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Thread: Questions about modelingpractices - quads and non-planar polygons

  1. #1

    Questions about modelingpractices - quads and non-planar polygons

    Hello All;
    I have had lightwave for a long time but haven't modeled anything that extensive yet. I am trying to learn and do more extensive work this summer. I am also working with a student and want to be sure I provide the most accurate information.

    I have two questions about modeling practices - I'd love to hear the thoughts of several or many of you.

    First, after going through many tutorials, I gather that all 3d models should use only four sided polygons. Is this a common practice in the industry ? Is there any case where you would use 3-sided polygons ? I believe when models are used in a game engine all 4-sided polygons are converted to 3-sided polygons.

    Next, as you are modeling something complex like a spaceship or a car, how much do you pay attention to whether any of your polygons are non-planar ? If some are, do you fix them ? From what I hear they should be avoided but some always seem to creep into my models - any way to prevent them ?

    Thanks all. Your responses are greatly appreciated.


  2. #2
    Super Member JonW's Avatar
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    Jul 2007
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    I avoid them altogether. Build correct the first time, not worth the time trying to fix them later. A recent architectural job, there was a radiosity issue & it wasn't rendering right. I try to be tidy when building things & thought all the polys were flat. Found 1 non-planar which was the problem. Could have saved myself 20 minutes looking for it. Thought it was every other problem than this!

    4 sided polys where you don't need 3 sided. Also avoid long narrow 3 sided polys.
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  3. #3
    It really depends of type of model you are after, There is some stuff that will take very long time to build if you use Quads only and no N-gons and yet if they aren't going to be deformed or so and side of mesh is flat you could get away with simple big flat Ngon and be done with it. Then there is organic stuff which needs deformations like characters where you really need to obey some rules about polyflow too and not just quads/tris. People are often scared about Triangles but they are pretty useful in some situations and there is no reason to avoid them at all cost if you know where to use them. If you are making SubDs mesh like Car or so then you need to use as much as possible quads but here and there few tris won't hurt, in some cases they are even needed and since car is not deforming mesh you can have them on flat areas just fine. in SubDs most problematic ones are "stars" i.e. areas where 5 polygons share singe point, those tend to make pinching at rendertime.

    So all in all you should decide what you are making and then we can give you ore specific guide . But if you use all Quads on everything you can't go wrong but you'll have hard time to do so and often unnecessary and just longer modeling process
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  4. #4
    You probably already have this basic info, but I'll add it just in case. It is often said that the only thing that matters is how the final result looks. The reason why they say that you should only use quads and you should not use non-planars is that non-planar polys and non-quad polys look ugly in the final image.

    For non-planar polys the reason they look ugly is very simple. They look like a hole. 3D renderers don't know how to render a non-planar poly so they will not render it at all. They will also let rays and other unexpected things go through, as Jon said.

    The reason why triangles and polys with more than 4 sides look ugly (only in subdivided models) is that the tension between the points is not evenly distributed and that will cause a visible crease when the surface is subdivided. In game models this doesn't matter because they are not going to be subdivided, so they are converted to triangles to make them more easily digested by the video card. When Lewis says that you should know where to put your triangles, he means that there are places in the model where you won't see the crease and/or that on flat(ish), non-deforming surfaces there won't be a crease.

  5. #5
    N-gons only if they are on flat ares, otherwise broken polygons on render: None, if you're going to use Subpatch. Ok if you use Catmull-Clark.

    Non-planar quads aren't usually a problem and even the Statistics flagged ones are filtered through a threshold in Options/General/Flatness, unless, to me at least, you are quite worried about reflections on flat surfaces and realize those aren't perfectly aligned.

    Tris are the most basic type of polygons and some apps, especially dedicated to simulations and tessellations will use them. The thing about tris is that they will present some shading and modeling issues. When you're modeling Quad Loops make your life much easier for both selections and operations, since there's a direction to follow, tris and ngons stop them. Same as valence 3 and 5 vertices stop edge loops, but people seem to be less bothered with that

    So All quads present many advantages for rendering, deformation and modeling. Some times wanting to have all-quads though could be more work and add more density than needed. Anyway ngons and tris, usually on symmetry meshes can cancel each other out.

    As for the spacing, it's usually good idea to break the geometry where it's long and thin. For Subdivision that's more critical, since it likes even spaced vertices so that's something to take into account if you are going to deform them. Usually leading to quite dense meshes, so subdivision ends up being just a way to have everything smooth on renders more than spar polygons.

    The rest is to learn a bit of topology, seeing a lot of references and practicing

    Last edited by probiner; 07-08-2013 at 09:45 AM.


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