A roller cone drill bit and a method for designing thereof. The changer control system includes a local database of metadata records containing information regarding the optical discs in the optical disc changer. In an exemplary embodiment, determination of eyewear may involve viewer interaction through visual inspection of displayed calibration images or by other means. The isotropic plasma etching through the shadow mask can create a curved dent in a planar substrate in a central portion of an area enclosed by the bottom opening. The determining unit determines whether or not the first file is a distinguishable file.