sclass G22CriticalPixelSearch is Steppable {
  // input
  settable IG22MasksHolder masksHolder;
  
  // internal
  int w, h;
  ItIt<Pt> pixelIterator;
  
  // output
  Pt lastPixelLookedAt;
  Entry[] pixelEntries;
  float[] pixelScores;
  
  *() {}
  *(IG22MasksHolder *masksHolder) {}
  
  class Entry {
    // child mask holders for pixel dark or pixel bright
    IG22MasksHolder darkChild, brightChild;
  }
  
  public bool step() {
    if (w == 0) {
      meta-for w also as h { w = masksHolder.maskSize().w(); }
      pixelEntries = new Entry[w*h];
      pixelScores = new float[w*h];
      arrayfill(pixelScores, -1);
      pixelIterator = new WeightlessShuffledIterator<Pt>(pixelsInImage(w, h));
    }
    
    if (!pixelIterator.hasNext()) false;
    lastPixelLookedAt = pixelIterator.next();
    true;
  }
}