sclass ScreenLens {
  double sleep = 0; // 0.2;
  int w = 300, h = 300;
  ImageSurface is;
  volatile BufferedImage image, imagePP; // pp = postProcessed
  bool keepAwayFromBorder = true, makeBot = false;
  bool showFPS = true;
  S botName = "Screen Lens.";
  Android3 bot; // exporting our functionality as bot to other VMs
  bool pack = true;
  new PerSecondCounter fps;
  javax.swing.Timer timer;
  
  O onImageChange;
  O postProcess; // func(BufferedImage) => BufferedImage
  
  ScreenLens start() {
    swingAndWait(r {
      is = alwaysOnTop(showImage(new RGBImage(w, h, Color.white)));
      timer = installTimer(is, toMS(sleep), r {
        Pt m = new Pt(mouseLocation());
        Rect screen = screenRect();
        if (keepAwayFromBorder) {
          m = new Pt(min(m.x, screen.x2()-w/2), min(m.y, screen.y2()-h/2));
          m = new Pt(max(m.x, w/2), max(m.y, h/2));
        }
        Rect r = intersectRects(screen, rectAround(m, w, h));
        image = shootScreen2(r);
        imagePP = callPostProcessor(postProcess, image);
        is.setImage(imagePP);
        if (pack) {
          packFrame(lens.is);
          moveToTopRightCorner(getFrame(lens.is));
        }
        pack = false;
        fps.inc();
        if (showFPS)
          setFrameTitle(is, "Screen Lens (" + iround(fps.get()) + " fps)");
        pcallF(onImageChange);
      });
    });
    addSizeChooser();
    if (makeBot) bot = makeBot(botName, this);
    this;
  }
  
  void addSizeChooser() {
    final JTextField tfSize = jTextField(w);
    addToWindow(is, jRightAlignedLine(jlabel("Size:"), jMinWidth(50, tfSize), jlabel("pixels")));
    onUpdate(tfSize, r {
      w = h = max(1, parseIntOpt(getTextTrim(tfSize)));
      pack = true;
    });
  }
  
  S answer(S s) {
    new Matches m;
    if "get image"
      ret image == null ? "null" : structure(new RGBImage(image));
    if "get post processed image"
      ret imagePP == null ? "null" : structure(new RGBImage(imagePP));
    if "running" ret yn(timer.isRunning());
    null;
  }
}