// draw faint one-pixel high stripes to indicate how big a pixel is
svoid drawPixelStripes(BufferedImage image) {
  int w = image.getWidth(), h = image.getHeight();
  var g = getGraphics(image);
  for y to h:
    drawLine(g, 0, y, w-1, y, even(y) ? Color.lightGray : Color.white);
}