In visible space algorithms we determine the surfaces which are visible to observer located at a particular point in 3D. There are two methods to achieve this: Object space method and image space method.
Object space method: It is implemented in physical coordinate system and used in line display algorithms. In this method, objects and parts of objects are compared to find out the visible surfaces.
Image Space method: It is implemented in screen coordinate system and used in hidden surface algorithms. In this method, each point is considered for the determination of visible surfaces.