Disney's Material Point Method

 Controlling the elements is a really hard feat in computer graphics. Whether its fog, rain, or snow, the particles and algorithms involved are hard to fully pull off and make look natural. When it comes to animating snow dynamics, I have not come across any sure ways of creating such a realistic effect before. This is because it is hard to simulate all the different kinds of snow like the density, wetness, solidity, liquidity, etc. This has all suddenly changed now thanks to Disney. This video isn't too new, but I came across it just recently. It's a very intriguing video that Disney put out addressing how they simulate snow in their animations. Take a look here

Now, after watching the video, you can see that Disney animators created something new to achieve the cool snow materials an effects. The technology they use is technically called material point method (MPM). MPM works great to simulate snow dynamics, addresses particle volumes, grid forces and collision response to help the animators create shockingly realistic snowball throws, drops and collisions. 

 The picture above is just an overview of the material point method (MPM). The top and the bottom rows are steps that operate on particles while the middle depicts grid-based operations. In the video, they present a unique snow simulation method utilizing a user-controllable elasto-plastic constitutive model integrated with both a Eulerian and Lagrangian Material Point Method as you can see from the picture above, which is taken from the video. The method is continuum based and its hybrid nature allows their animators to use a regular Cartesian grid to automate the self-collision and fracture between each particle. It also naturally allows them to derive a grid-based semi-implicit integration scheme that has conditioning independent of the number of Lagrangian particles. They can show how it all works by having the snow interact with each other and having it interact with characters as well.

 

This picture shows some of the physics calculations they use to create the specific properties they want. If they want a more powdery effect, they use the middle reference. If the snow is more liquid and wet they will use numbers according to the bottom image.

In conclusion, the basic idea behind the material point method is to use particles (material points) to track mass, momentum and deformation gradient. This causes a really cool effect. The math behind this method is extremely intense though. If you are interested in nature particles I suggest looking into this method a lot more intently because this can be applied to a lot of practices. It's great that Disney shares their up to date methods of work with the public similar to devs at GDC.