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Introduction
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The massachusetts Institute of Technology (M.I.T.) is recognised as the birth place of CAD. In 1962/3 the MIT unveiled the revolutionary SKETCHPAD system developed by Ivan Sutherland. This revolutionary system allowed the designer to interact graphically via a display screen and light pen, and was initially limited to a 2 dimensional system.
As CAD developed with 3 dimensional modelling and computer aided manufacturing systems, it was only in the 1980s that it became a practical commercial reality to use this new design tool in the day to day workplace.
At the present time, 2000AD, the now traditional CAD is well established and with the widespread availability of powerful/cheap computers and sophisticated wide ranging CAD, Graphics and Communications software, we would seem to witnessing an electronic design revolution.
Representing Objects
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The design process relies heavily upon the use of graphical images. Traditionally, this has been achieved through a combination of artist's impressions, technical drawings and photographs of existing
artifacts or landscapes.
The objects that exist in the natural world would appear to have an endless variety of shapes and textures whereas man-made objects would seem to adhere to some design constraints.
An artist using free hand techniques is able, within the constraints of the bone and joint linkages in the human hand and arm, to provide images that often time look very natural. In contrast to this, drafting produces images that are often described as " mechanical" in appearance. This is because the drafting process uses rules, templates and devices to produce desired shapes.
When artists produce paintings they do not include every piece of fine detail but include prominent features and sufficient of the detail to produce the desired effect. Even fine art artists only include sufficient detail to produce the effect, for example, a portrait artist does not include each and every hair follicle or individual eye lash.
Technical drafting requires that details of a proposed or existing design are drawn at specified scales. The implements used to mark the shapes on the paper are normally pencil or pen and they have definite line widths, for example, 0.5 millimetres. It would therefore become impossible to draw anything that is thinner or closer than the thickness of the pencil. So, there would appeared to be self regulating limits to the amount of detail which can be included in an artists drawing or painting and a draft-persons plans.
There are no such limits to technical drafting when using a cad system. Lines we can be represented with no thickness. Minute sub atomic dimensions can be detailed alongside galactic dimensions, which in reality is in-conceivable.
Coupled with these stated problems is one of the volume of data. There are limitations to the quantity of data that can effectively be handled or stored by a computer. This can be one of the overriding concerns when deciding upon a drawing structure and whether or not a drawing should be broken down into separate sub drawings and whether that amount of detail is actually necessary.
Form and Function
Each object has a FORM and a FUNCTION
We can look upon the FORM as the physical shape of the object and upon the FUNCTION
as the puropse of the FORM.
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For example:
a loud speaker has a form (its physical appearance) and a function has a purpose (the production of soundwaves)
a plan of a speaker has a form and the plan has a function which is to relate design information about the speaker.
Our studies during this module will concentrate on the FORM of an object with frequent reference to its function, because form, without function is of no use in the design process.
We have the ability to see and appreciate a great deal of visual detail and expect that the much acclaimed computer should be able to reproduce exactly what we know to exist. A hand looks fairly complicated at an initial glance and as we look closer we find that the detail gets even finer. Note the increased detail in the folds on the
knuckle. It would require a very complex model to describe the form of the hand in full
detail, ( and who says when full detail is reached?). A dermatologist may want
to look very closely at the surface of the skin but a Glove manufacturer may
only be concerned with the overall dimensional information.
In contrast to the hand, the form of the ring does not change to any appreciable
extent as it is magnified. It is a manufactured item, not perfect, but built within certain tolerances.
Its form is relatively simple and does not have great amounts of surface detail.
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We have to be sensible about the degree of detail we need to record. What is the
function of the image above. Perhaps to show the position and appearance of a
ring on a hand. Do we need to include details of skin folds and surface hair?
If the degree of detail shown above is important then it could be modelled. But! Is it reasonable to expect that degree of detail
for both hands, what about the face and the hair on the head, what about a crowd
of people? We have to keep in mind the function of the image!
A computer model of a proposed entrance hall in a building
The following image shows a view of a proposed lobby. It has a variety of shapes,
sunlight and reflections. The sunlight, shadows and colours have been calculated. It also has people!
How much detail should be included? What is the function of the image? Probably
to show how the design might look if it were built. People are included to show
the relative size and depth of the lobby.
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Do we need detailed models of these people? Is it important?
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We have to be aware of the limitations of the computer model, realising that there are some shapes that are very complex and would require enormous amounts of information to produce fully detailed models.
Description language
To be of any practical use, a CAD system must present the form of an object to a sufficient degree of
accuracy and detail, and be able to reproduce the details of the object each
time it is requested. To meet this requirement, it is necessary to use a language that is precise, reliable, well understood and widely available.
Everyday languages are far to imprecise to be used as a basis for an object description, so the mantle of object descriptor falls to the realm of MATHEMATICS.
Mathematics is a very well known and understood discipline, is able to describe a
range of
shapes and can infinitely repeat these shapes without error. Once defined, the mathematics of a shape can be manipulated to produce a wide range of
effects. CAD programs are built upon primitive shapes, that can be modelled precisely
in mathematical terms.