Full Version: mesh edges not aligned when exporting (Closed)

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So for example in this area of your model here:



I wouldn't be surprised if that topology there created a somewhat lumpy shape in that area. Probably not enough for you to notice it just looking at the wireframe or diffuse shaded render, but if you were to make that have a reflective material and then animate it, the reflections would probably squiggle around in that area instead of being very stable, while just exporting a mesh of the original model from MoI and not applying sub-d to it would give you a completely crisp and totally unwavering reflection in those same areas, as long as you got the good vertex normals coming through.

- Michael

Image Attachments:
subd_mutation.jpg 

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Does exist strictly nurbs renderers?

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Hi Pilou,

> Does exist strictly nurbs renderers?

You mean something that renders NURBS surfaces without turning it into polygons?

There are a few but it's not very common.

- Michael

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< without turning it into polygons?
Yes :)

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-> What would there be gained by applying sub-d smoothing

Hi Michael, most of the time I don't touch the topology exported by MoI but for example sometimes I find I need sub-polygon displacement on certain parts and I haven't planned at the start.

Marc

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Hi Flashfire,

I see you have gone for really low poly output. That can cause issues as already mentioned by Michael, but you do also have bad flow, which causes problems if sub-d was to be added.

I did not think you would actually attempt that, so I have cut up the model in MOI to show how I would do this. This is at the minimal I would cut the model up, although I would probably do this directly in a Poly modeller.



If I was then to add sub-d(for whatever reason), the model would maintain shape, although I would add hard edges to keep boundaries.




- Steve

Image Attachments:
block cut sd.png  block cut.png 

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Hi Marc,

> but for example sometimes I find I need sub-polygon
> displacement on certain parts and I haven't planned
> at the start.

Could you maybe show an example so I could understand what kind of displacement you're talking about? Do you mean something like painting on some ridges and raised areas?

One method that can work well for that is to import your model into a brush based displacement program - 3D-coat has a voxel method for sculpting like this, which is independent from the initial mesh topology since it converts from a mesh form into voxel form for doing the actual sculpting.

- Michael

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Hi Michael,

>>Could you maybe show an example so I could understand what kind of displacement you're talking about?<<

Some applications still use micro-poly displacement based on quads(a direct high sub-d). I know, some(including renderers) now use voxels.

- Steve

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Hi Michael,

I was thinking of images maps or shaders like noise applied to a surface in a displacement channel, a bit like a bump or normal.

Here's a example on simple cylinders that has been tweaked a little to keep their original shape while subdividing.

Marc

Image Attachments:
2010-11-23 - Displacement on nurb export.png 

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Hi Marc, but doesn't that render-time displacement that you're showing work on any kind of polygon model, not only one made up of just of quads?

In fact in your example there, it looks like the cylinder to the right has an n-gon cap on the top, not all quads on the top, yet you show it displaced in the rendering?

As far as I can tell, you can do "sub-polygon displacement" in Cinema4D on any kind of polygons - that means to dice any polygons up into smaller pieces before perturbing their vertices by the displacement, the "sub" in this case doesn't mean "subdivision surface" in the same way as a HyperNURBS smoothing.

Here's a quick test just to verify - here's a model that is not suitable for sub-d smoothing, it has not been retopologized into quads only, it's got just 1 big n-gon polygon on the top:



Yet it can still be rendered with a displacement maps with "sub-polygon displacement" enabled:




However, since displacement mapping works in Cinema4D by perturbing vertices around (instead of by perturbing micro polygon render samples which is a way some other renderers operate) you probably want to export such models out from MoI into more finely diced up pieces.

You can do this just at meshing time though, you don't need to retopologize your model into a subdivision surface friendly topology first, you just use the "Divide larger than" option when generating a mesh to dice big polygons up into small pieces (and you may as well just use Quads & triangles instead of n-gons), that looks like this:



Having more regularly spaced vertices like that will make more points available for the displacement mapper to push around. But note that this is just an option that you set when exporting the mesh from MoI, you don't need to totally reorganize the topology of your model for this purpose.


There are some render techniques that do combine displacement maps with subdivision surfaces as well (not sure if this applies to Cinema4D in particular though), but it is typical to be able to do displacement mapping on just regular polygon models. Just generate a diced up mesh if your renderer doesn't incorporate displacment directly into its sampling pipeline like RenderMan.

- Michael

Image Attachments:
marc_displacement1.jpg  marc_displacement2.jpg  marc_displacement3.jpg 

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Hi Marc, so in other words you can use the "Divide larger than" export option in MoI to subdivide (as in "dice up into smaller pieces") the generated mesh if you need a more even distribution of vertices.

That kind of subdivision though is not the same thing as producing a "subdivision surface" friendly topology.

- Michael

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Hi Michael,

Like said previously, this is a situation were I end up wanting displacement and is wasn't planned, or that it would very long to produce the geometry in a polygonal software.

Here are other examples with the cylinder,
When dicing very small, the vertex normal breaks make funky artefacts.
When dicing very small and unwelding vertices small openings appear.






Anyway this situation doesn't happens often and I'm a happy camper.
Just experimenting a bit. :-)

Thanks for the hints!!

Marc

Image Attachments:
Various Flavors.png 

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Hi Marc, you'll probably want to use a welded mesh so that the pieces don't jiggle apart.

Also are you just using the default UV coordinates for that displacement mapping? That's probably what is causing your unwanted disruption at the seam between the top surface and the side.

You'll need to get a continuous UV mapping on your shape if you want the displacement map to also behave as a continuous looking thing across the whole model.

If you were to apply a regular image map to that piece, that would probably show you a similar break between the pieces as well.

I think it's likely that the UV mapping on your other piece is the reason why it behaves how you want, not particularly something to do with subdivision surfaces...

- Michael

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Hi Michael, I didn't thought of unwrapping in the previous example, I used the default cylinder mapping because the default Uvmap stretched the texture.

I will try this later on.

Marc

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Steve Wow! How did you force Moi to make such uniform polys? I mean I tried to slice but had no luck
like that.

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Hi FlashFire,

Sorry, I missed your reply/question.

For that model I just used lines to trim the surfaces. For some models it can take longer to split up the surfaces than actually building the model. But if that is the type of output wanted, then that it what I do.

I did have a look at Topogun due to Michaels post (I had been meaning to look at that program for a while). The program is very easy to use, but find that it would take a lot longer to produce the type of output I want, than it takes me in MOI


- Steve

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I tried to trim the model using the models outlines, but I don't get your results.
I've used both a re-join surfaces approach and non rejoin.
Could you show me what am I doing wrong?

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You need to add lines to use for trimming.

Here is the start I made on that last model, I added a circular array of lines to trim the top surfaces. It is then a case of adding more trim lines where/if needed to make more cuts. It all depends on the surfaces if trimming is needed, as some surfaces will have good flow for the mesh output even without trimming.

Image Attachments:
trim.png 

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But how do you decide on how to trim just any model?
I've been poly modeling since 1993 ;)
I followed your example......IT doesn't magically do what you did.....

Image Attachments:
Clipboard-1.jpg  Clipboard-2.jpg 

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>>But how do you decide on how to trim just any model? <<

Thats like asking how do you decide to build a model. It depends on the model and output wanted.


>>I've been poly modeling since 1993 ;)<<

I was never really into poly modelling back then. I did try 3d studio and used truespace for a while when it came out, but I was modelling in CAD at work, so that became an hobby when Rhino was first released in beta.


>>I followed your example......IT doesn't magically do what you did.....<<


If it magically did, then I would not need to add trims.
The circular array was just the start, I added more trim lines to cut up the model. It took me about 30 minutes to add the lines/cut up, so its not very quick, but still quicker than some other methods.

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