static BWImage clusterImages(L<BWImage> images) {
  if (empty(images)) null;
  int n = l(images);
  int w = (int) sqrt(n), h = (n+w-1)/w;
  int cw = 0, ch = 0;
  for (BWImage i : images) {
    cw = max(cw, i.getWidth());
    ch = max(ch, i.getHeight());
  }
  // TODO: check if there are too many full black rows/cols in images
  int iw = 1+w*(cw+1), ih = 1+h*(ch+1);
  
  // light gray background to distinguish between image and spacing
  BWImage img = new BWImage(iw, ih, 0.95f);
  
  for (int x = 0; x < iw; x += cw+1)
    for y to ih:
      img.setPixel(x, y, 0f);
  for (int y = 0; y < ih; y += ch+1)
    for x to iw:
      img.setPixel(x, y, 0f);
  for i to n: {
    int x = i % w, y = i / w;
    BWImage im = images.get(i);
    copyBWImage(im, 0, 0, img,
      1+x*(cw+1)+(cw-im.getWidth())/2,
      1+y*(ch+1)+(ch-im.getHeight())/2);
  }
  ret img;
}