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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.