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michaelb-01

Grains cloth - bend resistance

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Using grains for cloth is extremely fast but since it’s not a fully fledged cloth solver it lacks control for creating realistic cloth sims. There is only really self collision and a basic stretch resistance through explicit constraints (through to avoid stretch you have to put constraint iterations very high..), but it doesn’t seem like there’s any concept of bend resistance in the explicit constraints.

Does anyone know of a way of creating bend resistance or faking it somehow?

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Hey Michael, not sure if it will fit for what you need but I've noticed that if you increase the simulation steps the object gets stiffer. The more you increase the harder it looks

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Hey Alvaro, I think (though I may be wrong) increasing the simulation substeps (and/or the constraint iterations) will effectively increase the stretch resistance (so make it stiffer in that sense) but it won't have any effect on how much the grains (cloth) can bend

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Hey Micheal,

I just spent 20 mins or so having a look with a simple setup (grid with granular sheet - falling onto a torus) with two grids one with default shelf settings then tweaking the other to compare.

The only way I could get it to be stiffer was by using the search radius within the grain source under explicit constraints;

My findings were - reduce the search radius = madness/broken - increase the search radius and you get less bend and stretch on the granular - was the only way I could find using the default pop grain setup, :) 

[ the constraints setup like any normal rbd constraints so the more interconnecting lines the less bend resistance I guess is the logic or what I think is the reasoning behind it!]

Edited by chrisdunham95
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Hi Chris,

That makes sense, I suppose thats the best approach for getting bend resistance with grains, thanks for looking into it!

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There's not a way for grains to do this by default. You'd need to add in a good amount of work but it is possible. Without thinking about it too much, you'd want your initial object your putting into the sim to have some sort of angular and position comparison to the array of points near to it. (I am imagining this as a sheet) Now if a cloth falls going down, perpendicular to the ground, you would need to add in force as its rest comparative position is changed from its current comparative position, this will enforce its "rest" shape, and so enforce non-bending. This is super primitive, for example, you'd now need to build into this the ability to rotate while retaining this comparative position, requiring it to additionally contain a rest quaternion which it can carry through to multiply into its rest position. Once this is all built, you could keep pushing, giving plasticity, by adapting the rest position and rotations if a sudden change in the current shape exists, or a gradual constant update of the rest attributes by the current shape, giving an almost viscous property. But yeah, this isn't really integrated into pops, but cloth does ;)

edit: quick hack workaround -- make your grain sheet about 5 grains thick

Edited by TheDude

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Thanks for your reply Jamie. Yes thats similar to what I had in mind but would be a fair amount of work.. Obviously cloth does all this but its much much slower and I always revert to nCloth instead.. But when you have very high res cloth geometry and collision meshes (especially animated) Houdini's cloth is too slow and nCloth can't handle that amount of geo so i've been looking for workarounds.

I had thought of adding another layer of grains but not ideal as it will slow it down a lot and there is not much control..

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Right, the only balancing act if you wanna do it the hack way is simply how many grains thick you make it along with constraint parameters -- That being said I've found cloth in houdini to be getting better and better each version, despite innumerable changes. Or maybe I'm just better at not doing dumb stuff :P

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