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1. IDEAS AND WORK ALREADY DONE.
The proposed work is based on the following ideas and work
already done:
i. Explicit 3D Object Representation.
An effective way of obtaining 2D mechanical drawings of a
three dimensional object is to derive the drawings from an explicit
computer model of the three dimensional object. The orthographic,
isometric and perspective projections of the object are obtained
automatically from the three dimensional description; with the
hidden lines of the object either eliminated, dashed or thinned;
and with the appropriate labels, dimensions, comments, and
arrowheads indicated. Figure-1 (all figures follow this section).
ii. Object Generation from Physical Description.
A convenient way of making an explicit computer model of an
object is to simulate the process of building the object; that is
the description of how to build an object is an implicit
representation of the object. For example it is easier to describe
Figure-2 as a dodecahedron with a regular five pointed star shaped
hole cut through it, than it is to draw the figure with a light pen
or to list the loci of its vertices.
iii. Language Extension.
Rather than developing new languages for geometric modeling
and mechanical drawing, we believe it is best to extend the old
languages: FORTRAN, ALGOL and LISP. The elements of language
extension include new data types for the language, general low
level primitives for manipulating the new data types, and a
convenient set of higher level operations. The division of the work
into high level operations defined in terms of low level primitives
is an important part of the design because it isolates the data
structure manipulating code.
iv. Object Representation from Physical Measurement,
Another way to get an explicit computer model of an object
is to derive it from measurements made on an actual physical object,
2D drawing, or picture. For example, the physical object might be a
clay model of the thing being designed. We believe that only the
lack of appropriate software is preventing the use of television
cameras as an inexpensive, accurate, and automatic means of entering
graphical data into a computer. Figure-3.
V. Prejudice against Pens for Interactive Graphics Control.
It has been our recent experience that a distinction should
be made between using a light pen (or sonic pen, Rand tablet, etc)
for graphics input and using it for graphics editing and control. We
observe that when adequate keyboard edit, control and language
conventions are provided the use of the light pen diminishs to the
point where it is only demonstrated to visitors who expect graphics
to involve light pens. One reason for this is that when an operator
can do something exactly in afew keystrokes he does not bother with
picking up the pen, aquiring the pen tracker, and drawing; a second
reason is that a pen is necessarily based on 2D screen coordinates
in which overlapping portions of a 3D drawing can not be directly
distinguished with a light pen. Pen based editing systems require a
keyboard or button box in any event, so we argue that an operator
who can control and alter a drawing with his hands always in the
locality of the keyboard will be more efficient than an operator who
has to use both a keyboard and a pen. However, the use of a pen (or
Rand Tablet) for graphics input, such as tracing chromosome
photographs into the computer, is justifiable and expedient but not
directly relevant to editing a 3D mechanical design. That is pens
are functionally replacible by either cameras or keybaords.
vi. Mechanical Simulation.
Information such as the degrees of freedom of motion are
included in the object description and can be used to get pictures
of objects in different positions, as is demonstrated in the
(enclosed) flip book animation of a mechanical arm turning a block
over. Mechanical information can also be used to constrain the shape
of a part in its desired place; or to find the space potentially
occupied by a moving part.
vii. Photometric Simulation.
Photometric information such as the location and nature of
light sources and the light scattering properties of the objects'
surfaces can be included in the model and used to compute the actual
appearance of solid opaque objects. Figure-4.