anchor.diff:        if (yPos < 0) yPos += _gameSizeHeight;
   
         _starPositions[i] = new Point(xPos, yPos);
  +    }
  +  }
  +
  +  void update(double dt) {
  +    if (_autoScroll) {
  +      move(dt * 100.0, 0.0);
       }
     }
   }
  
positive.code: 
  // Background starfield
  
  class StarField extends Node {
    Image _image;
    int _numStars;
    List<Point> _starPositions;
    List<double> _starScales;
    List<Rect> _rects;
  
negative.code:     ..isAntiAlias = false
      ..setTransferMode(sky.TransferMode.plus);
  
    StarField(SpriteSheet spriteSheet, this._numStars, [this._autoScroll = false]) : super(new Size(1024.0, 1024.0)) {
      _starPositions = [];
      _starScales = [];
      _colors = [];
  