Squishy Volume With nCloth

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Old 03 March 2013   #1
Squishy Volume With nCloth

Hi all!

I'm trying to simulate a squishy volume with nCloth that more or less realistically maintains its volume while being wedged between a regular nCloth object and a collider, but am having some trouble understanding the relevant settings under "Pressure" as well as how the other dynamic attributes affect a closed volume simulated with "Volume Tracking".

The Help-Docs are not really helping my "artist brain", and I would very much appreciate if someone could point me to some more in depth reference or provide simple, easy to understand explanations of the attributes and how they work with each other when simulating a volume rather than a flat cloth object. I have a fairly good understanding of how to do the latter, but haven't really tried to sim a volume with nCloth before. My initial attempts with "Water Volume"-preset didn't get me very far. Interestingly I got close to what I want by tweaking the Putty-preset, but its all been shot-in-the-dark trial and error so far, since don't really understand how to tweak the attributes to react more like a realistic volume of ie fatty tissue. If anybody can help me out here, that would be great!

"Knowing is not enough, we must apply. Willing is not enough, we must do."-Bruce Lee

Old 04 April 2013   #2
Just a quick bump hoping someone able to help out sees this and also to add an update from my end.

I found some good explanations of the general nCloth attributes in the book "Mastering Maya 8.5", but particularly what exactly the settings in the Pressure roll-out do still eludes me and I still find myself unintuitively shooting in the dark with adjustments of "Air Tightness" and "Incrompressibility". If I raise "Air Tightness" beyond 1.25, the volume inflates beyond reason when compressed. If I leave it at 1.2 the object looses at least 10% of it's volume, as checked with the appropriate MEL command. I just cant seem to find the settings for exact volume preservation. How sensitive exactly are these attributes/should I try really tiny numbers or crank all of them up?

Looks like I might have to turn to DMM after all, which I guess does simulate a volume more accurately via a tetrahedral mesh. Problem is, I really want a 2-way interaction with cloth, which might be hard to do that way.

"Knowing is not enough, we must apply. Willing is not enough, we must do."-Bruce Lee

Last edited by TheRazorsEdge : 04 April 2013 at 12:18 PM.
Old 04 April 2013   #3
The exact volume preservation should happen with air tightness at 1.0 and sealHoles = on. Keep in mind that this is a simulation of trapped air in a cloth bag. If you want something more like jello you may need a different simulation, although this combined with high stretch/compression/bend resistance can get jello-like effects. With lower stretch/bend you get something more like a water filled balloon.

Depending on the simulation you may also need to increase incompressibility and/or substeps on the nucleus node.

This also relies on your geometry being fairly clean... things like lamina faces or non-manifold geometry can mess up the determination of interior volume.

Also for more rigid sorts of object you might try creating a non-colliding cloth tetrahedron around the center of your object then doing a component to component constraint between the two. Change the constraint connectionMethod to maxDistance and make the max distance large( will create a lot of links.. you may wish to turn off display connections). Make the constraint strength something like .1 or less and make the mass on the tetrahedron cloth higher. A stronger constraint strength will make the mesh fully rigid, but a high mass on the tetrahedron is required to keep things stable.

Last edited by Duncan : 04 April 2013 at 05:47 PM.
Old 04 April 2013   #4
Thank you very much for the detailed reply, Duncan!

The object I'm using is a closed, clean-geo volume, a good part of which is constrained to a passive collider on one side of the volume to keep it more or less in place on the collider, which is animated. I do have "Seal Holes" enabled and am currently simulating at 12 substeps with Scale Space at 0.01 for real world scale and have been jacking up stretch/compression/bend resistance/incompressibility to fairly high values, I think around 200-300 for the resistance values, but am not sure right now regarding incompressibility. The object itself deforms more or less as desired right now, it just won't maintain it's volume when compressed. Judging from your explanations, I'm guessing a higher substeps value might help and will try that next.

What would you consider appropriately high values for stretch/compression/bend resistance/incompressibility when trying to simulate a volume-maintaining object? I'm asking because in some cases these values appear to require really high values or they are rather sensitive to tiny changes. Finding just the right range to begin playing with can sometimes take quite some trial&error time.

I'll be sure try your other suggestion, but am not quite sure what exactly you mean by "create non-colliding cloth tetrahedron". Do I simply create a tetrahedron primitive, snap that to the objects center, make it nCloth, disable collisions and so forth ....?

I was even playing with the idea of filling the volume with blobby nParticles and see what that gets me. lol

Thank you very much again for your input. It's much appreciated.

"Knowing is not enough, we must apply. Willing is not enough, we must do."-Bruce Lee

Last edited by TheRazorsEdge : 04 April 2013 at 09:47 AM.
Old 04 April 2013   #5
Quote: The object itself deforms more or less as desired right now, it just won't maintain it's volume when compressed.

As yourself how the shape would behave if it was an air filled balloon but wrapped in non-stretchy cloth. If it couldn't expand the way you want due to the non-stretch of the cloth you might need to make the stretch resistance less. However your object will not hold its shape as well. To enforce the volume preservation more increase the incompressibility attribute.

A weak constraint to a tetrahedron might allow more deformation when squished but better preserve the overall shape. Just make a tetrahedron( platonic solid ), make it nCloth, turn off collision, make its mass high, say 50, put it roughly in the center of your object and create a component to component constraint between it and your object. Set the type to max distance on the constraint and make max distance high and the constraint strength weak, say .1. Then make the stretch compression resistance on your object lower.

I've tried filling cloth objects with particles before. It can sort of work, but you need fairly high bend resistance to keep the surface of the object from looking bumpy.
Old 04 April 2013   #6
Thanks a lot for the additional details and tips. I'm going to give those a try next time I'm working on the simulation.

"Knowing is not enough, we must apply. Willing is not enough, we must do."-Bruce Lee

Old 04 April 2013   #7
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