static void drawBoxes(BufferedImage image, L<Rectangle> boxes, Color color, float opaqueness) {
  Graphics2D g = image.createGraphics();
  g.setColor(colorWithAlpha(color, opaqueness));
  for (Rectangle r : boxes)
    g.drawRect(r.x-1, r.y-1, r.width+2, r.height+2);
  g.dispose();
}