static RGBImage mergeImagePartsVertically(RGBImage img, L<Rect> rects) {
  RGB spaceColor = new RGB(0.9f);
  int spacing = imageMergeSpacing();
  if (l(rects) == 0) ret new RGBImage(1, 1, spaceColor);
  int h = intSum(rects, "h")+(l(rects)-1)*spacing, w = intMax(rects, "w");
  RGBImage out = new RGBImage(w, h, spaceColor);
  int y = 0;
  for i over rects: {
    Rect r = rects.get(i);
    int x = (w-r.w)/2;
    copyRGBImage(img, r.x, r.y, out, x, y, r.w, r.h);
    y += r.h + spacing;
  }
  ret out;
}

Began life as a copy of #1006078