Follow

When I save out to CATIA V5, why do some parts have more surfaces than the original parts?

CATIA V5 States the following requirement:

BRep Continuity Compliance 
Imported geometry that is not compliant with the continuity requirement (C2 continuous) of CATIA V5 formats are split on discontinuities. Based on continuity requirements, the topology is modified accordingly.

In other words, some CAD systems represent surface math differently than others.  There is no correct method.  CATIA V5 does not allow “lower degree continuity” to exist in a surface.  For CATIA V5, this requirement is technically called sub-C2 internal discontinuities.  Anywhere this lower degree continuity is found, CATIA V5 will automatically place an edge in that location.  Depending on the part being saved to CATIA V5, you will end up with more surfaces than the original part had.

It is critical to understand that even if there are more surfaces, the original surface math is preserved and the converted part will still be mathematically identical.  Keep in mind the two following items:

1) The surfaces that are split are still mathematically identical.  It's just that in some CAD systems sub-C2 internal discontinuities are permitted and in CATIA V5 they are not.
2) The term "discontinuity" is not a negative term when speaking of 3D solid math.  It's a term that is analogous to the meaning "a change in curvature derivatives."

It is possible that a translator from another company that does not use the native CATIA V5 Kernel (called CNext) for CATIA V5 translations will not have surface splits in these locations.  However, even though these parts may appear visually identical to the original model, the fact is that their surface math has been changed.  In CATIA V5, you would actually be looking at a non-identical copy of the original model.  When inquiring to Dassault Systemes regarding this behavior, they state precisely this:

"Even we can generate no splits but that would require us to alter the tolerance which V5 uses to fit data - thereby altering the translated data."

“…thereby altering the translated data” is the important quote here.  The TransMagic mandate is to never change the original design intent of surfaces or solids - this could result in errors in manufacturing downstream.

Comments