I found out how to do a super dense mesh in ViaCAD - you can do it by right-clicking on a surface and picking "Change object type" to a mesh type, that has some controls including edge length control which can be used to make a really dense mesh object. I did that with the original blend surface and it definitely does not show the same kinds of problems as what you get from export/import.

So I'm reasonably sure that it is indeed different than the exported one and that it's probably treated as a special kind of surface in ACIS and when it gets that special treatment it's able to look nicer than it does when converted to NURBS.

Anyway, it's pretty good for me to know about this particular inter-op problem with ACIS,

- Michael

And in ViaCAD I am seeing some operations that end up using the NURBS surface, offsetting does seem to do it - if I take the nice looking N-sided patch by itself and thicken it into a solid with a thickness of 10 units, those same bumpy areas appear in the offset surface (here converted into a dense mesh):



So it seems that sometimes you get things calculated off of the ideal logical blend surface, and other times stuff happens on the approximated NURBS surface that's along with it.

- Michael

I know this isn't what you were looking for but this kinda gets you there:
(i patched a bunch of surfaces together and joined, although I realize this process is generally frowned upon)

Hi Michael.

Another splendid explanation as to what is going on here - thank you.

So it looks like ACIS can make nice surfaces, but is apt to throw out a lot of the good work further down the line. I think that may explain why I've noticed changes to (complex-ish) faces of solids when I've done something like join them within ViaCAD.

My question is whether this hugely degrading 'feature' is entirely down to ACIS, or is likely to be some unfortunate error in how ACIS is being implemented? A case of "Snatching defeat from the jaws of victory", if ever I saw it!

http://doc.spatial.com/qref/ACIS/html/modules.html for anyone with an interest and can comprehend it :)



Cheers.
Martin (2).

Hi Martin,

> My question is whether this hugely degrading 'feature' is entirely down to ACIS, or is likely to be some
> unfortunate error in how ACIS is being implemented?

Well, the "error" part is just that the actual ACIS "fit NURBS surface through n-sided boundary" mechanism seems to have a bunch of the problems that are just pretty typical to that class of operation which is a tendency to have bumps and wiggles in the generated result. Wiggles are pretty much a natural byproduct of constraint based fitting mechanisms.

But they are able to mask that problem in many cases because they do have a nicely performing "analytic n-sided patch" surface which is able to be used instead of the NURBS surface for quite a few operations. Any operation within ACIS that is able to use that analytic surface instead of the NURBS surface will behave really nicely. Any operation that actually needs to operate only on a NURBS surface rather than a more abstract analytic surface (export being a prime example) will then have the less nice fitted result come into play.

Whether this is good or not I suppose is open to some interpretation and dependent on what in particular is being done - I guess if you only end up doing the particular things where the analytic surface is able to be used then the lesser quality one is totally masked and never comes into play and you could say that is a good result for that particular case. If you do end up doing something that needs the NURBS result then this sudden quality shift is probably pretty surprising though. And it's bad news for NURBS data exchange certainly.

- Michael

Hi Michael.

In one way I was hoping you were going to say that it was poor integration of ACIS into ViaCAD, because that can be worked around.

If ACIS really is this flawed - and I don't doubt your word, I just can't quite believe this chasm of quality exists within it - then it does seem to make the notion of moving files between applications very hit and miss and not something to be done without some serious thought first.

I can understand a poor input surface being responsible for generating a poor output surface (GIGO), but I do struggle a bit with good going to bad, especially if it's not flagged in some way.

As a last grasp at this Michael, do you (or anyone else, reading this) have some other ACIS based software that you could try importing the SAT file contained in my zip attachment, and check whether/how the problems persist there?



Cheers.
Martin (2).

I didnt have another ACIS based system to look with, but here's a quick look at ParaSolids Handeling of the file. (Autodesk failed to load the SAT file)



Note how converting the surface to NURBS actually has Parasolids reconstruct the trim boundries completely different.

It's a good view to see how different kernels will do things differently. Interoperability has always been a burden. All the understanding will go along way in helping people get results. Parasolids will do the same thing to other kernels. The higher end fancy stuff doesnt go around "generic" or you wouldnt need higher end fancy stuff.

It's a good lesson for anyone who would "blame" a software. ACIS isnt really doing anything "BAD". If you keep the analytic surface analytic surface, then it's perfect!

Hi Burr, can you try importing the version attached here without any fillet into Parasolids, then do a fillet with a setback to generate a new fillet there and then export that back to STEP and post it here? It would be interesting to be able to compare the Parasolid generated n-sided patch at the juncture area.

- Michael

Hi Martin,

> In one way I was hoping you were going to say that it was poor integration of ACIS into
> ViaCAD, because that can be worked around.

Well I can't say for certain that is not the case since it's not like I have any specific information on the internals of ViaCAD.

But it seems doubtful that it would be a problem specific to only ViaCAD rather than ACIS itself.


> As a last grasp at this Michael, do you (or anyone else, reading this) have some other ACIS based software
> that you could try importing the SAT file contained in my zip attachment, and check whether/how the
> problems persist there?

I gave it a quick try with Alibre, and it's hard to see for certain but it appeared to open with the lumpy surface in place from the start, which probably means that Alibre converts these kinds of analytic surfaces into plain NURBS , either that or it uses the NURBS version of the surface for its display.

- Michael

Hi Michael,

FYI here's a couple from NX the 'NX_default_fillet.3dm' is using the default filleter with setback in NX, to get an n-sided surface that's a seperate feature shown in 'NX_n_patch.3dm'.

Cheers

Thanks Danny. I couldnt provide the result because i use a version which requires me to pay to export out a model. I was going to ask you anyway :o

I didnt do the fillet (lack of understanding). The autodesk filleter made the star fillet method the same as the nx file Danny Posted.

Here's a video of doing the original patch to show some of the results (Not the same as reviewing the file, but an idea of how they are doing it)



It looks to be somewhat the same as Rhino's. The other thing to note is doing the patch ion the extended part to get a smooth surface. (Kindof one of your mantras Michael).

Anyway, Cant provide the file, but maybe some others with solidworks or something could present something.

Hi Danny, thanks for posting the NX examples!

That's interesting that the default filleter uses the star method. That result does have some of those little star-pattern bumps in it.

The N-sided patch result looks really nice! No little wiggles or bumps in there.

- Michael

> ...The N-sided patch result looks really nice! No little wiggles or bumps in there.

> - Michael

Hi to all.

Yes it does look nice, and with a rather ironic twist, because if I export Danny's file from MoI as a SAT file and then import into ViaCAD (ViaCAD Pro actually, which is where the surface analysis shots come from), it comes in pretty well (GRRR)!

It's a pity ViaCAD can't export its own files with similar accuracy (GRRRRR)!

I'll think about taking the issue to the ViaCAD forum as it's not really a MoI problem, but I must say that this is a nice place to chat about these things - I always feel I've learned something here, so thanks to everyone for helping out :)

 

Cheers.
Martin (2).

So now what is the best way for simulate the best result in Moi ? 6 stars ?

Hi Burr,

> Thanks Danny. I couldnt provide the result because
> i use a version which requires me to pay to export
> out a model. I was going to ask you anyway :o

No Problem, I was familiar with Powershape, I used it at the previous job I had, but then Powershape was using the ACIS kernel, now they're using the Parasolid kernel which I believe they changed to in the past few years. Is that Powershape-e you're using? I read somewhere they brought out a 'pay as you export/translate' software model, pretty cool if you want to learn the software without the outlay and risking trying to download a pirate copy, I think the other vendors like Solidworks and Seimens NX should release something along that model.

Cheers

Hi Michael,

> The N-sided patch result looks really nice!
> No little wiggles or bumps in there.

Yeah, the thing I like about NX's n-sided surface it has a lot of control options to achieve the desired result, not that I use it much in my line of work but hey it's good to know it's there.

-

The Solidworks version is almost identical to the NX version except I noticed the SW version has a looser control point structure, It's possible to tweak the patch further in both software's.

-

Hey Danny,
"""""""""". Is that Powershape-e you're using? """"""""""
Yes... It's very cool to have access to the software. At least they lowered the price for exchange (The cost of using the translator per model). It's something like $79 now.

""""""""""but then Powershape was using the ACIS kernel, now they're using the Parasolid kernel """"""

I think they had a proprietary kindof version (Heavily modified) ACIS. They actually still have BOTH! You can convert between them. You can see me do it in the video. They call it "power surfaces" or something like that... Side note, they even use MESH.

""""""the thing I like about NX's n-sided surface it has a lot of control options to achieve the desired result""""""""

I think thats whats getting the result. But maybe alot of "how to use those" will go along way ;-)

Hi Pilou,

> So now what is the best way for simulate the best result in Moi ? 6 stars ?

The best way is to not do it in MoI - isntead do this particular shape in one of those other CAD programs and then bring the result into MoI.

That will probably be the best way to do it until MoI gets its own n-sided blend tool.

- Michael

Hi Danny,

So in both these cases for NX and SolidWorks, did you have to manually delete the "default fillet" star-structured corner and then construct the n-sided patch as a separate step? Is there no way for the initial filleting to make n-sided corner patches from the start?


Although the SolidWorks generated n-sided patch is not lumpy, it also looks to be significantly not G1 at its connection to the boundary surfaces, I think it's actually too light and maybe needs to have parameters fiddled with to give it more control points to work with in order to hug the boundary's normals better.



- Michael