Wasn't it jessy who wrote:
>thanks,Hasan3,
>I just saw the information about normal vector.it says that the
>normal_vectors{}  is the vertex's normal and If the number of normals
>equals the number of vertices then the normal_indices{} list is optional
>and the indexes from the face_indices{} list are used instead.
>   I don't understand the normal_indices{} is record for what?HOw it relate
>to normal_vectors{}?it seems   like face_indices{} in the example.
>   if I can get the face's normal,can use it?how?thanks.

If you want an object to be smooth everywhere, then each vertex should
only have one normal. The normals on all the faces that meet at the
point match, so there's a smooth transition from one face to another.

On the other hand, if you want your object to have sharp edges somewhere
(e.g. around the edge of the circle in the middle of your shape) then
points on the edge should have two normals (or more if you had wanted
sharp points, which you don't). A face outside the circle would have one
normal at an edge point, and a face inside the circle has a different
normal at the same point. This gives a discontinuity in the normals and
creates a sharp edge.

It is also possible to achieve the same effect by duplicating the edge
points. You could have two vertices at the same location, one with a
normal that matches the inside slope and one that matches the outside
slope. 

-- 
Mike Williams
Gentleman of Leisure

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