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View Full Version : Strange Question about Non-Planar



kremesch73
08-31-2007, 10:00 PM
I've done a search about a question that's been bothering me for a while now, and even moreso since I started using Modo. It seems that whenever I model in Modo and bring it into LW, it's riddled with non-planar surfaces that I simply can't seem to avoid. Also, when I use metaform subdivision within LW itself, all the surfaces become non-planar. But if I simply keep sub-patch on, it doesn't recognise any non-planar surfaces.

So far, from what I gather, if you change the flatness limit in your options in LW to a higher number, statistics will become more forgiving about what it considers to be planar vs. non-planar. I've set mine to 25% to see this for myself and it seems correct.

So, my question is, exactly what is the repercussion, if any, by setting it to a higher percentage? What impact will the non-planar surfaces have if I I keep my settings at 25% vs. 5%? The reason I'm asking is because I really hate spending a lot of time on a model to find out that I've done nothing but make a huge, unforgiving mess. I've always been the type to keep everything at default settings to force myself to learn how to work with the way things simply are. However, times are changing, and I figure, I might as well too.

I hope I'm making sense. But the topic is simply confusing me lately, and I'm hoping I can get a little more clarification about the does and don'ts in this particular area.

jin choung
08-31-2007, 10:47 PM
first-

definition: non planar polygon is a polygon whose verts don't (and can't) lie on a single plane.

only QUADS and higher ("ngons") can be non planar. by geometric definition, there's no way that you can arrange 3 verts so that a single plane cannot go through all of them.

so all triangles are planar.

the issue of whether the 'w' statistics panel considers something non-planar or not is really a non issue.

THE ONLY THING THAT MATTERS IS: whether it is "so" non-planar that it will not render properly.

but as i said, if this becomes an issue, simply turning everything to triangles will alleviate your render woes.

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i like your question because it's asking something fundamental - what is the significance of non-planar polys.

quick answer is: THINGS THAT SHOULD BE FLAT, S H O U L D be flat! if you model a table that should be flat and it's non-planar, you did a bad job.

BUT

if your model is supposed to be a curved surface (which you are going for if you metaform), then the planar-ness of your base mesh is inconsequential because it is likely that you are going for a curved surface that is not supposed to be flat and will most likely be turned into subds anyway (or it is so heavily subdivided that it will render correctly anyway).

ultimately, the only "wrong" is that if it does not RENDER CORRECTLY (not true if you are involved in manufacturing or video games but i'm assuming you're not).

if it renders correctly, if it looks right, you are golden.

jin

kremesch73
09-01-2007, 11:22 AM
Thanks. That clears a lot up for me. I've never run into problems rendering and always wondered why non-planar seemed like an issue in certain circumstances and always tried to avoid them without ever really knowing why.

I love your answer by the way. It was very thorough and makes a lot of sense.

Dave Jerrard
09-02-2007, 08:24 PM
So far, from what I gather, if you change the flatness limit in your options in LW to a higher number, statistics will become more forgiving about what it considers to be planar vs. non-planar. I've set mine to 25% to see this for myself and it seems correct.I hope you don't plan to do any serious modeling with that setting.


So, my question is, exactly what is the repercussion, if any, by setting it to a higher percentage? What impact will the non-planar surfaces have if I I keep my settings at 25% vs. 5%? The reason I'm asking is because I really hate spending a lot of time on a model to find out that I've done nothing but make a huge, unforgiving mess. I've always been the type to keep everything at default settings to force myself to learn how to work with the way things simply are. However, times are changing, and I figure, I might as well too.The Flatness Limit in modeler is a threshold for what it considers acceptably flat or not. A higher level means that more non-planars are considered ok. This only kicks in when you use the Stats Panel to select Non-planars. The setting has no other relevance anywhere else - it doesn't affect renders, it doesn't change the geometry, nothing; it's only a threshold for the filter when you select non-planars. Previously, before the new render engine was included and we only had the Classic Camera, the default value of 0.5% was just barely ok (though I always felt it was way too high). The classic Camera was pretty forgiving with non-planars in this range, UNLESS you were using ray tracing. With Ray Tracing, non-planars would frequently result in small gaps in the surface that you'd see in reflections, light leaks in shadows, and errors in refractive surfaces. Ray tracing is pretty unforgiving with non-planars. Since you can render without ray tracing with the Classic Camera, these errors don't always show up.

The Classic Camera is going away though. The new cameras we got with 9.0 all use a new render engine that always does ray tracing, so you have to be more careful with the geometry than before. They use a ray tracing engine, and as such, they're not going to be forgiving with bad geometry. The same problems I described above - leaks in shadows, gaps in reflections and refraction - will always be a threat, but even more so now. Gaps in a surface that were previously only visible on a surface as seen in a reflection can now be visible directly.

The default Flatness Limit isn't low enough to catch all the non-planars that will cause problems. Even barely non-planar polygons will appear to snap, flicker and leave gaps in the model, and I've had to fix a fair number of models because of this. For the past several years, I've had this set to 0.01%, and I haven't had to deal with non-planar render errors in my models ever again. Raising this value higher is just asking for trouble. With a higher value, when you look for non-planars, you're only going to find the worst offenders, while there's hundreds, if not thousands of other polygons that will cause just as much trouble in a render. You want to find ALL the troublemakers that you can and triple them, not just a few and ignore the rest. I strongly suggest you lower that threshold if you want to have good, clean models.


He Who Is Shocked When He Sees People Using The Default Value.