   mesh normal
  a mesh consist of some submeshs.some submesh has reverse normal.if D3DCULL_NONE do not be set, you will not see it .if D3DCULL_NONE is set. all is ok.but if you set a direct light, and enable light. what happen?? you guess.you will see a darker mesh. but why ?   
  Because the normal is pointing away from the light, so the face isn't lit. Remember that lighting is done as L dot N (Light vector dot Normal vector), so if the normal is facing away from the light, you'll get a darker triangle.The best way to fix it would be to fix the normals so they're not backwards. Also, not culling back-facing triangles could cause some performance issues and is best avoided for this purpose.   ;  You might want to try googling/searching for "two sided lighting" - I know Nvidia put out some samples a few years back (it was one of the cool new things that vertex shaders made easier). That should allow you to have a single triangle that can be lit from both sides - although typically its used so you can have different textures/colours without doubling the geometry.hthJack   