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At present, object-oriented programming is emerging from the research labora­ tories and invading into the field of industrial applications. More and more products have been implemented with the aid of object-oriented programming techniques and tools, usually as extensions of traditional languages in hybrid development systems. Some of the better known examples are OSF-Motif, News, Objective-C on the NeXT computer, the C extension C++, and CLOS an object­ oriented extension of LISP. All of these developments incorporate interactive graphics. Effective object-oriented systems in combination with a graphics kernel­ does it mean that the field of computer graphics has now become merely an aspect of the object-oriented world? We do not think so. In spite of interesting individual developments, there are still no sound object-oriented graphics sys­ tems available. If it is desired to develop a complex graphics application embed­ ded in a window-oriented system then it is still necessary to work with elemen­ tary tools. What is to be displayed and interactively modified inside a window must be specified with a set of graphics primitives at a low level, or has to be written with a standardized graphics kernel system such as GKS or PHIGS, i. e., by kernels specified and implemented in a non-object-oriented style. With the terms GKS and PHIGS we enter the world of international graphics standards. GKS and PHIGS constitute systems, not mere collections of graphics primitives.

Eurographics, the European Association for Computer Graphics, has always been an important forum for discussions and presentation of results concerning the first ISO Graphical Standard, GKS (the Graphical Kernel System) and later of its three-dimensional extension, GKS-3D. This book is a collection of those articles which have appeared within the framework of Eurographics in the past 5 years, and which still contain, even after several years, valid and interesting results concerning the problems arising in connection with GKS. Some of these papers help the reader to gain a deeper understanding of the standard; others deal with general implementation problems, and finally there are some presentations of specific algorithms usable also for a GKS or GKS-3D implementation. The book may be of a particular interest to those specialists who intend to implement a GKS package or some similar graphics subsystem and who can therefore make direct use of the experiences reflected in this collection. The book should also be a valuable supplement in university courses concerned with teaching the principles of implementing device-independent computer graphics.

TO COMPUTER GRAPHICS BASED ONGKS Part I gives an introduction to basic concepts of computer graph­ ics and to the principles and concepts of GKS. The aims of this part are twofold: to provide the beginner with an overview of the terminology and concepts of computer graphics, based on GKS, and to give the computer graphics expert an introduc­ tion to the GKS standard. In the early chapters of this part, the main areas of computer graphics, the various classes of com­ puter graphics users, the interfaces of GKS and its underlying design concepts are discussed and important terms are defined. The later chapters give an informal introduction to the main concepts of GKS and their interrelationships: output, attributes, coordinate systems, transformations, input, segments, metafile, state lists, and error handling. This introduction to the GKS framework will prepare the ground for the detailed description of 2D GKS functions in Part III and the 3D extensions to GKS in Part IV. 1 WHAT IS COMPUTER GRAPHICS? 1. 1 Defmition of Computer Graphics The Data Processing Vocabulary of the International Organization for Stan­ dardization (ISO) [ISO 84] defines Computer Graphics as follows:'Methods and techniques for converting data to and from a graphic display via computer.'This definition refers to three basic components of any computer graphics system - namely'data','computer', and'display'.