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Solid Modeling Database Issues: Database Errors & Reliability  Don
LaCourse, 3DCADTips.com
Don Meagher, Octree Corporation
December 7, 2004
reprinted by permission from
3D CAD databases form a complex array of explicit and implied relations
between data sets. They can and will become corrupt resulting is an unusable
model file. All 3D CAD users should read these tips to help identify and
resolve database errors.
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Sources of Database
Errors
Database related failures usually occur when a solid modeling system is
unable to handle certain geometric situations that arise infrequently
such as objects with self intersections, faces that meet at very small
angles, or objects with isolated parts. While such errors can result
from programming bugs and oversights, they can also be due to the
inexact nature of geometric calculations.
-
Undetected Errors
A new topological model (set of connections between new and existing
faces, edges and vertices) results from solid modeling operations such
as Boolean operations. It is usually computed from the geometric models
(how the elements of the objects being combined intersect). An error can
introduce internal contradictions within a solid modeling database.
Serious problems can arise if such errors are undetected for some period
while subsequent operations are performed (and perhaps design decisions
made). If a database failure is encountered, try undoing several
operations and modifying the modeling approach. This is one reason why a
powerful undo feature is so important.
-
Parametric Methods
Solid modeling systems often use extensive procedures to detect internal
errors. Such procedures are, however, difficult to implement, lower
performance, and usually can not guarantee that all possible errors will
be detected. Solid modeling systems may force limits on a design by
imposing various constraints. When using parametric methods, for
example, the designer may be called upon to first specify an object's
topology, often by drawing it. Any parameter changes that would violate
this fixed topology are rejected. This approach greatly reduces
failures, but can also reduce system flexibility and capability.
-
Disabling Error Checking
Achieving a high level of reliability becomes increasingly difficult as:
(1) designs increase in complexity, (2) the operations demanded by the
design community become more sophisticated, and (3) designers work at
higher levels and become less intimately familiar with the details of
their designs. Designers are sometimes allowed to disable certain levels
of error checking to improve performance. This should be used with
caution. Performance means nothing if the resulting design is invalid.
It is good practice to perform a complete error analysis off-line after
major design changes.
-
Correcting Database
Errors
If detected, database errors can often be corrected. Sometimes the data
set can be modified automatically to remove the problem (e.g., slightly
moving an edge or vertex). Or the system may attempt to undo the
operation that caused the error and ask the designer to try something
else. Clearly, systems that fail or require manual intervention will
become less usable as designs become more complex and the design process
becomes more automated.
-
Reliability Precautions
No solid modeling system should be considered 100% reliable. Steps
should always be taken to facilitate recovery from errors (regular
backups, etc.), especially after installing new program releases.
-
Reliability & Geometry
Reliability is enhanced when you are intimately familiar with the
design. Operations that cause difficult geometric situations should
always be avoided when possible.
About the Authors Don Meagher is a pioneer of volumetric
visualization and the inventor of the "octree" database structure. He
founded Octree Corporation, which provides 3D technology solutions to GE and
other companies.
Don LaCourse is Editor and Founder of
3DCADTips.com, Principal Partner of eDocHelp,
and Contributing Editor for Cadalyst Magazine. Don has over 25 years of
experience in design, documentation and writing about CAD/CAM/CAE issues.
3DCADTips.com is a free, independent and objective information resource for
3D CAD users, administrators, and managers developed exclusively by eDocHelp
and in partnership with Cadalyst Magazine, TenLinks.com and many other 3D
CAD sites.
Its goal is simple but challenging - to continuously provide a wide range of
valuable tips on 3D CAD, CAM and CAE and to bring all application tips on
the web under one roof where they can be contents, indexed, searched and
browsed by you.
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