Simulating Sticky Cotton in Houdini

Introduction

Hello! I am José León Molfino, I’m a 3D Artist and Generalist based in Buenos Aires, Argentina. I’m the co-owner of a studio boutique Stato with my partner and wife. During the past months, we’ve been working on several animation pieces for ZARA, L’Occitane, Auth0, and NFL among others.

Houdini

I’ve been jumping between commercial work and personal R&D since the last interview. I usually post on Twitter any Houdini stuff I come up with during that process. I’ve been trying to push myself into learning new USD workflows inside the software. Solaris and LOPs for scene assembling, management, and rendering are some of the best latest advancements from SideFX for me.

I’ve been using the old OBJ and ROP contexts for scene rendering for so long, but I’m currently in the process of moving all those workflows into Solaris, struggling a bit in the process, but in the end, it helps to keep scene stuff sorted out in more efficient ways.

The Sticky Cotton Simulation

It all began with a commercial project of a cotton flower (not yet released) I’d been working on. I watched a lot of cotton flower footage, also playing with real cotton material trying to understand what it’s made of. My goal was to create a realistic flat piece of cotton. So first, I collected a few references from the internet.

The main shape is pretty basic. A flat plane with a bunch of holes. Then I created a group for all the holes edge points which I would use to weld both sheets together.

I also pinned the top points and moved them along the Z-axis to create the force that will tear them apart. The other side was just a symmetrical plane. I used a vellum cloth and vellum weld constraints to set up vellum attributes.

The simulation was pretty straightforward. With all the constraints and weld attributes, I only increased the substeps to 3 and let the solver do the magic.

Usually, for this kind of material, I would use a high-density volume mixed with scattered hairs to make it look like cotton. But in this case, for such a thin sheet, I decided to use just curve splines, not volumes.

The first thing I did was freeze the very last frame of the simulation and set a rest state to work with. I had to give those flat planes some subtle thickness to work with, it was super simple with “Labs Thicken”, a couple of VDB nodes to smooth it out, and finally converting again to polygons.

For the curves, I first tried using just Houdini’s hair generation tools and guide process nodes (Guide Advect, Frizz, Bend, etc.) in order to scatter hair guides over the surface. It actually looked okay but more like hairy skin, not like cotton.

So I realized cotton is mostly very long and thin fibers all stuck and pressed together. A few years ago, Simon Fiedler did a great Houdini Hive Talk, where he explained and broke down a setup to generate continuous curves wrapping around an object. So I used that setup to create cotton fibers along the flat surface. 

I scattered a bunch of initial points and ran the algorithm through tons of iterations until the object got fully covered by curves.

I then added some noise displacement and resample to smooth out the curves. I ended up with 85k unique curves and 5 million points.

Finally, the Point Deform SOP was used to skin and deform those two pieces of cotton based on the vellum simulation.

Rendering

For the scene render, I used Redshift. Rendering hair in Houdini-Redshift is pretty straightforward. You just need to convert the splines to NURBS curves, and Redshift will recognize and render them as hair curves. I only added a “width” attribute with a very low value so the fibers look super thin. The material is very basic also. It’s a new Redshift standard material with a subtle grayscale ramp along the curve and a bit of reflection. A single frame at 1600x1600px took about 1.5 minutes to render out on two 3080Ti cards.

Only one HDRI was used to light the scene. As usual, the one from the Maxim Roz Library.

Tips for Beginners

For similar projects, I would recommend diving into Vellum inside Houdini, there are plenty of tutorials about it on YouTube. For the first steps, I’d highly recommend all the Vellum node videos from Paul Esteves, and for more advanced stuff – John Lynch's H17 and H18 masterclasses are really useful learning sources.

For Houdini in general, I always recommend Matt Estela's cgwiki. He keeps the site up-to-date with all new features in Houdini, with .gif examples, great explanations, and .hip files.

José León Molfino, 3D Artist & Generalist

Interview conducted by Arti Burton