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Computer aided design is the modeling of physical systems on computers, allowing both interactive and automatic analysis of design variants, and the expression of designs in a form suitable for manufacturing. I think that this definition encompasses all of the types of work that is subsumed under the CAD umbrella, in all the various areas of application.

This definition implies that simulation is a far more important part of CAD than design description. I believe that this is true. Also, computer graphics has nothing at all to do with CAD, except as the servant of design, simulation, or presentation.

What Should a CAD System Be?

To best fulfill the definition of CAD given above, a CAD system should be a computer system that allows modeling of physical systems. To date, modeling has been done almost entirely with hard-coded dedicated systems usable only for one form of design: there's not a lot in common between ANSYS and SPICE. But, after all, the physical universe is a unified place with common rules, and it's not at all clear that one should have to write tens of thousands of lines of FORTRAN just to get started on a general-purpose modeler.

CAD systems to date have developed into general-purpose tools that understand geometry. From MacDraw to Medusa there is a continuum of knowledge about geometry and operations on either 2D or 3D primitives. What knowledge of reality exists is usually welded on as an afterthought (the very word ``attribute'' indicates how reality takes a second seat to geometric description).

A typical CAD system offering has a geometry processor with attached database, providing a ``common design database''. Analysis and simulation sits on top of this core, embodied in a host of separate programs which intercommunicate, if at all, only by passing information through the database. If one wants to create a new analysis program, ``well, we have a FORTRAN compiler and library that lets you read the database''.

Need it be this way? Can we not imagine a geometry-based CAD system evolving into a system which describes physical objects, and knows about the various ways in which they interact (and can be taught about interactions as we define new forms of geometry today)? Such a system would encompass all of what a CAD system does today, and would provide a common user interface and model for working with reality represented in a database.

What Does Simulation Have To Do With It?

Alan Kay, delivering the keynote speech at the Second West Coast Computer Faire in 1978 said, ``we decided to focus on simulation in Smalltalk, because that's the only really interesting thing to do with a computer''. When I heard this, I was aghast: ``Simulation'', I thought, ``why in the world would people want to use personal computers to model throughput in a machine shop, or to calculate the number of toilets in a football stadium''. Certainly any rational person wanted a personal computer to do real computer science on it: to write operating systems and compilers so that others could use them to write programs, and...well, I hadn't thought that out completely.

``Simulation'' had come to mean (at least in the computer science lexicon), a specific kind of modeling of systems, usually done in an odd simulation language such as Simscript or Simula.