How to use C++ display the normal direction of a face?

Hi everybody,
I’m using the Rhino 5 C++ SDK. Could somebody tell me how to display normal direction of a surface ?

Does this help?

hi Dale, thanks for your help. In addition to know the normal direction, the direction can be displayed on Screen. For example use Rhino,
1.choose a surface, "_Dir"
3.white arrow will be showed.
4. press mouse left button, it can change direction of surface
Third and fourth points above what I want.
I do not know what function I can use. Sorry my English is not good, if you still do not know what I mean, I’ll try to explain.

Here is an example of how to draw mesh face normals:

You can use this an an example of how you might draw surface normals.

To change the direction of a face normal, use ON_BrepFace::Reverse(). See opennurbs_brep.h for details.

Thank you very much.