iges file compare

I was also considering "converting" back to actual mathematical
surfaces from polygon input data.

These might come in handy:
http://www.iges5x.org/
http://www.nist.gov/iges/
http://www.iges5x.org/archives/version5x/

IIRC I have the full IGES spec of versions 2 & 3 in the files ...
both had "copious data" IIRC -- it's not new.

 

Hi Jeff,

Yes an iges file can hold a polygon mesh, but all it will do is convert
each tri into a planar nurbs surface that duplicates the original
polygon. (garbage in garbage out!) I've never used iges files for that
purpose. In the old days it was all we had for data translation and
there are still a few vendors around who accept nothing else. I've
only used iges files to export solid or surface models to other
systems, always nurb or b-rep based etc. The whole reason I started
this thread was because a recent project required iges output. I don't
think I've used iges from solidworks for a long time so to be on the
safe side, I re-imported my own iges files and compared them to the
original solidworks parts for verification and found discrepencies, I
was surprised, but chalked it up to minute rounding errors out in the
6-8 decimal place range.

Zander

 

it's not new.

It's not. I am. (Well, not really new; just a bit slow.)

 

I've never used iges files for that purpose.

My experience is limited to building polyface mesh models in acad and exporting
to Femap. That's my only use for the pesky critters.

Very good idea. If for no other reason than to "sanity check" the translation.
If the source system can't read it back in there's little hope...

Sorry to munge up your thread bantering with Cliff. `;*)

 

It's not up to IGES to do any converting.

 

You probably should ask SW about that one. Sounds like
they may have a problem ... perhaps in not using enough
decimal places in their IGES files ... or in their own
software someplace.

 

Don't worry about it <G>.

 

These may be of interest:
http://www.math.uga.edu/~mjlai/nurbs/chui.html
http://atrey.karlin.mff.cuni.cz/projekty/vrr/doc2/manual/progman_html/Rational-Bezier-curves.html

There is much more if you search.

 

Thanks, Cliff. I don't pretend to comprehend the math (not the language,
structures, abstractions, calculations; dropped on my head or maybe it Was the
pot) but think

"We have chosen rational Bézier curves to be the base, because they can exactly
represent both conic sections and nonrational Bézier curves. They are also an
equivalent to NURBS (non-uniform rational Bézier splines) used in professional
CAD systems."

answers my question in it's present form. (... Are Catia v4's native reps
Beziers? Maybe my bad.) (Also thought "basis splines" was the BS in NURBS?
Basis splines [the basic segments], Bezier equations? or is their acronym
expansion incorrect?)

It's an idle time pursuit; understanding the mysteries of less than perfect
Catia v4 translations. Rooted in more mundane procedural or
not-platform-specific causes contradictory to "prevailing wisdom" perhaps?

 

It would seem that you can go Bezier ==> NURBS and get the exact
same mathematics but if you try NURBS ==> Bezier it ends up as
multiple Bezier segments except in "special cases".

Basis Splines are polynomials ......
Note the difference in form of the two equations/series. The NURBS
can reduce to the Bezier it looked like under special conditions.

http://www.google.com/search?hl=en&ie=ISO-8859-1&q="Basis+Spline"

IIRC There's a section in the back of the IGES spec on spline
functions but don't hold me to it. If so, it's also probably somewhere
on those IGES Sites that I posted a day or so ago.

I might suggest Faux & Pratt's "Computational Geometry for Design
and Manufacture" (which may be out of print) though it may be a bit
dated.

http://www.amazon.com/exec/obidos/ASIN/0131516558/categoricalgeome/104-8571785-7539117

Looks like some are available yet.

If you want to pay me the big bucks I'll do some actual work <G>.

 

If you want to pay me the big bucks I'll do some actual work <G>.

Thanks, I'll get back with you on that.

[ Nah, seriously, thanks for all. `;^) ]