Help Center - Import 3

I see bad shading on imported objects.

Change the Hard Shading Angle in the object's Stats dialogue. This will recalculate the shading and usually fixes any shading problems such as dark polygons, flickering polygons or missing polygons.

If two surfaces are in the exact same position the OpenGL renderer won't know which one to display. This can be fixed by moving one of the objects so the two surfaces are no longer in the exact same position.