Creased edge being generated between sections of a Scuplted Loft.

[J_MacMillan]

Hi,

Hoping that you might be able to help me out. I am working with the Loft tool in the Sculpt workspace creating an exercise for students. It looks like regardless of the sections that I loft between I am getting an edge that is being created. Visually, I can’t tell that it is there initially. Even with the analysis tools it looks smooth. It only is noticeable once I add another adjacent feature like Fill Hole.

The model that I am using is a little more complex with multiple section that don't have equal entities, but it happens on a simple Loft as well (see attached image). It definitely seems to be related to the Loft. I would really like to explain why this is happening to students. There seems to be a darker control edge where the crease appears.

Thanks,
Jennifer

[saito.kh]

Hi @J_MacMillan,

Thank you for posting in the Fusion 360 community!

I can see this crease in my testing as well and I found this is known issue (internal ID FUS-18441) reported to the development team.

I will discuss internally and then someone in the team or I will circle back here for a better explanation on this issue - thank you for bringing it to our attention!

Thanks!

[colin.smith]

Hi @J_MacMillan, 

I'm not sure which fill hole operation is used, (they all seem to have an issue dealing with that creased edge properly) but this issue can be avoided/minimized by manually filling the hole.

If you look at the images below, you can see that I've used Edit Form with the Alt key held down to scale the end of the cone surface in towards the centre of the circle. 

This continues the creased edge into the end cap and you don't see the tangent break that is in your example image. 

Hope that helps, 

Colin

[J_MacMillan]

Hi Kanehiko and Colin,

Thanks for the replies. I need to fully close the hole so tried using the Extrude (ALT key) to close the hole. It can collapse down on itself by entering 0. The only issue with this is that each point collapses on one another and they aren't automatically merged. If I then try to move faces I see that they aren't attached. Additionally, if I try and weld the vertices in the middle the crease appears. Any other ideas for closing the hole? Faces, Fill Hole also generate the crease.

Thanks
Jennifer

[colin.smith]

Hi @J_MacMillan

To close the end, select Weld Vert and put a box select around the area where all the Verts come together.

Colin

[J_MacMillan]

Hi Colin,

I had tried that but when I do the creased edge comes back. It definitely seems to be something strange that is happening.

Thanks

Jen

[TrippyLighting · Accepted solution]

As @colin.smith has already stated, closing a hole manually the best way to do this , at least for circular holes. I can confirm that the star-collapse appears to introduce the creasing.

Here's a screencast that shows how to close the hole manually using the bridge tool and 2 vertex welds. It's actually not more complicated that the semi automated hole closing tool.

Even if closing the hole with the collapsed star and center vertex welding would work without the creasing, you'd still have to delete every second edge on the star to get a clean quad topology.

[J_MacMillan]

Peter, this looks like the best solution! I am going to go with this method to close the hole. Thanks everyone!

[TrippyLighting]

Actually, looking at this agin just before closing this out it appears the the loft code generates an asymmetric cone. The crease that is observed is there right from the beginning in the lofted cone before anything else is done.

[laughingcreek · Accepted solution]

EDIT:  oops! peter beat me to it.

That darker edge is a creased edge.  For some reason t-splines generates it to maintain the intended geometry.  It's thinking of that edge as the boundary edge of the surface it uses to generate the t-splines.  You will see it on many automatically generated t-spline surfaces, particular lofts. esp if it has some complexity to it.  Some times the boundary areas aren't even conected as a single t-spline body, but rather have their vertices occupy the same spot. 

if you uncrease that edge, you'll notice the surface moves off being a circle there (which can be fixed by manually moving it back).  Closing the circle using good topography techniques like  @TrippyLighting shows doesn't prevent the line from being creased, it's already there!   It's just that good topology gives you good geometry.

Here's a screen cast showing some of the things mentioned above-

[J_MacMillan]

Hi Peter,

Interesting.. I didn't go that far to see that the crease still exists by further manipulating it. I think that this all goes back to what Kanehiko mentioned above that this is an issue with Loft (internal ID FUS-18441). Your approach of Bridge + Weld Vertices didn't make the edge visible, whereas all other options I tried to use to close the hole very obviously revealed the crease. When I Finish Form and the solid is automatically created it doesn't look like the crease is there and surface curvature testing seems ok.

Jennifer

[J_MacMillan]

Thanks for the explanation @laughingcreek. I see what you mean about some edges not even attaching I haven't seen that yet. Thanks for the video explanation. The model that I am working on seems to be giving good results now with the Bridge + Weld Vertices. I also can use Uncrease before even starting the hole closure and then I Match to the circular sketch at the opening and it gives a smooth result with Fill Hole. Thanks Everyone!