Get objects from a Graphics3dGroup

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Dear all,

I would like to trasnform a 3d object into a set of faces (or triangles). For that, I take a 3d object and convert it to a STL file following this procedure.

The short questions is: how to get the 3d objects from a Graphics3dGroup?

The long question : I don't understand the nature of the 3d graphics. For instance:

G1 = polygon3d([(0,0,0),(1,0,0),(2,1,1)])
print(type(G1))
G2 = polygon3d([(0,0,0),(1,0,0),(2,1,1)]) + polygon3d([(1,2,3),(1,0,0),(2,1,1)])
print(type(G2))
G3 = sphere((0,0,0),1)
print(type(G3))

1. In the first case it seems to be a set of faces and it's possible to convert it to a STL file.
2. In the second case it is a group of 3d object and -as a group- it has no face.
3. The sphere is a TransformGroup and has no face.

How to get the faces of the last two situations?

Thanks for yours answers.

Arnaud