code: public void learnFern(boolean positive, ImageRectangle r) {
  
  		float rectWidth = r.getWidth();
  		float rectHeight = r.getHeight();
  
  		float c_x = r.x0+(rectWidth-1)/2f;
  		float c_y = r.y0+(rectHeight-1)/2f;
  
  		for( int i = 0; i < ferns.length; i++ ) {
  
  			// first learn it with no noise
  			int value = computeFernValue(c_x, c_y, rectWidth, rectHeight,ferns[i]);
  			TldFernFeature f = managers[i].lookupFern(value);
  			increment(f,positive);
  		}
  	}
docs: Learns a fern from the specified region.  No noise is added.