Setting Up Fluid Simulations in Houdini

Quentin Lengele showed how to make a fluid simulation in Houdini, talked about necessary hardware, and the advantages of Redshift. Check out his Lava Simulation here.

Life Updates

In 2016, after 20 years of working for agencies, I started a freelance carrier. I’m still developing games and apps but I recently discovered Houdini and Redshift (a year ago) and I must admit I fell in love with Houdini. I spent almost all my free time between wife, children, and projects in it. Houdini is really amazing. I wonder how I could have worked during all these years and ignoring it. This software solution allows me to quickly create things I have in mind and I became addicted to short animations like the ones I post on my social networks.

Setting Fluid Simulations in Houdini

FLIP (Fluid Implicit Particles) are quite easy to use in software like Houdini but we must say a big thanks to their engineers for that. They are amazing, I literally love them. As a software engineer myself, I did code some GPU accelerated particle systems for real-time purpose with compute shaders and parallel computing (see my blog). I must admit FLIP isn’t an easy subject.

Basically, the main idea of FLIP is to simulate a bunch of particles with the Navier-Stokes equations to get the physics right, like a real fluid.

Here are a few tips to set up this kind of sim in Houdini:

You’ll start by creating a simple sphere and apply the “FLIP Fluid from Object” on the “Particle Fluid” tab.

Quite easy, as it creates a DOPNetwork for you with a FLIP Solver in it.

Then, to be able to collide these particles inside a sphere, you need to create a hollow geometry that will be your fluid container. To do that, I use a simple boolean with 2 spheres:

After that, you need to refer to this new container as a static object in your DOPNetwork. Pay attention to the order of the merge node.

Now, if you press play, you see your FLIP particles colliding with the sphere container:

To be able to generate a wave or a certain shape in your sim, you can vary the shape of the particle emission geometry and the position of it. Make it bigger or spawn it more on the side of the container. There are plenty of ways to do it and get what you’re looking for:

Once your simulation of particles looks OK, you must now use the ParticleToFluidSurface node to generate your mesh. This process can be heavy when you have lots of particles and it isn’t GPU accelerated so having a good CPU with lots of cores is very helpful here.

All the baked frames:

Now we have to shade this thing with a Redshift material. Quite simple: you can use a common RS Material you create under SHOP. Choose the Tint Glass preset and adjust the sub-surface parameters. It should look like this:

Finally, you get a rough fluid animation inside a sphere:

Hardware Setup

Simulations like that are quite heavy and I had to spend some money to be able to do the sims quicker. Also, having lots of GPUs isn’t enough. High-Speed RAM and Hard Drive, multi-cores CPU are also very important if you don’t want to get asleep while you’re working.

Here is my rig:

AMD Ryzen ThreadRipper x1950
2x GeForce RTX 2080 Ti
128gb RAM DDR4
2x 1Tera M.2 SSD NVMe

About Redshift

I’m no expert in this field unfortunately but I tried many renderers, as a hobby. I must admit I felt very comfortable when I met Redshift last year. From my point of view, RS is the most elegant renderer on the market for indie guys… and not only, for sure. In terms of UI, clarity of tools, it’s very good and God, surely it’s super fast for rendering if you have decent GPUs.

Otherwise, rendering volumes are still very heavy, even with my 2 new cards. I’ll surely invest in 2 additional GPUs and I’m very excited about the coming of Redshift 3.0 features.

Quentin Lengele, Software Engineer & 3D Generalist

Interview conducted by Kirill Tokarev