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Using displacement on a mesh with inconsistent poly density

Posted: Sun Jul 07, 2013 11:14 pm
by itsallgoode9
I've noticed something that I'm not sure if is a bug or a technical limitation. If I have a model where topology density isn't consistent I will have disappearing faces when using displacement in the areas of the model that are very dense. This is an issue especially on hard surface objects where sharp edges will have a very high poly density due to additional edge loops, compared to areas of flatness on the same model, which will have relatively low poly density. Theoretically I could just add extra edge loops in the lower density area so the whole model is more consistent, but that's not really a good way to work, or even a realistic solution in alot of cases

Is there any rhyme or reason to what makes this happen? am i doing something wrong with the model or is this a bug with displacement in maxwell?

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Tue Jul 09, 2013 3:56 pm
by tom
Yes, Maxwell doesn't do retopo on your base mesh. Imagine you have a 20 cm bbox model and a short edge like 0.1 mm. It would tesselate the whole thing until it crashes so it's user's responsibility to provide a properly/evenly tesselated base mesh.

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Tue Jul 09, 2013 4:37 pm
by Mihai
I don't think there's something any other renderer can do differently. If you have a model in polygons, the displacement will subdivide those until the specified subd level is reached. So it won't work very well if the model has very uneven concentrations of polygons. Have you tried it with a different renderer?

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Wed Jul 10, 2013 9:27 am
by itsallgoode9
it's not a question about retoplogizing models when using displacement. It's asking why there are render errors, specifically disappearing faces, if your tessellation is too high. Trying to figure out if that's a bug or normal issue of displacement in general. I'll post a sample picture when I have some time.

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Wed Jul 10, 2013 5:59 pm
by Bubbaloo
I have noticed errors when the displacement result has areas of self intersection.

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Wed Jul 10, 2013 7:41 pm
by itsallgoode9
ah, that could be it. i tend to really notice the problem on edges when there's high density, which could make sense with what you're saying.

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Wed Jul 10, 2013 11:31 pm
by Maximus3D
It has probably something to do with the renderer internally tries to subdivide the polygons to generate the displaced surface and when the polygondistribution is uneven it will crack open the displaced surface and polygons will go missing, be flipped or hidden behind other polygons. You will get the same results in Zbrush if you import a CAD model or a scanned model and try to add displacement to it without building a new topology on it first. The uneven distribution and size of polygons on the surface will cause problems as soon as the displacement is applied.

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Fri Jul 12, 2013 12:37 pm
by tom
itsallgoode9 wrote:it's not a question about retoplogizing models when using displacement. It's asking why there are render errors, specifically disappearing faces, if your tessellation is too high. Trying to figure out if that's a bug or normal issue of displacement in general.
I understand your frustration but there is simply nothing to be figured. It's just like Maximus explained above and we all buy it.

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Fri Jul 12, 2013 9:05 pm
by itsallgoode9
yeah that's fine tom, it's not that i don't "buy it", I was just trying to understand where the issue lies...be it something inherent with displacement across all platforms or if it was something to do with the way maxwell handles it. makes sense now :)

Re: Using displacement on a mesh with inconsistent poly dens

Posted: Mon Jul 15, 2013 1:37 pm
by tom
Great. :) Maybe I confused you when I said retopo. Because, sometimes it's the best way to avoid this issue if the model has a heavily irregular polygon distribution and triangle aspect.