class ImageWindow : public TopWindow {
   Image img;
   Image cache;

   void SetCache() { if (IsNull(cache) || cache.GetSize() != GetSize()) cache = Rescale(GetSize(), GetSize()); }
public:
   virtual void Paint(Draw &w) { SetCache(); w.DrawImage(0, 0, cache); }
   void SetImage(const Image &i) { img = i; cache.Clear(); }
};