When CG animated movies first began appearing 25 years ago, Pixar was at the forefront with pioneering shorts such as Luxo Jr. and Knick Knack. They continue leading the way with feature films such as The Incredibles, Wall-E, and Up which have quickly become beloved around the world. Along the way, the Toy Story films have been the heart and soul of the Pixar experience with Toy Story 3 bringing the trilogy to a dramatic yet satisfying conclusion.
Employing many of the top minds in computer graphics, Pixar has a tradition of building proprietary tools which they can develop and improve as needed. At the same time, they also work with off-the-shelf solutions and in the last few years this has included using Houdini and its procedural, node-based approach. Pixar has integrated Houdini on top of their proprietary tools and has recently increased its deployment of Houdini by acquiring a site license of Houdini Batch to support future projects.
Talking Procedural Trash
For Toy Story 3, Pixar used Houdini to help them achieve some of the movie’s most memorable VFX. In one of these scenes, the drama builds to a climax as Woody, Buzz and friends find themselves in a garbage incinerator being dragged towards a fiery doom. Also being dragged into the pit are millions of pieces of broken-up trash which needed to be simulated realistically.
For this sequence Pixar integrated Houdini into a workflow involving RenderMan and an in-house simulator. First the Pixar VFX artists treated each item of trash as an individual particle that could be procedurally animated using Houdini particle networks. The particles were then exported using a custom output operator before being rendered with a special RenderMan plug-in that procedurally generated more trash pieces around each particle. With this technique, it was possible to create the illusion of millions of trash pieces flowing down the sides of the incinerator.
Using their own rigid body system working outside of Houdini, Pixar’s artists simulated the foreground trash that interacts with the characters. To keep resources running at an optimal level when handling the mid-ground and background trash, they used Houdini to create and procedurally animate a single, flowing surface that covered the entire incinerator funnel. They shaded the flowing surface to look like it was comprised of millions of trash pieces.
In another scene, Pixar’s artists were assigned the task of animating plastic shopping bags that flutter through a wasteland of discarded trash. This additional detail was being added to an already approved sequence of shots where a dump truck empties its haul of trash.
This sequence was simulated using Pixar’s proprietary tools and the bags needed to be easily integrated into the shot without re-running the simulation from scratch, which would have been too costly in terms of time and resources. With Houdini, they could simulate the floating plastic bags without having to modify the already approved look of the shot by importing a cache of rigid bodies from the simulation using propriety tools. With a simple network of geometry nodes, they turned it into an approximate volume representation.
Using the volume representation, they could procedurally scatter plastic bags on top of the surface. Applying a special geometry solver to the volume representation made it easy to determine which rigid body objects remained on the bottom layer of the pouring trash throughout the entire simulation. This approach also gave them the opportunity to cull out a large percentage of unnecessary rigid bodies freeing up valuable resources.
When it came time to fine-tune the simulation, Houdini provided Pixar’s artists with the ability to achieve rapid iterations thanks to a heavily trimmed simulation cache and procedural placement.
Little Details that Matter
Throughout Toy Story 3’s production, Houdini also came in handy as a post-processing tool to use with existing simulations, such as the scene where Barbie torments Ken by tearing apart his collection of vintage clothing.
What Pixar required for the job was an extremely flexible way of post-processing all of the cloth in this sequence, down to the point of painting specific details that would follow the simulation, say for example, sculpting shapes to push cloth into Barbie's hands. By using Houdini’s surface operators and a few custom digital assets, they could quickly create rigs for each of the various types of deformations that were needed and then layer them as much as necessary.
In another shot in Toy Story 3 , Jessie, the cow-girl character has her head dipped into paint that is then used as a paint brush by one of the human characters.
By creating a rough set of paint particles using Autodesk’s Maya that were then post-processed in Houdini, they could make the particles resemble real paint. In Houdini, the points were projected onto the paper and then dressed up with thickness and opacity. The gaps were then filled procedurally.
To Infinity and Beyond
This year marks Pixar’s 25th anniversary. With each project, they continue to raise the bar and provide inspiration for the industry. With Houdini playing an increasingly larger role in their VFX pipeline, we look forward to seeing how Pixar will use it to delight future audiences.