Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!seismo!rpics!chassin
From: chassin@rpics.UUCP
Newsgroups: comp.graphics,comp.ai
Subject: Re: Solid Modeling
Message-ID: <1055@rpics.RPI.EDU>
Date: Sat, 28-Mar-87 23:14:36 EST
Article-I.D.: rpics.1055
Posted: Sat Mar 28 23:14:36 1987
Date-Received: Sun, 29-Mar-87 17:38:03 EST
References: <798@ssc-bee.ssc-vax.UUCP>
Lines: 103
Keywords: 3D Graphics, Robotics, Automation
Xref: utgpu comp.graphics:418 comp.ai:340
Summary: Let us begin...

In article <798@ssc-bee.ssc-vax.UUCP>, thornton@ssc-vax.UUCP (Ken Thornton) writes:
> 
> 
> Unfortunately, there is no CAD/CAM, Robotics, or Automation newsgroups
> so I decided to post here.

I guess it as good a place as any...

> 
> I am interested in hearing from people who know about solid modeling systems
> and have experience using them. Specific questions I'm interested in are:
> 
> What is generally preferred, constructive solid geometry (CSG) representations
> or boundary represesentations (B-rep)?
> 
> Of the available commercial systems, is CSG or B-rep more predominant?

Preference really depends on application (see below), as for predominance, it
depends on what system you using. B-rep modeling is the predominant form
of geometric data representation on microcomputers. This is mainly because
of memory/speed restrictions that have existed since the dawn of micros 
(things are changing but not yet enough, and not fast enough). CSG is
far more common on minis and mainframes for the same reasons, but also
because data is much more easily manipulated, and more logically in terms
of geometric thinking (unions, intersections, cutting, etc). My preference
(as an architect) is to use CSG for conceptual manipulations, and B-rep
for detailed representations. Each have their limitations, and if anyone
is interested, we can discuss these at great length sometime later.

> 
> I am specifically interested in generating procedures for a robotic vision
> system to automatically inspect a part, given a solid model of the part.
> In addition to the actual part model, it would be necessary to have 
> information about specific features, relationships between features,
> feature tolerances, and object surface reflectance. From what I understand,
> commercial systems do not provide this information in the output file
> representation of the part.

CSG seems to me to be the most readily applied to this type of work. The
reason is that CSG can naturally indicate whether two parts geomtrically
intersect each other, for example. However surface features like color
and reflectances are not inherently applied to CSG modeling, although I imagine
this could be developed, and might even be worth while. B-rep seems to be
a bit more of a problem in terms of manipulating relationships between parts.

I think that you have another problem when you get involved with robotic vision,
and this is something that I've never thought about in terms of robotics, but
I am working on in terms of architectonics (architectural modeling of sorts).
That is that you will need to create some sort of algorithm for generating a
3D model from 2D information received by the cameras. Essentially the idea
is to analyse a pair of images, extract the boundary data, assemble a 2D
'image' for each view, project the two images together into a 3D 'image',
and finally take the resulting B-rep data and convert it to CSG type data,
which can then be correlated with the previous frame and the motor algorithms
to properly direct the parts into their desired positions. Piece of cake, eh...

Each of these steps involve some very complicated and SLOW computing. I've
worked out the basics for the first 4 steps, but have a long way to go
still. In any case I would love to talk more about the ins and outs of this
type of analysis because this is the main focus of my work for the next
year or so. By the way, it's all being done on a Sun 2/120 and 2 AT clones...
...wish me luck!!!

I know there are some people who have already done some work in these areas,
but it has always amazed me how little is in fact published. I have NO, get
that, NO references relating to 3D reconstructions other than the following,
and these have nothing to do with computer application thereof:

	Wittcower & Carter, "The perspective of Piero della Francesca's
	Flagellation", COURTAULD INSTITUTES, vol.16, 1953

In this article the authors explain the method they used for reconstruct
the actual architectural space that Piero painted. The mathematics of
perspective are treated, and discussed.

Since this is obviously not directly related to the subject I would
greatly appreciate any sources anyone might know of. They are rare, and
those that I have found, uninspiring.

So, anyway, I encourage further discussion of this topic as it is a very
difficult one, and it will, I believe, in the long run test what we
computer graphics buffs are really made of. This problem goes beyond
simply one of analysis, to become one of representation and ordering. The
results, or lack thereof, will reveal much more about how we perceive and
order what we see. This is the heart of the problem.


_____________________

David P. Chassin
Rensselaer Polytechnic Institute	              |
School of Architecture			            __+__
Troy, NY   12181			           /  _  \
USA						   | | | |
					  /=======/   =   \=======\
(518) 266-6461				  |   _   |   _   |   _   |
					  |  | |  |  | |  |  | |  |
chassin@csv.rpi.edu			  |   =   |  | |  |   =   |
=======================================================================
The above is my opinion, and mine alone. The organization I belong to
may refute these statements at any time. They are however more likely
to take credit for them.
=======================================================================