[vtkusers] How to get the visible face?

[Larry]

Hello, everyone!
Suppose I have a polydata, which is consisted of triangles, I'd like to get the faces which can be seen by the camera.
That's to say, there is a point M, and an object whose faces is a set of triangles, How can I get the faces that can be seen by the point M? and how can I get the faces that is not seen by the point M?

Thanks!
Larry
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