How to draw one parallel path through finger gesture?

[shaozhenged]

I want to draw one red path which parallel or follow the white path.

Just like the video show:

parallel_pen.mp4

I search all the documents and tutorials that I can found,I am wonder if there was a simple way to implement it?

My implementation method is to calculate the normal vector of each point on the path and obtain new parallel points,the new path is not really smooth.

I would greatly appreciate it if anyone could provide some advice.