说明：  边界跟踪程序，可以跟踪闭合的边界和非闭合的边界。读者可以根据自己需要存储其跟踪方向以及跟踪路径。
(Boundary tracking program, you can track the closed borders and the non-closed borders. Readers can track the direction of their own need to store and track the path.)