srecord noeq G22RegionThinner(IImageRegion originalRegion) is Steppable {
  Rect bounds;
  
  // true = part of region
  // index is in bounds coordinates
  bool[] pixels;
  
  IImageRegion region = new {
    public Rect bounds() { ret bounds; }
  
    public bool contains(int x, int y) {
      ret containsPt(bounds, x, y) && pixels[idx(x, y)];
    }
    
    public Iterator<Pt> pixelIterator() {
      ret iff_null(new IF0<Pt> {
        int idx = 0;
        
        public Pt get() {
          for (; idx < pixels.length; idx++)
            if (pixels[idx])
              ret idxToPt(idx);
          null;
        }
      });
    }
  };
  
  int idx(int x, int y) {
    ret (y-bounds.y)*bounds.w+x-bounds.x;
  }
  
  Pt idxToPt(int idx) {
    ret pt(bounds.x+(idx % bounds.w), bounds.y+idx/bounds.w);
  }
  
  void init {
    if (bounds != null) ret;
    boudns = originalRegion.bounds();
    pixels = new byte[area(bounds)];
    for (Pt p : originalRegion.pixelIterator())
      pixels[idx(p.x, p.y)] = true;
  }
  
  public bool step() {
    init();
    false;
  }
}