topic Re: Bug Report: Surface Patch not functioning properly on 3D splines in Fusion Support Forum

Bug Report: Surface Patch not functioning properly on 3D splines

OceanHydroAU — Fri, 27 Nov 2020 23:27:01 GMT

It's missing some points out, so the patch doesn't match the sketch properly:-

This spline is slightly curved around an imaginary cylinder - if I project the spline to a flat plane, the patch works happily:

but that's no good for me of course - I need that wrap, and this bug should be squashed anyhow.

Here is the sketch in question:

https://a360.co/2Va2dfh

Here is the screencast: https://autode.sk/37cka2o

Re: Bug Report: Surface Patch not functioning properly on 3D splines

jeff_strater — Sat, 28 Nov 2020 01:07:46 GMT

I can reproduce this, and created bug FUS-75503 for this issue. Not quite sure what the issue is, since an Extrude of that same curve does follow the curve fine.

Re: Bug Report: Surface Patch not functioning properly on 3D splines

OceanHydroAU — Sun, 29 Nov 2020 06:33:06 GMT

I found something else which might be the same problem, but which manifests itself differently.

These 2 near-identical profiles here: Sketch is here: https://a360.co/2KQJZ0z - the one on the left refuses to "patch" at all.

I made a screen-cast of it before I realized the problem was one of the profiles (I thought it was a loft problem at first): https://autode.sk/39CdjSX

I've successfully patched several hundred of almost exactly the same thing today - it's just a few that give problems, and mostly (only?) ones with greater amounts of difference in the Z of their points.

Re: Bug Report: Surface Patch not functioning properly on 3D splines

OceanHydroAU — Tue, 01 Dec 2020 15:59:43 GMT

This might be the same bug:

https://forums.autodesk.com/t5/fusion-360-support/bug-report-fusion-360-3d-surface-patch-getting-closed-splines/td-p/9907212

Or if not, would be convenient to fix at the same time.

Re: Bug Report: Surface Patch not functioning properly on 3D splines

TrippyLighting — Tue, 01 Dec 2020 17:11:46 GMT

I would not call this pe-se a bug. Fusion 360 creates a trimmed NURBS surface in cases like this. I verified this in another application by looking a the CV distribution.

For curves that lie flat on a surface that is straight forward and only the simple surface with 4 control points and a trimming loop are needed

However, the airfoil curve does not lie on a plane. It is double curved, so to speak. As such the simple surface that needs to be trimmed is not simple whatsoever.

The airfoil curve is densely and unevenly populated with too many control points (both are deadly when working with NURBS surfaces) , which results in bad curvature.

Now Fusion 360 will have a hard time trying to match that mess with a NURBS surface.

Re: Bug Report: Surface Patch not functioning properly on 3D splines

OceanHydroAU — Tue, 01 Dec 2020 22:15:49 GMT

Hi - "too many control points" is not the meaning I expect you're trying to convey.

As you may know, badly spaced control points can lead to curious mathematical loops - which is why it is important to have *more* points in areas of tight curvature, but not to suddenly have too few thereafter (which would trip the loop conditions).

Re: Bug Report: Surface Patch not functioning properly on 3D splines

TrippyLighting — Tue, 01 Dec 2020 23:56:00 GMT

Check the curvature of the splines with the curvature comb tool in the inspection menu. That provides you with all the info you need. The curvature is garbage!

Then perhaps check out the Autodesk Alias Theory builders to learn the basics how technical surfacing works.

Then, at some point in time the practical aspects of technical surface generation may catch up with your mathematical knowledge.

Re: Bug Report: Surface Patch not functioning properly on 3D splines

OceanHydroAU — Tue, 08 Dec 2020 00:40:03 GMT

The curvature appears to be irrelevant. This bug still crops up on "perfect" splines in about 5% of cases. It might be related to the angle that the starting and ending points arrive together at perhaps? It's so random it's hard to work out why 95% work and why those 5% fail.

Worse, in about 10% of those 5% - it crashes Fusion 360 dead (locks up forever)

Try surface-lofting these 3 examples to observe the problems - one of the inner splines on the top-left prop blade is broken, and the tip of the bottom-right is too. https://a360.co/2VOwKiW

Fusion 360 crashes for me after I've done the top and middle blade, then try to do the last one.

Re: Bug Report: Surface Patch not functioning properly on 3D splines

TrippyLighting — Tue, 08 Dec 2020 16:02:29 GMT

This is a different problem than initially presented.

I believe in this case the difficulty for FUiosn 360 is to determine the proper location of the twist control handles.

It's a closed spline so might need some user input to work, if it works at all.

In respect to your previous comments, the software you are using is used for technical surfacing in the area of product and industrial design. I had linked to the Autodesk Alias Theory builders to provide you with a background on how that usually works. That is the background for my earlier statements of too many spline control points. Less is more!

Perhaps it's time for your practical knowledge of technical surfacing to catch up with your purely theoretical musings.

Re: Bug Report: Surface Patch not functioning properly on 3D splines

OceanHydroAU — Tue, 08 Dec 2020 22:23:59 GMT

I can't use less points, because Autodesk will not tell us the exact algorithm they're using for fit-point splines and their end conditions, so I'm forced to convert from my internal 17-point control-point format into sufficient points to faithfully reproduce the right shapes.

As you might know, control-point splines don't wrap around cylinders neatly (plus the projection-only implementation makes them frustrating for users to try and work with).

In this broken case, I moved the tail point around slightly, and the problem went away. I gave up trying to work out exactly which move steps solved it, because it took too long (Fusion360 kept locking up).

Can you give me a clue about what might be missing in my practical knowledge of surfacing [is that even the right term - this is a "bent" 2D spline]? I did read the entirety of the Alias Theory Builders reference you suggested, but keep in mind that I'm doing engineering: the majority of that reference is for artists. Pretty joins and Class-A surfaces etc don't interest me: perfect behaviour in a flow does.

Re: Bug Report: Surface Patch not functioning properly on 3D splines

OceanHydroAU — Wed, 09 Dec 2020 05:05:35 GMT

It looks like they fixed this in todays update? - I can no longer reproduce these bugs...