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;

  }

}