Surface Blending - Unexpected pinching

 From:  Michael Gibson
1546.5 In reply to 1546.1 
Hi PaQ,

This is a problem that can happen with both 2-rail sweep and blend (which are related), especially on longer guide rails that go around a sharp turn.

What happens is the motion along each rail can be going at slightly different "speeds", when it goes around a bend that can lead to a kind of slanting connection between them.

Here is a more visual description:

You can see there where the different profiles for the blend are getting slanted. That makes them longer which makes the blending action between them kind of scale up in that area, that is what causes the pinched result here.

The main problem here is that the connections between the 2 rails are based on the parameterization of the rail instead of being based purely on 3D distance traveled. So if the parameterization does not match up very well it can cause this slanting. It can be particularly noticeable around a bend.

Steve, you wrote about the object scale - actually the object size in this case seems to be fine. MoI has a tolerance of 0.001 units, so it is not good to try and model stuff that has features in it that close to that level of size, but this is well enough above that.

By chance it happens that if you scale up before doing the intersections that result in those rail curves, it will create a denser edge curve that happens to have a better matching parameterization between the 2 of them.

One other thing you can do to work around this problem right now is to dice up the edge curves into some smaller pieces, like say 4 sections instead of one long edge. To do this select the edges and run Edit/Trim and push "Add trim points" to add points to split the edges at. Then blending between smaller pieces at a time will basically force better connection points between the pieces.

This is definitely an area that I have targeted for improvement, but it isn't just a simple fix, it needs a fair amount of re-work in the guts of the sweeper. Right now I have been planning on doing this with a batch of other improvements to sweep which I hope to include with the first v2 beta.

- Michael