sclass ImageSurface_HighlightRegion > ImageSurfaceMouseHandler is G2Drawable {
  FastRegions_BWImage regions;
  int iHighlightedRegion;
  Pt lastPt;
  RegionPainter regionPainter = new RegionBoundingBoxPainter;
  
  *(ImageSurface is) {
    if (containsInstance(is.tools, getClass())) ret;
    register(is);
    is.overlays.add(this);
  }
  
  public void close :: after {
    is.overlays.remove(this);
  }

  public void mouseMoved aka mouseEntered(MouseEvent e) {
    lastPt = getPt(e);
    update();
  }
  
  public void mouseExited(MouseEvent e) {
    lastPt = null;
    update();
  }
  
  // TODO: enter, exit
  
  int newRegion() {
    if (lastPt == null || regions == null) ret 0;
    ret regions.regionAt(lastPt.x, lastPt.y);
  }
  
  void update {
    int r = newRegion();
    if (iHighlightedRegion != r) {
      iHighlightedRegion = r;
      is.repaint();
    }
  }
  
  void setRegions(FastRegions_BWImage regions) {
    this.regions = regions;
    iHighlightedRegion = -abs(iHighlightedRegion);
    update();
  }
  
  public void drawOn(Graphics2D g) {
    if (iHighlightedRegion == 0 || regionPainter == null) ret;
    regionPainter.zoomX(is.getZoomX());
    regionPainter.zoomY(is.getZoomY());
    regionPainter.g(g); afterwards regionPainter.g(null);
    regionPainter.regions(regions).iRegion(iHighlightedRegion);
    regionPainter.drawRegion();
  }
  
  swappable void highlightRegion(int iRegion) {
    Rect r = regions.regionBounds(iRegion);
    g.setColor(withOpacity(.75, Color.red));
    temp tempSetStroke(g, new BasicStroke(lineWidth));
    drawRect(g, growRectTopAndLeft(scaleRect_floor(growRectRightAndBottom(r), is.getZoomX(), is.getZoomY())));
  }
}