code: public static List<Contour> contourExternal(GrayU8 input, ConnectRule rule ) {
  		BinaryContourFinder alg = FactoryBinaryContourFinder.linearExternal();
  		alg.setConnectRule(rule);
  		alg.process(input);
  
  		return convertContours(alg);
  	}
docs: Finds the external contours only in the image
  @param input Input binary image.  Not modified.
  @param rule Connectivity rule.  Can be 4 or 8.  8 is more commonly used.
  @return List of found contours for each blob.