// by default returns dark regions as foreground

sclass G22ForegroundFinder {
  // input
  settable L<IImageRegion> allRegions;
  
  // internal
  gettable Rect borderRect;
  
  // output
  Set<IImageRegion> foregroundRegions;
  
  *() {}
  *(L<IImageRegion> *allRegions) {}
  
  Set<IImageRegion> foregroundRegions aka get() {
    run();
    ret foregroundRegions;
  }
  
  L<IImageRegion> backgroundRegions() {
    ret listMinusSet(allRegions, foregroundRegions());
  }
  
  run {
    if (foregroundRegions != null) ret;
    borderRect = mergeRects(map(allRegions, r -> r.bounds()));
    foregroundRegions = new LinkedHashSet;
    for (r : allRegions)
      if (isForegroundRegion(r))
        foregroundRegions.add(r);
  }
  
  swappable bool isForegroundRegion(IImageRegion region) {
    ret defaultDecolorizer().toGrayScale(region.color()) < 128;
  }
}