modify a function and call it in the main function

[zhenghui]

I want to implement the border tracing algorithm

[Stewbond]

Your URLs aren't complete, but yes we can help:
Take a look at this tutorial. It will do a better job at explaining functions than I can in a single forum post:
http://cplusplus.com/doc/tutorial/functions/

Really, you need to ask yourself, what are the inputs to my function? Those will be the arguments. You also need to ask, "what is the output of my function"? That will be the return value.