public MedianPointAccumulator(SerializationInfo info, StreamingContext context)
 {
   _max_entries = info.GetValue<int>("_max_entries");
   _grid = new DensePixelGrid<PerPixel>();
 }
 /// <summary>
 /// Construct empty accumulator
 /// </summary>
 public MedianPointAccumulator() {
   _max_entries = 1000;
   _grid = new DensePixelGrid<PerPixel>();
 }