GEODERM: geometric shape design system using an entity- relationship model D Krishnan and L M Patnaik they also lack application independency 8. In most mech- GEODERM, a microcomputer-based solid modeller, which anical design applications, there is a need to generate incorporates the parametric object model, is discussed. The mechanical parts having the same topology but different entity-relationship model, which is used to describe the dimensions. This necessitates the need to represent shape conceptual schema of the geometric database, is also pre- topology and dimension at a conceptual level. sented. Three of the four modules of GEODERM, which In robot motion simulation, it is often necessary to have been implemented are described in some detail. They maintain relationships between object parts like the robot are the Solid Definition Language (SDL), the Solid Manipu- arm and the base. These relationships should be easily lotion Language (SML) and the User-System Interface. accessible to a user who has no knowledge about the data structure details involved. Also since the geometric modeller computer-aided design, solid modeller, geometric database, could be used in a variety of applications, it should be entity-relationship model possible to add a new application program without modify- ing the internal representations. All these requirements Most present day computer-aided design systems incor- emphasize the need for a high level conceptual represen- porate a solid modeller as one of their main components, tation of a solid object. Thus the conceptual parametric The main operations performed in such a system include object model proposed by Kawagoe and Managaki s'~ has engineering analysis, drafting and computer-aided manu- been adopted in GEODERM (geometric shape design facturing. Such operations should be based on a single system using the entity-relationship model), the solid representation or multiple consistent representations of a modeller presented in t'his paper, to allow a high level shape 3D object. Object representations in CAD systems should representation. Such a representation also removes the pre- be able to generate all the design data required during a viously mentioned defects of the existing solid modellers. design cycle of an object. CAD systems incorporating a We incorporate some modifications to the original pard- solid modeller with interactive graphics facilities find exten- metric object model. These modifications include the use sire applications in mechanical product design, numerically of Euler operators in modelling B-rap objects to result in controlled machining of parts and finite element mesh a conservation of memory space. They also include the analysis, addition of the octree representation to the internal repre- Ever since the establishment of the fact that a simple sentation schemes to facilitate faster display and property wireframe representation of a solid is not adequate for most calculation of the solid objects. The well known E-R purposes such as volume calculation and hidden-line removal, model 1° which is a conceptual model for database applica- a number of other representation schemes have been pro- tions has been used to describe the parametric object model posed to model solids in a more effective manner. Signifi- in case of GEODERM. We also present solid definition and cant among solid modelling systems based on these schemes manipulation languages for this model. are BUILD 1, PADL 2, TIPS 3, GLIDE 4 , GEOMAP s and One of the most recent developments in computer GWB 6 . Most of these systems use either the CSG (construc- graphics is the emergence of the microcomputer-based tire solid geometry) 7 or the B-rap (boundary representa- intelligent workstations. We chose to implement our soft- tion) T approach as an internal representation of the solid, ware on an Intel Series III microcomputer development The CSG approach models a solid as a combination of system with a keyboard input. This will make easier the primitive solids, expressed as an ordered binary tree development of a powerful CAD workstation supporting wherein the terminal nodes denote primitive solids and the solid modelling features on a graphics subsystem with 8086 nonterminal nodes denote operations such as union, inter- processor, 82720 graphics display controller and CORE section and difference etc to be performed on the primitive standard graphics software 11. solids. The B-rep method describes a solid through a set of entities such as faces, edges and vertices. These and other unambiguous representations of solids have been discussed PARAMETRIC OBJECT MODEL in great detail in Requicha 7. In this model s'9, an object class P is represented as a Most of the above systems lack the ability to represent sextuple objects parametrically, to incorporate more semantic infor- mation regarding relationships between object parts, and P = {T, D, O, V, R, C ) School of Automation, Indian Institute of Science, Bangalore 560 012, India where T stands for topologies which represent relationships volume 18 number 4 may 1986 0010-4485/86/040207-12 $03.00 © 1986 Butterworth & Co (Publishers) Ltd 207