Hi Armin, to answer another part of your questions:

> And the width of the extension is exactly where that trimmed
> surface gets split. Not sure if this is supposed to happen?

Yeah if I understand you correctly that part is supposed to happen. Doing a cutting operation with a planar curve is the equivalent of extruding that curve out into a surface. When you do an extrude of a curve made up of multiple segments, you will get one surface per segment for the extrusion result.


> Anyways after trimming the part with a new curve (arc), the
> curved surface looked ok, no more split lines, and of course the
> fillet command works now. I attached that file, maybe you can see
> why this extended curve behaves that way, and maybe this is just
> the way it is?

Yeah, that is just the way that curve extension currently works - if you extend a line, then you will get a longer single line, and if you extend an arc you will get a longer single arc.

But if you extend just a "general" curve like this one you have here (looks like it was probably the result of some surface intersections or something like that), then the extension is a line segment that is added as an additional segment to the curve, so you get a multi-segment curve in that case which you can see if you use Edit/Separate on it to break it into different pieces.

I do want to add some options to extend in the future though, it should be possible to add an option that will make a "smooth" extension that could make lengthen that piece without making it segmented. I'm not quite sure when that will happen yet.


> I also noticed, after turning on the points for that particular curve,
> there are a lot (relative :) ) of points, why don't they cause the trimmed
> surface to split?

It is possible for a curve to have a lot of points internal to it but still be a single segment as long as the curve has a high degree of smoothness at each of those internal points.


> Does this also mean DO NOT USE EXTENDED CURVES FOR TRIMMING?

Well, not necessarily... It kind of depends on the situation.

But for filleting, things that are broken into more segments can cause problems. The way it causes problems is that the filleter will try to produce a fillet surface for every edge that you have, and then when it has produced all these fillet fragments, it has to try and trim them to one another, trimming surfaces that are tangent when they touch each other tends to be a difficult area of calculation, unfortunately. So currently if you just generally reduce the number of fragments and separate pieces in your model it will tend to avoid some of the areas where the filleter has difficulty in.

- Michael