For generation of realistic graphic displays, we have to identify those parts of a scene that are visible from a chosen viewing position. There are many algorithms called visible surface algorithms developed to solve this problem. In early days of computer graphics visible surface algorithms were called hidden line or hidden surface algorithms.
In a given set of 3D objects and viewing specification, we wish to determine which lines or surfaces of the objects are visible, so that we can display only the visible lines or surfaces. This process is known as hidden surfaces or hidden line elimination, or visible surface determination. The hidden line or hidden surface algorithm determines the lines, edges, surfaces or volumes that are visible or invisible to an observer located at a specific point in space. These algorithms are broadly classified according to whether they deal with object definitions directly or with their projected images. These two approaches are called object-space methods or object precision methods and image-space methods, respectively.
Object-space Method: Object-space method is implemented in the physical coordinate system in which objects are described. It compares objects and parts of objects to each other within the scene definition to determine which surfaces, as a whole, we should label as visible. Object-space methods are generally used in line-display algorithms.
Image-Space Method: Image space method is implemented in the screen coordinate system in which the objects are viewed. In an image-space algorithm, visibility is decided point by point at each pixel position on the view plane. Most hidden line/surface algorithms use image-space methods.