The evolution of computer-aided design has been largely related to developments in computer graphics. Of course, CAD encompasses much more than computer graphics, as we shall discuss in the remainder of this chapter. However, ICG forms the essential technological foundation for computer-aided design.
One of the significant initial projects in the area of computer graphics was the development of the APT language at the Massachusetts Institute of Technology in the middle and late 1950s. APT is an acronym for Automatically Programmed Tools, and this project was concerned with developing a convenient way to define geometry elements for numerical control part programming using the computer. We discuss numerical control Breitling Replica(http://www.luv-replica.com/GoodsBrand/Breitling_Replica_Watches-4.html) programming, and in particular the APT language. Although the development of APT was an important milestone in the field of computer graphics, the early use of APT was not accomplished interactively.
Another concept which took form during the late 1950s was the "light pen.” The idea for this device came about during research on the processing of radar data for a defense project called SAGE (Semi-Automatic Ground Environment system). The purpose of the project was to develop a system to analyze radar data and to present possible bomber targets on a CRT display. To save time in displaying the interceptor aircraft against the bombers, the notion of using a light pen to identify a particular sector of the CRT screen was developed.
The same principle can be applied to three-dimensional cases. A wire¬frame model in 3D space is, in fact, a computer graphic model. The basic form uses wire frames to represent the object. In this form, the object is displayed by interconnecting lines, as shown in Fig. 6.3. A wire-frame model is also represented in the computer by tables of lines and points. The main advantage in using wire-frame models in 3D graphics is fast generation of graphic objects. This is because no additional evaluations for geometric and topological integrity and for shading of surfaces are carried out. Because of the lack of integrity evaluations, the model may be ambiguous. Also, some invalid objects may be created.
Even three-dimensional wire-frame representations of an object are sometimes inadequate for complicated shapes. Wire-frame models can be enhanced by several different methods. Fig. 6.4 shows the same object shown in the previous figure but with two possible improvements. The first uses dashed lines to portray the rear edges of the object, those which would be invisible from the front. The second enhancement removes the Cell decomposition;
To use geometric modeling, the designer constructs the graphical image of the object on the CRT screen of the ICG system by inputting three types of commands to the computer. The first type of command generates basic geometric elements such as points, lines and circles. The second command type is used to accomplish scaling, rotation, or other IWC Replica Watches(http://www.imitatewatch.com/GoodsBrand/Replica-IWC-Watches-45.html) transformations of these elements. The third type of command causes the various elements to be joined into the desired shape of the object being created on the ICG system. During this geometric modeling process, the computer converts the commands into a mathematical model, stores it in the computer data files, and displays it as an image on the CRT screen. The model can subsequently be called from the data files for review, analysis, or alteration.
During the early 1960s, Ivan Sutherland worked on a project at MIT called "Sketchpad" and presented a paper on some of his results at the Fall Joint Computer Conference in 1963. The Sketchpad project is significant because it represents one of the first demonstrations of the creation and manipulation of images in real time on a CRT screen. To many observers, it marks the beginning of interactive computer graphics.
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