Andrew,

I still cannot reproduce the errors you are getting in Modo. Could you upload the .lxo file so I can compare it with mine?

Hello guys, Hi Michael

About : CentroidTriangulation

I have some time to waste, so I decide to start a discussion about this Modo internal triangulation problem (on modo beta forum), with good examples etc.

Because I have a new computer here, I had to re-install MoI, and I start some export test, and ... I get all kind of ugly shading glitches when using 'only triangle' in MoI export.

Hopefully the 'deja-vu' result makes me remember this famous 'CentroidTriangulation' option, ... and as expected, if I disable it, everything comes out perfectly.

 

But I'm wondering, why is this option ON by default in MoI .ini ? , because it introduces so many glitches when using triangles only export, especilay in fillet area ?
Is there any example where this option creates 'better shading' result, using triangles only export ?

@PaQ not about this centroïd
Do you know this French Modo forum ? http://modo.logy.fr/

Hi PaQ - can you post some more information about the ugly shading glitches with "triangles only" part with centroid triangulation enabled? Is it with a somewhat rough/low poly count type mesh generation?


> Is there any example where this option creates 'better shading' result, using triangles only export ?

re: Centroid style on by default - it's more about polygon structure for stuff like displacement I guess, not so much shading specifically.

For some situations it makes a much cleaner mesh structure with the centroid style triangulation, the best example is the top cap of a cylinder, if that is meshed with a centroid point added to the center like slices of pie, it makes for all regular shaped triangles. If it is triangulated using only by connecting together points on the outside of the n-gon then the triangles will kind of zig-zag across the face and will be of different sizes. But actually MoI analyzes the n-gon and if it is an exact regular polygon shape like a cylinder end cap it will use centroid style on it no matter what.

Let me see if I can find the thread that got me started with turning centroid triangulation on by default for other cases as well... I think someone mentioned that ZBrush triangulated all n-gons that way and that they liked that type of polygon structure better.... Ok here it is:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1024.1

I guess the main reason is that it tends to make kind of more squat triangles instead of longer skinnier triangles. The downside can be with lower density meshing it basically introduces a new sample point in the middle of the polygon and when that is evaluated on the actual curved surface it can make a kind of "poking out" spot...

- Michael

Hi Michael, here's an exemple ...



Mostly every fillets show some glitches , a bit like the previous posts I made.

It's not a really big problem as I know how to resolve it (editing the .ini, using subdivide larger than option, etc), but in the other hand I expect 'triangle only' to produce the best shading quality possible and I would not really care that much about 'topology aesthetic' in that export mode ... dont you think ?


@Pilou, thanks for the link, I have follow a webinar last time with greg from luxology, really great to hear some french :)

Hi PaQ, so that's basically that same "low polygon" density kind of problem that I was describing earlier - the fillet piece there gets a low number of divisions along one direction despite the angle setting of 6 because the angle setting is based purely off of curvature and will not divide things that are very shallowly curved but long.

What you get in that case is an n-gon that's pretty non-planar and then introducing a centroid point in the middle of that does not work very well.

The real fix is probably more like adding in some additional refinement mechanism that kicks in automatically to avoid making a pretty non-planar n-gon to begin with. Basically if your n-gons are not torqued then centroid triangulation would not cause that kind of problem.

Right now for that kind of geometry you have to use those various additional settings like "Divide larger than" to force that fillet to get subdivided more in the long direction than what is currently happening by the angle measurement alone. On shapes that are shallowly curved like that you basically will get low-poly artifacts even with a high angle setting.

Non-centroid triangulation does tend to not mess things up on non-planar n-gons as much as centroid ones do, but it's still better for those types of things to get subdivided more overall so that the final n-gons that get triangulated are not so non-planar in the first place.

But part of the difficulty though is that the non-planarity of the n-gons isn't really there during the subdivision process itself which works only on "underlying surfaces". In a case like you have here the non-planar n-gon is formed only after the trimming boundaries are introduced onto the surface, that happens after the subdivision process is all complete so it's not very easy for the subdivider to take that in particular into account at least with the current architecture. But some other metric that would look for things that were fairly long but only shallowly curved and divides those surfaces additionally would help, it's just that some of these things like what exactly "fairly long" means in some algorithmic way that applies to any kind of model is kind of difficult to quantify. And also it's a problem for actual desired low poly output if something kicks in with too many subdivisions totally automatically.


Anyway, to make a long story short - in your model here your fillets are getting low-poly artifacts on them because they are not divided enough in their long directions, they need to get divided more to make the n-gons less warped and once they are divided more either centroid or non-centroid triangulation will work ok on them. Right now you have to force such subdivisions yourself by using some of those other settings, I do not yet have an automatic way of ensuring that this kind of situation gets automatically subdivided.

- Michael

Hi Michael,

- The real fix is probably more like adding in some additional refinement mechanism that kicks in automatically to avoid making a pretty non-planar n-gon to begin with. Basically if your n-gons are not torqued then centroid triangulation would not cause that kind of problem.

Would be indeed great if that kind of problem can be fixed in an 'automatic way' without adding to much geometry.
Maybe I'm too greedy about polycount too :)

Hi PaQ,

> Would be indeed great if that kind of problem can be fixed in an 'automatic way'
> without adding to much geometry.
> Maybe I'm too greedy about polycount too :)

That "not adding too much" part is where it gets particularly tricky, especially trying to come up with a mechanism that can gradually do it in increments along with the slider being moved...

In the meantime maybe it is best if I turned centroid triangulation off by default since it has a tendency to not handle these types of situations very well. Another thing I was thinking about was to maybe have it turn off when the base n-gon was over a particular aspect ratio like 10 to 1 or so...

- Michael

Hi Michael,

Yes this centroid triangulation is, from my pov, a little problem, especially since this thread.

I was about to suggest some modo users to use triangle output format if they really want to get the best possible
shading out ot MoI (until modo can handle ngone tesselation better, not a MoI fault here).

But having to edit the moi .ini file to avoid glitches from this centroid triangulation can look a little bit tricky too.

Well that's up to you now !