Why are some surfaces of the 3D model upside down or invisible in Eyecad VR?

If you can't see some faces of the 3D model in Eyecad VR, these are probably flipped. Control the direction of the mesh in SketchUp, Cinema 4D, Rhinoceros or in the most popular 3D modeling software, they all have the function that allows you to reverse the direction of the 3D model mesh normals. Usually, you can correct the direction of the normal of the mesh by right-clicking on it and selecting the inversion option (the latter can have different names in each 3D modeling software). In SketchUp and in most software of solid modeling, all faces of the 3D model are visible from both sides. In Eyecad VR the back of the mesh is removed from rendering due to performance issues.