   Lighting wierdness (XNA)
  Having a bit of a problem using a shader under XNA yet it works in RenderMonkey and FX Composer with the exact same model.If you imagine a coin spinning around its vertical axis, one side will be lit, the other side (when it comes into view at the same angle as the first) remains dark.If I flip the model over, then as it spins the previously dark side now lights and the other doesn't.  However, the light is fixed in the same place as the camera, so I expect both sides of the coin to light.  With the same parameters the same .FX works fine in FX Composer and both sides lighting equally well as I rotate the model, regardless of orientation or the axis of spin.I've checked my meshes and the normals look, well, perfectly normal - perpendicular to the face, all unit length : )So, that leaves me with the tangent frame, which XNA is apparently computing for me (Checked the box "Generate Tangent Frame" in the "content processor" category).  I'm pretty sure this is where the problem is as it is the only thing which I have not verified.Any suggestions ?EDIT: 'coin' is a simplification.  Actually thisis an object with a slightly U shaped profile and the origin point is thus external to the actual mesh.I mention this in case it upsets the tangent frame calculation.-Gary[Edited by - GaryCh on May 31, 2010 7:42:21 AM]