Continuous surface breaks, why?

[Anonymous]

Hi there,

First of all, regarding the issue I'm about to present, I actually believe that it didn't exist a couple months ago, so I'm really hoping it's an update issue and not a core problem, because we're talking about one of the most critical feature in my opinion.
I'm an industrial designer, and build most of things directly with "complex" surfacing. And with "perfect" surface continuity as often as I can, so broken zebra really bother me...

I build a file to show you guys the issue I'm meeting: Sweep Curvature Break 

Basically, I have a corner path built with continuous spline, and when I sweep a profile on it, some keep the continuity and some doesn't...obviously the one which don't are the one I need...please see below:

1. Straight line: No problem!

2. Half Circle: Great!

3. Arc: What the f...? How come a "special arc" (half circle) is fine when a random version is not?

4. (Complexe) Spline: It's starting to get ugly...

I'm a heavy Mac yet Solidworks user, so Fusion 360 is pretty much a dream comes true...if it can perform continuity and if I can trust it...
So hopefully someone can either help or let me know if this is a temporary issue.

Thanks!

[Johnc911]

Try this. Right click the body and set detail control to fixed high. Does that change anything?

[TrippyLighting]

Can you show the results of similar experiments in SW ?

I don't see a break in the surface. I do see the zebra stripes changing in width, meaning there is a change in curvature, but there's no break or discontinuity. I believe is to  be expected when using a sweep.

I see this same behavior in another CAD software.

Sometimes it helps to right-click on the body in the browser and set the display detail control from "adaptive" to "fixed/high".

[Johnc911]

Also set your zebra stripes to high resolution.

[Anonymous]

First of all, thanks both of you for jumping in the discussion.
I am not sure I would fully adopt Fusion 360 unless I can solve this problem.

So first I tried the "simpler" solution, switching the display detail control from adaptive to fixed high.
I can only notice a difference if I set up to high level, see screenshot below.
The zebra gets much smoother, which is nice, but it doesn't improve the surface, just the Fusion 360 display rendering.
But the continuity breaks are still there...

However, as you can see, I still have continuity break on the zebra...

Then, just to make sure, I took the CAD to Keyshot in order to double check the surfaces.
Same issue, as shown below:

Finally, I rebuilt it on Solidworks, and I agree with you, I remember having similar issue with Sweep before, but as you can see on the screenshot attached, the result is definitely flawless with Solidworks compared to Fusion 360...

So unless I missed something, I'm back to where I was...
It is also true that this slight continuity break won't be visible in 99% of the case while manufacturing, but as an Industrial Designer, I need my renderings to show perfect continuity so...

[Johnc911]

Thanks for the follow up. I hope someone from autodesk will address this matter. Even if it’s expected behavior from fusion. I would want to know what to expect vs say solid works in terms of surface quality. I like especially that you compared both results in a third party render engine. Nice work.

[TrippyLighting]

I don't know what I was looking at earlier but only now I looked at he areas you had circled in red.

So I exported the Fusion 360 surfaces as a STEP file and opened the STEP in ZW3D, my other CAD workhorse  and I can confirm these discontinuities.

I tried to eliminate these with lofts in Fusion 360 where G2 conditions can be specified. To no avail.

With the native tools in ZW3D I don't encounter this problem.

Unfortunately at this time I don't have a good solution for this quite puzzling situation..

[Johnc911]

Are you seeing these surface breaks in the Fusion renderer? I'm not.

I exported your references as IGES surfaces and brought them into Rhino. The discontinuity is clear in Rhino for all except the straight surface. All of the others have discontinuity. Zooming in, you can see the breaks in the surface as gaps. The worst gaps are in the spline curve.

Now I understand that even in Rhino zooming in to very high resolution to see gaps of a few microns might not be 100% accurate visually. But I think it gives a clue.

I am thinking that Fusion uses too high default stitch tolerance when building these surfaces. As far as I know we don't have control over the tolerances for building surfaces with say a sweep.

I tried unstiching your reference surfaces and re-stitch with .0000001 mm tolerance and it's hard to tell if it makes the discontinuity any less.

[TrippyLighting]

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@Johnc911 wrote:

Now I understand that even in Rhino zooming in to very high resolution to see gaps of a few microns might not be 100% accurate visually. But I think it gives a clue.

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A word of caution here. The zebra stripe tool works based one tessellated data. That is the case even in high end surfacing software such as Autodesk Alias. That might account for some of this.

[Anonymous]

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@Johnc911 wrote:

Are you seeing these surface breaks in the Fusion renderer? I'm not.

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It is very small but yes it is visible in Fusion Renderer as well, which makes sense.

@TrippyLighting 

How would you build that same corner in Fusion 360? You probably know what usually works best with that software.

[TrippyLighting]

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@Anonymous wrote:

How would you build that same corner in Fusion 360? You probably know what usually works best with that software.

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I would also use a sweep as that is the right tool to create that geometry. However' I've tried this with aloft and blend curves - what a pain those are in Fusion 360 - but as soon as I added a blend curve to take control of the center section of the loft this crack also appears and seemed to be even more pronounced than with the sweep tool.

@jeff_strater @ryan.bales this would be important to get to the bottom of!