 function, some polygon may disapear with no reason.
   The normal Point3D is used to compute the polygon visibility, and if you don't update the normal Point3D after the vertices modifications, there's an error.
   To fix that problem, Sandy3D offers that new property appeared in 3.0.3 release, which once set to true, automatically update the normal Point3Ds for you.
   As a performance warning, don't set this value to true if your model geometry isn't animated.</p>
   </div>
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