Several non-manifold data structures using the Ordered Topological Representation Method have been proposed to meet the increasing need for non-manifold modeling in CAD systems. However, a detailed comparison of characteristics, merits, and demerits of each data structure has not been made. In this paper, several reference topological entities are introduced to compare non-manifold data structures. The boundary information for B-rep non-manifold representation is clearly defined. Characteristics of topological entities, adjacency relationships, treatment of special cases, and storage requirements are compared. Ideas to improve the time and space efficiency of each data structure are proposed. By the observations a new data structure is proposed.
Existing non-manifold data structures, which use the ordered topological representation method, are designed based on a model which is the highest topological entity. Their non-manifold information is always included in edges and vertices even if they are in the manifold situation. Thus they require large storage spaces than manifold data structures. The proposed data structure reduces its storage space by removing unnecessary information stored in edges and vertices. Topological information is classified into manifold and non-manifold information. The important non-manifold information is radial cycles and disk cycles. The proposed data structure always stores manifold information. For the non-manifold situation, the edge stores radial cycles, and the vertex stores disk cycles. The storage space can be reduced in the later stage of CAD design when the ratio of non-manifold to manifold entities is small.
