How to make kinetic sand using obi fluid?

[Ronnie]

I want to make kinetic sand using fluid and wanna cut it using obi collider. Also how can i keep the fluids held together in a shape for example box? Here is the link for reference. Also please give a little detail explanation I'm new using this asset. Thanks https://youtu.be/mjMW51EFNVw

[josemendez]

I want to make kinetic sand using fluid and wanna cut it using obi collider. Also how can i keep the fluids held together in a shape for example box? Here is the link for reference. Also please give a little detail explanation I'm new using this asset. Thanks https://youtu.be/mjMW51EFNVw

Hi,

Kinetic sand has extremely high viscosity, as it is more of a solid than a liquid. Such high viscosity values generally cannot be simulated in realtime as they require really small time steps.

There's extensions to particle-based methods that enable somewhat fast high-viscosity fluids, but still they're way too expensive for realtime use:
https://www.youtube.com/watch?v=-RALL7WCDag

So the takeaway message is: this cannot be done in realtime, not with Obi, not with any physics simulator I've ever heard of. Even offline simulations typically take several minutes per frame.

[Ronnie]

Hi,

Kinetic sand has extremely high viscosity, as it is more of a solid than a liquid. Such high viscosity values generally cannot be simulated in realtime as they require really small time steps.

There's extensions to particle-based methods that enable somewhat fast high-viscosity fluids, but still they're way too expensive for realtime use:
https://www.youtube.com/watch?v=-RALL7WCDag

So the takeaway message is: this cannot be done in realtime, not with Obi, not with any physics simulator I've ever heard of. Even offline simulations typically take several minutes per frame.

In your video I saw a part called Diffusion which showed same sand as the kinetic. How to have that? And will that be mobile friendly

[josemendez]

In your video I saw a part called Diffusion which showed same sand as the kinetic. How to have that? And will that be mobile friendly

You mean in the promotional video? That would be the "Raclette" sample scene at 0:48, I guess?:


This particular scene runs at roughly 60 fps on a quad-core CPU w/hyperthreading, so unfortunately won't run as fast on an average mobile device.

Still, the viscosity shown is way lower than that of kinetic sand (the sand keeps its given shape much better). In addition to that, kinetic sand is a non-newtonian fluid. This means its viscosity properties change according to stress: if you gently push the sand, it will exhibit low viscosity, acting more fluid-like. If you compact it, or violently hit it, its apparent viscosity will increase substantially, behaving more like a solid. This trait is what allows you to compact sand then cut it with a knife, or gently pour it from your hands for a fluffy, soft look. Obi cannot simulate non-newtonian fluids: viscosity is fixed and independent of stress.

Don't even try to simulate this in a mobile device. Even in offline sims, using a powerful desktop computer, its quite hard and extremely costly to pull this off. It takes RealFlow (state of the art fluid simulator for VFX) around 10 hours to simulate just a few seconds worth of kinetic-sand like behavior. Doing this in a mobile device, in less than 16 ms/frame is... well, impossible.