Creating Boiling Potion FX in Houdini

Introduction

Hello! My name is Shipra Pal. I am an FX Artist from Mumbai, India. I started my 3D career about 5 years ago. In my first years, I mostly used Blender. I started learning Houdini about 1 year ago.

Boiling Potion: Inspiration

I like scrolling Pinterest for inspiration and concepts on a regular basis. Once I found an image from Hearthstone that grabbed my attention. I've always liked magical and fantasy designs. So, I decided to create this for my personal project in my free time. My goal was to learn boiling liquid simulation in Houdini.

Modeling & Texturing

I started creating the pot model in Blender. After the basic shape, I did some sculpting to achieve details like dents to get a look closer to the reference. After that, I used Substance Painter to create the textures. Same process for the wooden planks.
For the backdrop, I heavily used Megascan assets that were added at the end. 

Boiling Liquid Simulation

I used Houdini for my FX elements. For the boiling liquid, I set up a simple particle system with an upward motion and copied spheres (with animated mountain noise) to particles to create bubbles as well as added some size variation. Later, I converted that into VDB to add them as a volume velocity in my flip solver. I got the basic idea from this awesome tutorial by João Pereira:

Later, I created my liquid mesh and exported it as an alembic sequence. After that, I imported it into Blender and created a wetmap in Blender’s dynamic paint system which I later added to the shader to make it more appealing.

Steam & Fire

For the steam, I animated two spheres in random motion and added them as a source emitter for Pyro. Your source emitter is very important for simulations. You need to set up your emitter with some awesome noise and random normals/velocity to drive your simulation. And then you are ready to give it a try with Houdini’s shelf tools basic setup. For beginners, it's very helpful to create a flow. Then play with your settings and value, add some tweaks here and there to get a result you want and you’re good to go. The same method was used for fire: I gave some animated noise to the emitter, set up my flames, and added collision objects!

Rendering

For the rendering part, I wanted to use Blender fully. But for the fire, Houdini’s output looked better, thus I decided to render fire and steam in Houdini. For the rest of the sequence, I used Blender and Cycles render engine. I used a couple of render layers (View Layers in Blender’s terms) for separate light groups. I went for a dark mysterious lighting and most of the focus was on the boiling potion.

Since I used still camera, I saved quite a lot of render time. For all the static objects, I rendered a single frame. With RTX Card and Optix enabled in Blender, a single frame render took only around 1 min 46 sec. Fire and steam were rendered overnight in Houdini.  And for the potion, I used Blender with denoise which took only 12 sec per frame.

Since I rendered the floor and pot in a single frame, there was no animated light impact from fire. I used Nuke to add impact by blurring the fire layer and overlay with the pot and floor which did the job well enough.

As an FX artist, I could’ve just rendered the simulation and called it a day, but in my opinion, a little scene with lighting and mood would be more appealing to the viewers. And since there are such sources as Megascans, it’s so easy to just grab some models and build up a scene quickly.

Shipra Pal, FX Artist

Interview conducted by Arti Sergeev