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headus 3D scans

SubD in practice

 
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bencowellthomas



Posts: 9
Joined: 30 Jun 2015

PostPosted: Fri Dec 11, 2015 2:51 am    Post subject: SubD in practice Reply with quote

Hi,

Since Max now has open subdiv and Mari 3 has just implemented subdiv surfaces I thought I'd look into this workflow more closely. Like most people using Max I tend to just model polys and throw on a turbosmooth modifier and call that a subdiv workflow .. but I guess strictly speaking it's not.

I understand the theory of how UVLayout works with subdivs, but having run a few experiments I'm not sure why in practice I'd ever want to unwrap as subdivs, when I can unwrap as poly and then in Max or Maya I can simply subdivide later, choosing how the software handles the uv boundaries.

Does anyone unwrap using subd regularly and can they explain to me why you'd work this way ?

Many thanks
ben
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headus
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Joined: 24 Mar 2005
Location: Perth, Australia

PostPosted: Fri Dec 11, 2015 7:07 pm    Post subject: Reply with quote

I'm coming from the VFX side of things where there is no "simply subdivide later" ... the geometry is pretty much always kept as light as possible, and textured and rigged and animated like that, and only every subdivided by the modeler for display purposes, or by the renderer as required, probably to sub-pixel resolution.

So in the context of that pipeline, you would always be loading your mesh into UVLayout as a SUBD because its never actually turned into subdivided geometry as a model.

And the reason then for loading the mesh as a SUBD is so that the UVs get optimized based on the shape of the final subdivided surface, and not the shape of the SUBD cage.

Looking at the sequence of images below, first up is the SUBD surface as it would end up when rendered ... nice and smooth.

Next is the SUBD cage drawn over the top ... the blue lines show the underlying geometry that defines the smooth shape, and its this "chunky" cage that is the only geometry stored by the modeler.

Then that is shown as a shaded surface ... see how spiky it is, but as you know, all that get rounded off when its subdivided in the renderer. But if I loaded that spiky mesh into UVLayout, the UVs would be incorrectly optimized. Certain areas would be given too much area (e.g. the spikes down the spine).

Finally that SUBD cage is shown loaded into UVLayout as a "SUBD". Everything gets round off, and is much closer to the shape of the final subdivided surface. UVs optimized for this shape will be less distorted when applied back onto the SUBD surface.

Hopefully that helps explain why ... if not, let me know and I can split it up into smaller steps.

Phil
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bencowellthomas



Posts: 9
Joined: 30 Jun 2015

PostPosted: Mon Dec 14, 2015 1:50 am    Post subject: Reply with quote

Hi Phil

Thanks for the detailed response. This makes sense.

I'm getting some uv distortion when I send my meshes back into Max after unwrapping in UV Layout that I don't seem to match the subdivided mesh, it's as though the UV subdivision algorithm doesn't match the polygonal one, this might be as I have to cut borders within the uv mesh and so the UVs are able to smooth more than the actual mesh.

I'll port up a few pictures later to illustrate what I mean.

Do you think at some point in the future UV Layout would get Open subdiv as an option for subdivision ?.

Many thanks
ben
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headus
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Joined: 24 Mar 2005
Location: Perth, Australia

PostPosted: Mon Dec 14, 2015 5:47 pm    Post subject: Reply with quote

"I'll port up a few pictures later to illustrate what I mean."

Yes, thanks, that'll make it easier to see whats going on. You should have a look at the "Calc Subdivision Targets" tool though ...

http://www.uvlayout.com/doc/User_Guide:_Optimize#pro6

On more complex geometry that does extra processing to reduce distortion further, and might be what you need in this case.

"Do you think at some point in the future UV Layout would get Open subdiv as an option for subdivision ?."

As I understand it, Open Subdiv is a GPU accelerated implementation of the standard catmull-clark subdivision scheme, so its not producing different smoothed surfaces, its just doing it faster where interactive manipulation of the surfaces is required. I'm pretty happy with the performance of my own subdiv code ... its fast enough for what UVLayout needs ... so I wont be jumping ship to Open Subdiv at this point.

Phil
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