Abstract: This paper proposes a hierarchical model for developing CAD application system, analyzes the hierarchical structure characteristics of the model, proposes a solution to the special problems in graphic programming, and develops a general hierarchical model processing function library.
Keywords: CAD hierarchical model modeling transformation
Hierarchic Model for Developing CAD Application System
Zhou Houbin et al
Abstract:A hierarchic model for developing CAD application system is presented. The features of the hierarchic structure of the model are analyzed. The special problems in the graph programming are discussed and solved. A universal function library for handling the hierarchic model is developed.
Keywords:CAD hierarchic model modeling transformation
I. Introduction
In the development of CAD application systems, the programming of various design drawings is often an important content. Due to the many structural details involved in engineering and mechanical design drawings, the mutual relationship is complicated, so the corresponding graphic programming work is often very cumbersome. In addition, the graphics program compiled according to the conventional programming ideas is not refined enough from the programming style and is not easy to maintain. Therefore, under the premise of following the principle of uniform programming, it is of great practical significance to rationally organize the program structure according to a new concept and idea to improve development efficiency and program quality. The idea of ​​model hierarchy proposed in this paper is an effective method to solve this problem, which is very suitable for application in engineering and mechanical CAD graphics programming. In fact, it is also an important part of the PHIGS graphics standard promulgated by ISO. Based on this, the author developed a general-purpose C language function library for hierarchical model processing, which can work with existing graphics software packages and has been applied to the development of multiple CAD application systems, and has received good results.
Second, the model's hierarchical structure and modeling transformation
1. Hierarchy of geometric models
The hierarchy of the model is expressed as: constructing a higher-level entity with basic components, and constructing a higher-level entity with a higher-level entity. In engineering design, the relationship between layers is actually the relationship between objects and sub-objects, parts and sub-components, graphics and sub-graphics. Hierarchical relationships also reflect the assembly call relationships between them. The components in the hierarchy not only have their own geometric shape, but also the position and relationship of the components to each other. The geometry of the component itself is independent of other components and can be defined on a local coordinate system that it owns. This coordinate system is called the master coordinate system. The high-level component can perform an instance transformation on the body coordinate definition of the low-level component to form an instance of the component in the body coordinate system of the high-level component. If the high-level component is a root object, an instance of the component is obtained on the world coordinate system. The relative positional relationship between components is determined by these reference transformations. In addition, in the body coordinate system of the component, in order to facilitate the geometric processing of the component itself, some coordinate transformations can be introduced, which is another kind of transformation different from the reference transformation. Both types of transformations are used to construct object models, collectively referred to as modeling transformations.
The graph mechanism is introduced in the GKS graphics standard, which can be used to construct a single-layer object model, and its modeling function is very limited. The newer PHIGS graphics standard enables the construction of object models of any level by introducing structures and their nested reference mechanisms.
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