So these algorithms are line based instead of surface based. In the wireframe model, these are used to determine a visible line. These methods generally decide visible surface. After comparison visible, invisible or hardly visible surface is determined. Object space methods: In this method, various parts of objects are compared. Types of hidden surface detection algorithms Hidden surface algorithm bears a strong resemblance to two-dimensional scan conversions.A hidden surface algorithm is generally designed to exploit one or more of these coherence properties to increase efficiency.Object coherence results from relationships between different objects or between separate parts of the same objects.Frame coherence in a sequence of images designed to show motion recognizes that successive frames are very similar.Scan line coherence arises because the display of a scan line in a raster image is usually very similar to the display of the preceding scan line.Different types of coherence are related to different forms of order or regularity in the image.Hidden line and Hidden surface algorithms capitalize on various forms of coherence to reduce the computing required to generate an image.Geometric sorting locates objects that lie near the observer and are therefore visible.Just as alphabetical sorting is used to differentiate words near the beginning of the alphabet from those near the ends.All use some form of geometric sorting to distinguish visible parts of objects from those that are hidden.The algorithm operates on different kinds of scene models, generate various forms of output or cater to images of different complexities.To remove these parts to create a more realistic image, we must apply a hidden line or hidden surface algorithm to set of objects.Instead, all parts of every object, including many parts that should be invisible are displayed.
In the computer generation, no such automatic elimination takes place when objects are projected onto the screen coordinate system.In real life, the opaque material of these objects obstructs the light rays from hidden parts and prevents us from seeing them.One of the most challenging problems in computer graphics is the removal of hidden parts from images of solid objects.
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