  swing {
    MouseAdapter ma = new MouseAdapter {
      public void mouseMoved(MouseEvent e) { pick(e); }
      public void mouseEntered(MouseEvent e) { pick(e); }
      public void mouseExited(MouseEvent e) { pick(null); }
      
      void pick(MouseEvent e) {
        if (e == null) callF(onColor, null);
        else callF(onColor, getImage(is.pointFromEvent(e));
      }
    };
    c.addMouseMotionListener(ma);
    c.addMouseListener(ma);
  }
}