About meshes topology

[Reaversword]

Respect Vray for Maya render engine, what kind of meshes are considered "illegal"?.

We should use always triangle meshes?, quads?. How many Ngons are permitted?.

Concave polygons its a problem?.

[snivlem]

I get ugly meshes from Architects via sketch-up all the time and I've only ever seen one object out of hundreds of millions that was corrupted. It's pretty flexible I'd say.

[vlado]

It would be best to avoid meshes that have duplicate polygons (polygons occupying the exact same space) even if their normals point in different directions. Other than that, there aren't any restrictions. Everything gets converted to triangles for rendering anyways.

Best regards,
Vlado

[zeronove]

It would be best to avoid meshes that have duplicate polygons (polygons occupying the exact same space) even if their normals point in different directions. Other than that, there aren't any restrictions. Everything gets converted to triangles for rendering anyways.

Best regards,
Vlado

emh, could you elaborate this a little bit more?
I mean, I know about the general guideline "overlapping faces are a big no" but I've never deeply investigated it, so:
- SEPARATE meshes with with some faces or polygons overlapping each other is a real issue or not?
ex. windows frame (object A) and walls (object B)
- SAME meshes with some faces or polygons PARTIALLY overlapping each other is a real issue or not?
ex windows frame and windows glass (separate elements inside the same object C but the frame face is usually larger that glass one)

[instinct]

The issue isnt overlapping objects but co-planar faces. And if coplanar faces are present they are always a real issue, no matter if in the same object or different ones (e.g. an accidently cloned object or bad CAD geometry)

Regards,
Thorsten

[zeronove]

if coplanar faces are present they are always a real issue,

I know; I'm just trying to understand if there is a difference among EXACTLY co-planar face and PARTIALLY co-planar faces or if they return the same problems.
To me, I'm not a programmer, they seem a little bit different cases: in the first one there is no way to know in advance which is the correct way (without guessing); in the second one maybe it's easier

[instinct]

Depends on what you mean with PARTIALLY coplanar. If you mean you have to faces that are for example both on the floorplane and parts of it overlap then these parts will present the same issues to the renderer.

Regards,
Thorsten

[zeronove]

a good example could be:
table legs onto a a floor plane: objects are different but I've four legs polygons (the bottom of the legs) overlapping the larger polygon on the floor object, so what's better? moving up the table object a couple of mm? and, what is the real issue? a visible one (artifacts or whatever) or longer render times? in that case how much longer?

[instinct]

depending on the setup this kind of issue might not pose a problem because the problematic areas arent visible. The issue is both visually and in rendertimes. I dont think that in this case rendertime would be added as the problematic areas would be skipped altogether. It still might create issues with GI tho i guess (splotches around the legs ?) cant tell for sure tho.

Regards,
Thorsten

[dlparisi]

a good example could be:
table legs onto a a floor plane: objects are different but I've four legs polygons (the bottom of the legs) overlapping the larger polygon on the floor object, so what's better? moving up the table object a couple of mm?

I, like a lot of other people I'm sure, have conditions like this all over my archvis models - nearly everything rests on something else. I've never noticed any problems related to it other than when something is either transparent (glass meets the window frame- just give it a little push) or something is really wrong like duplicate geometry. I think it's like thorsten says and the overlapping areas (like the table legs) are hidden (and ignored) from the render and GI anyways so these areas never pose any kid of problem, except when they are transparent.

[zeronove]

that's exactly my point but, let me say, it's common sense
what I would like to know is, if possible, what is the amount rendertime lost? I mean, we all are awared to avoid overlapping faces, fine ...but it could be a lot of extra work to take care of this so we have our tables resting on the floor; let's say that we have a looot of issue like this in our scene, if we loose a lot of rendertime it could worth the extra work otherwise, maybe not.

maybe it's a stupid theoretical thought but it's just to understand deeper how the render engine works

[Reaversword]

Ok, thanks for the info.

... appears I need to take a look deeper to a pair of objects topology...


it's interesting the conversation about surfaces on surfaces. About it, if we leave a glass of water over a table, there is any error in final render, still being the glass bottom exactly over the polygon face... Appears Vlado refers only at double face between the same vertex in the same mesh. For other side, Vray is prepared to intersect lightly two surfaces (at least, with different iors, for make classical glass full of water), so I guess then should be no problem / performance lost with a table on the floor.

[mike_kennedy]

Will try to make it clear.
Co-planar faces occupying the same space in the same object.
For example:

A: Sketch up models imported into Max.

B: 2 copies of the same object occuping the same space. (you copied a model and forgot to move it or hide it)

Basically for polygons occupying Exactly the same plane with the same size. The same polygon twice

This throws Mental Ray for a loop too in many cases.

For cases where there are co-planer faces that are really close but not indentical you can try the
global switch roll out "secondary Ray Bias" and set it to some thing like "0.001" if you see artifacts other than then ones I mentioned.
I have only seen errors in recent years from Sketchup models or the copying issue.

Cheers

Mike K

[snivlem]

There's an option in FBX for SKetchUP to export single faces. In Maya use Cleanup Polygons and check lamina faces to also further eliminate the "doubled up polygons'