Full Version: SubD polygon to nurbs convertion issues

Hi all, I'm currently working on a project that involves a lot of polygon to nurbs conversions, we are using Fusion 360 as a converter, between Cinema 4D and MOI. It's a rather annoying process, so I'm happy MOI is implementing that functionality directly :)
But I have some issues, F360 and MOI don't give the same results, and I don't know if it is a bug, or if it is normal differences.
Here is a test I made two simple rectangles, one that is simply 1 quadrangle, the other is subdivided:
https://postimg.cc/SXCyq4Sh
Converting those in Fusion 360 I get the same exact result, a clean rectangle, with all corners staying corners :
https://postimg.cc/Q9DjdzCW
Converting them in MOI, the 1st one is ok, the subdivided one is different, the surface is subdivided and the corners are rounded:
https://postimg.cc/r09M09RZ
Can someone comment on this?

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yes it's normal it's import an OBJ Format for Subdivision! So result is SUbdivided! ;)

For no Subdivision Use the Max Smirnov plugin : Import OBJ (but don't works with massive files ! )
You will find it here : http://moi.maxsm.net/media/files/ (middle of page)

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Hi marc, so the rounded corners on borders is something that normally happens in any polygon modeling program when applying subdivision.

You should see the same result in Cinema4D if you were to apply subdivision smoothing inside there like this:





So the rounding that you see in MoI is just trying to behave the same as polygon modeling programs do when performing subdivision smoothing.

It is better in general to avoid having large open borders though because the behavior changes a little bit in those areas and there can be only G1/Tangent continuity between pieces in those border areas instead of the regular G2/Curvature continuity. When you have a closed mesh it helps make totally uniform higher level continuity behavior throughout.

It could be possible in the future to put in an option to make MoI's subdivision at borders be clamped down like the Fusion360 result that you show. But that would be different behavior than what you would normally see in any polygon modeling program.

- Michael

Image Attachments:
subd_open_edges1.jpg  subd_open_edges2.jpg 

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Just for info, ''corner is always corner" is a frequent option, usually default one if 3d app supports it. Here's with - without, using OSD (Pixar Open Subdivs) in Maya LT. OSD is only supported subdiv method in latest Blender 2.8 as well, while I think they didn't enabled ''corner is always corner" in Blender 2.8, yet (UI option is present, anyway). Obviously there is ''additional story'' about these corners...

Image Attachments:
subdiv_corner.png 

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Hi anto, I'd guess it's probably only an option in apps using OpenSubdiv ... ?

Do you know of any apps not using OpenSubdiv that have the option for that?

For example I don't see any such option in Silo.

- Michael

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It was default for Catmull-Clark subdivs in Softimage XSI, long time before introduction of OSD. Other than that, anything Autodesk DCC, that is, Maya and 3ds Max is now OSD with sharp corners as default. So f360 is following that, probably. Blender too, while is not clear is it supposed to be default option, their betas are changing everyday.

P.S. regarding Silo, it always been popular in Maya community - I think Silo followed some older Maya methods (there were many of them, btw).

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Fusion360 and NPower (developers of Cybor3d, Powersurfacing etc) allow for edge weighting which may not be exactly the same effect but it does give some control over the rounding effect typically seen with subdivision.

I have not played with Rhino WIP 7.0 but McNeel has been moving toward integrating subdivision modeling. Not sure if there is a function similar to edge weighting in there now though?

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Thanks everyone for the replies :D
Actually I was first surprised by F360 behavior at corners, as indeed it's not what you would expect from subdivided polys, then I found it really useful and time saving in my workflow, so I was a little disappointed to not have that in this new MOI feature. So I guess I'll have to hope the clamped down option will be implemented some day ;)
Thanks for this amazing program Michael!

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Hi marc, so for the next MoI v4 beta there will be an additional option under Options > Import/Export > SubD import options > "Open boundaries: Keep corners" or "Open boundaries: Rounded".

The default will be "Keep corners" since that seems to be more common than I had thought and it also actually makes a simpler result too. But you'll be able to set it to Open boundaries: Rounded which will then give the current version behavior.

- Michael

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Hi Michael, thanks so much for this. It is really cool that you take it into consideration like that. :D

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Hi Michael, I've just installed the latest beta, and I'm really grateful you implemented the option like you said, I can now skip Fusion 360 entirely, thanks again for this ! :D

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