Two copies of the same pair of surfaces generate different intersection curves


I think I’ve found something strange. I have a pair of intersecting surfaces and I copied them to create two identical pairs of surfaces, differing only by their X and Y coordinates.

When I perform an Intersect call on each pair, the resulting curve is different (the right one appears to be incomplete in that it doesn’t extend far enough to the lower-left):

Am I going crazy? Does anyone know why this happens and/or how I can spot when this will occur so I can work around it?

On a related note, something else that’s confusing: when I use the left-pair’s intersection curve to split the swept surface, it results in 3 surfaces, not 2.

Any help or insight on these observations would be much appreciated.


IntersectionAnomaly.3dm (152.9 KB)

Hi Nic - in V6, the curves are the same, and incomplete. You can get more of a curve if you insert a bunch of knots in the U direction using the Automatic option a few times but the curve always stops at the lowermost - nearest the singularity - knot. A bug, I’ll add it to the pile, thanks for the example.


Thanks @pascal - using v5, is there an approach that will reliably split the smaller surface by the larger one for both pairs?

(The Split command only succeeds for one of them, not both)

Hi Nic - I understand that it is weird… my guess is that there is just a very slight difference way out at the end of some calculated number that is the difference between a full intersection and a partial one. I mean, this is a bug, really… it just needs to get fixed, though I am fairly certain the developer will say it is one of the harder cases, there being a singularity and tangency, or durn near it, at the same location.