How can I determine the surface normals of meshes created with the 3DFACE and RULESURF commands?
Normals are determined by the direction in which a face is drawn in AutoCAD based on a right-handed coordinate system. For example, if you draw the face counterclockwise, the normals point outward. If you draw the face clockwise, the normals point inward. When rendering the drawing, you can disregard faces with normals pointing away from your viewpoint by checking the “Discard Back Faces” option in the Rendering Preferences dialog box. The AutoLISP routine re3dface.lsp can be used to revert the direction order for a 3D face.
