2828	 Hierarchical Geometric Models for Visible Surface Algorithms	 The geometric structure inherent in the definition of the shapes of three-dimensional objects and environments is used not just to define their relative motion and placement but also to assist in solving many other problems of systems for producing pictures by computer. By using an extension of traditional structure information or a geometric hierarchy five significant improvements to current techniques are possible. First the range of complexity of an environment is greatly increased while the visible complexity of any given scene is kept within a fixed upper limit. Second a meaningful way is provided to vary the amount of detail presented in a scene. Third clipping becomes a very fast logarithmic search for the resolvable parts of the environment within the field of view. Fourth frame to frame coherence and clipping define a graphical working set or fraction of the total structure that should be present in primary store for immediate access by the visible surface algorithm. Finally the geometric structure suggests a recursive descent visible surface algorithm in which the computation time potentially grows linearly with the visible complexity of the scene. visible surface algorithms hidden surface algorithms hierarchical data structures geometric models
