Breakdown: Building Fluid and Particle Simulation in Houdini

Aleix Plancheria Mosquera did a detailed breakdown of his waves simulation project made in Houdini, discussed his lighting and shading approach and shared tricks on using the FLIP tool. 

Introduction and Career

My name is Aleix Plancheria Mosquera, and I'm 23 years old. I'm from Lliça d'Amunt, a small village near to Barcelona. I was always interested in new technologies and also everything related to them. As I like video games, VFX, and the 3D world, this passion for the 3D world comes from my childhood when I used to see fantastic worlds at the cinema like Pixar and Disney movies. 

I started my education with IT systems and networks course but then, I realized that even though I liked it, I prefer a field that allows me to be more creative. That was the moment that I decided to try the 3D world, so I took Higher Education on 3D Animation in Enti-UB. Right after that, I started my masters on VFX in FX Animation, and now I'm finishing it.

During my whole education, I have had a lot of good instructors, with whom I have had the opportunity to acquire enough knowledge in order to understand, what was the path that I wanted to take. 

Inspiration and References

I started this simulation by looking for references because I think references are the key if you try to recreate something real in the 3D world. Then, I tried some setups in low resolution in order to see the main shapes which I was looking for. 

My main goal in this project was to recreate little curly waves crashing into the rocky wall with a bit of splash. I actually did this project to push my experience and increase my technical knowledge.

My main reference here is the beach from Carlos Parmentier. I don't want to do exactly the same thing but just the concept of the mockup beach with little waves crashing into the rocky wall, so I took some references of beaches in the same weather conditions.

About the FLIP tool

I'm not an expert in this tool, I'm just a student, but I think FLIP is a hard tool that is available on Houdini for large simulations of liquid. It has different setups to recreate beaches, the middle of the sea, lava, or any other crazy liquid simulations that you can think of. The key is creativity and the ability to use this kind of tool to recreate an idea. 

FLIP is Fluid Implicit Particles, it is a simulation that mixes fluids and particles. It works by simulating a spray of particles using Navier-Stokes equations, so they try to be as similar as possible to real-life physics. 

The first time I used this tool, I saw too many nodes and too many options available. Also, you could make custom setups to add custom forces, also, in a short time you get used to this tool, and now I really like it! Getting a nice mesh depends on the number of particles and on setting up the right voxels and then smoothing it properly using a mask. 

Tricks on Using the Tool

First of all, you have to pay attention to the scale of the scene that you want to recreate. Then you need to take a look at the particle separation. With these two steps, you can control the number of particles in your scene. The scale of the scene is important because it determines how the physics are going to influence your simulation: the bigger the scene is, the more particles you are going to have. Next, try to do a simulation with a low amount of particles (low res) to get the shape of your simulation.

When you have the particle simulation ready and cached, it is good to check the mesh and to look from a delete with a bounding box in order to check a little portion of our simulation. This way, we get changes faster.

Also, you have to check the thin parts of the mesh. If you don't have enough particles, these parts will flicker and create holes.

Modeling

I started modeling the floor and also looking for good photogrammetry in order to incorporate it. I always look at Sketchfab because there are some good meshes to incorporate into my scene. My aim is the simulation that is why I don't model everything, and sometimes I use 3D models from others. In the beach project, I modeled the shore of the beach in Houdini. Starting with a subdivided grid and painting an RGB mask to give some noise, also using Attribute VOP to give the terrain some detail. After I did a volume extrude, which is used to create a volume out of the surface geometry but I left it because it looks nice and gives an aspect of a mockup. 

Lighting and Shading

The challenge of building this kind of simulation with such dimensions is to be patient because it is a large and heavy process to simulate. So, your machine must be capable to process it. Shading and lighting are two things that come together. The lighting is very important because the shading depends on how the lighting is penetrating the materials, this is what is going to make your simulation look real. I recommend trying different setups for your lights until you get the result you are looking for.

Particles Workflow

From a student's point of view and taking into account that it was my first project with this tool, I think that I used an accurate approach. The number of particles used in this project was 13.303.846, from my point of view, it seems like a good quantity of particles. 

Waves and Foam Simulation

I started the simulation with the ocean spectrum and setting up the behavior according to my preferences. I did the curl waves by setting up the wind and the wave instancing, this is where you tell how they will behave. 

Regarding the foam, I decided to render particles instead of letting the volume because I believe it looks better on the render. I adjusted the mask to set where I want it to appear and set the size and amount of particles. 

The Biggest Challenges

The hardest part of this project was to process the simulation, it took 4 days to process it, also to generate this simulation to get the look that I was looking for. I started with low-resolution particles and making flipbooks to see how the simulation was behaving. The most important part of controlling this kind of simulation is to control the behavior, the fast movements, and the splashes. I would like to do a pyro sim to build my reel to look for a job. 

Aleix Plancheria Mosquera, VFX and FX Animation Artist

Interview conducted by Ellie Harisova

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    Breakdown: Building Fluid and Particle Simulation in Houdini