From: mgkst1+@pitt.edu (Michael G Koopman)
Newsgroups: comp.graphics,comp.graphics.visualization,comp.graphics.animation
Subject: STEP/IGES/PDES, 3D geometry and graphics standards
Summary: Intro to CAD and Geometry standards 101
Keywords: Geometry standards, data exchange
Date: 18 Nov 92 04:09:30 GMT
Followup-To: poster
Distribution: na
Organization: Concurrent Technologies Corporation
Disclaimer:  Don't confuse me with someone who speaks for the company.

In article <1992Nov10.044003.25748@midway.uchicago.edu> chrisw@fciad2.bsd.uchicago.edu (chris williams) writes:
>   IGES is a fairly bloated format, and a good example of
>design-by-commitee. It contains _everyones_ favorite entities
>and lacks limits that would encourage a designer to convert his/her
>design elements to a common denominator.

IGES is yesterday's news.  The Initial Grapics Exchange Specification
(IGES) was never intended to be the premiere standard for data
exchange, as its name reveals.  It is an ANSI standard to facilitate
exchange of CAD drawings.  A much more complete geometric and
topological representation standard is contained in the ISO Standard
for Exchange of Product Model Data (STEP) Part 42.  STEP is ISO 10303
under the auspices of ISO TC184/SC4.

STEP Part 42 allows for many representations of its major
subdivisions: Geometry, Topology or Geometric Models.  In its
diversity Part 42 is a better common denominator.  By requiring
exchange of geometry to occur as a tesselated triangular mesh, as
Chris Williams suggests, a representation can become excessively
"bloated" and the richness of the original representation may be
defeated.  Loss of accuracy in a geometry model should be avoided
unless necessary, e.g. until rendering, IMHO.

Conversion of standard exchange file representations to "your favorite
representation" is the activity of a conversion package such as those
CAD vendors have written to utilize IGES.  Failure of these packages
to deliver what you want is more likely the fault of the vendor than
of the standard.  Part 42 is a superset of most modeling tool
representations with a few exceptions, such as non-manifold geometries
(not included) and tolerance information which may be covered by
separate specifications.

STEP Part 46, Visual Presentation should be of great interest to most
of this newsgroup's readers.  This resource includes such entities as
cameras, light sources, material appearance and texture info, text and
glyph descriptors, view and scene descriptions, view area relationship
hierarchies, and geometry rendering tolerances (which might be
considered implicit level of detail info).

PDES is the U.S. voluntary effort concerned with the development of
STEP.  For further information you may contact ISO, NIST or direct
specific questions to me at the CALS Shared Resource Center.  A good
overview of STEP is contained in "Materials STEP into the Future," by
John Rumble Jr. and Joseph Carpenter Jr. of NIST in Advanced Materials
and Processes 10/92 published by the ASM.

Mike Koopman
Concurrent Technologies Corporation     phone:   +1-814-269-2637
1450 Scalp Avenue                       telefax: +1-814-269-2666
Johnstown, PA  15904                    e-mail:  koopman@server1.ctc.com
USA
-- 
Mike Koopman
Concurrent Technologies Corporation     phone:   +1-814-269-2637
1450 Scalp Avenue                       telefax: +1-814-269-2666
Johnstown, PA  15904                    e-mail:  koopman@server1.ctc.com