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