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;

}