Posted June 02, 2015

In Big Hero 6, Disney Animation introduces us to Hiro, a highly intelligent teenager who uses his robot-building skills to hustle the local bot-battle scene in San Fransokyo. Devastated by a family tragedy, he finds new purpose as part of an elite team of collegiate superheros. By his side is Baymax, a robot designed for enhanced medical diagnostics and treatment who Hiro mods into the ultimate battle bot. 

Over the course of the movie, the Big Hero 6 team comes together to take on a kabuki-masked villain in an attempt to regain control of tiny, yet powerful, microbots that had been stolen from Hiro. These microbots are the wave of the future as they work in harmony to create anything a person desires with both amazing and destructive results.

All of the tech being brought to life in this movie needed to be realized in a way that supported the creative process. Whether they were forming large swarms of microbots, adding smoke and contrails to Baymax’s rocket thrusters or bringing an alternate dimension to life, artists at Disney turned to Houdini’s procedural toolset to provide an efficient workflow, rapid iterations and high-quality VFX.


We knew at an early stage of this project that the microbots were going to be one of the biggest challenges for the effects department

Henrik F?ä?lt, Effects Lead

The microbots were, at their simplest, a central sphere with two legs connected using magnetics. This gave them a wide range of motion and multiple ways to cooperate and connect to other microbots. Each of them were controlled using a neuro-cranial transmitter which translates the free flow of ideas into physical form. Each microbot is about 1.5 inches tall, which meant that large numbers were needed to create the movement and scale indicated in the storyboards. The bots made an appearance in close to 300 shots and in many instances, tens of millions were on screen at once.

“We knew at an early stage of this project that the microbots were going to be one of the biggest challenges for the effects department” says Henrik F?ä?lt : Effects Lead at Disney Animation. “With hundreds of microbot shots to accomplish and very specific art direction and animation control needs, we did not opt for a flocking or fluid simulation based workflow. Instead, we used a layered procedural approach ideal for Houdini’s surface operator (SOPs) context”.

To control the bots, the team designed a workflow based on using artist-generated guide or base curves to direct the movement of the microbots as a swarm. Relying heavily on VEX, artist DJ Byun then created a collection of Houdini Digital Assets (HDA’s) to control how the microbots would organize themselves along these curves. This made it possible to block out basic animation and iterate fast. By taking advantage of Houdini’s procedural workflow, these simplified animations could be used to generate a sophisticated aesthetic. 

The guide curves were constructed in a variety of ways, depending on the needs of the shot. Sometimes they were derived from proxy animation provided by the animation department, sometimes from particle simulation trails or they were hand drawn. In some cases, they were driven by a rig such as the one used when Yokai rides the microbots through the streets of San Fransokyo.

One characteristic of the trails is that they would have a circuit-board feel to them. Alexey Stomakhin at Disney Animation wrote a custom tool using the Houdini Developer’s Kit [HDK] to convert smooth curves into curves obeying the rules of a (3D) circuit board. The tool could control how much the output curve would deviate from the input curve which would then define how frequently the microbots would turn. The tool also had controls for what angles to favor. Typically, curves that turn 45 degrees gave a more appealing aesthetic over 90 degree turns.

A Modular Approach

Smaller modular toolsets built using Houdini Digital Assets were created to help establish the look of the microbots as they move through a variety of different curve types. Taking this approach also helped to break the complexity of the work into smaller and more manageable chunks. 

About 20 tools were created altogether to assist with the animation and movement of the microbots. They can be categorized into the following: 

  • Pattern Generation | Circuit Board Generation: ?Assets that could take one or more input curve and generates new curves around the input curves following circuit board logic. These pattern generation tools had a wide range of user controls such as animation, offset from input curve and curve bend frequency.
  • Bundling | Miter Copy: Assets designed to handle the duplication of input curves mainly to fill volume and create certain aesthetics.
  • Walk Cycle | Flipper: Assets used to assist in the generation of animation. The microbots moved along these curves by passing each other over themselves using their magnetic legs, best described as mechanical crowd surfing. This walk cycle was defined in VEX code and encapsulated in a HDA with numerous controls over speed, rhythm, bending angles and variation.
  • Animating | Carving: ?In large masses and in particular fast moving collections, applying the walk cycle to all microbots at once gave a very noisy erratic look. To resolve this they developed these assets to “slide” the microbots along generated curves (also with controls over speed, rhythm and variation) and then apply the walk cycle sparsely, mostly in edges of the microbot masses.
  • Rig: Using these assets, they created higher level rigs for artists to use as a starting point for their shots. These rigs included lava which was initially designed for microbots to transport along the ground - but extended to work on any surface, and a rig for fast moving spiky tentacle masses. 

All of these tools were driven by similar interfaces and passed along attributes representing the interface setting to the node below to allow for a modular workflow where assets could be connected in different orders allowing different looks and functionality for larger rigs.

First Flight | Generating Smoke And Contrails

Our main challenge was coming up with a simple solution that would work for 100+ shots

Blair Pierpont, Effects Artist

When Hiro upgrades Baymax by equipping him with wings and thruster rockets, it leads to scenes of them ripping through the skies of San Fransokyo approaching speeds of Mach 1. In preparation for these flight sequences, artists researched various reference materials of fighter jet thrusters and reviewed them with directors Don Hall and Chris Williams to land on a design that was visually interesting, yet still grounded in reality. They also discussed where smoke and contrails would naturally exist when Baymax made sharp turns, quick adjustments to throttle, or acrobatics. Heat distortion needed to be added to thrusters as well.

“Our main challenge was coming up with a simple solution that would work for 100+ shots, 40 of them in the first flight sequence,” says Blair Pierpont, Effects Artist at Disney Animation. “Because of Baymax’s high speed, the length of his smoke trails could end up being kilometers in length, depending on the shot. The directors wanted more control over when smoke and contrails appeared, so it could accentuate the character animator’s work. The first flight sequence also needed to be delivered in less than a month.”

Disney Animation again developed a modular setup for Baymax’s thrusters, splitting them into flares, smoke, and contrails. The smoke and contrails setup utilized Houdini’s CHOPs, VDB, and Volume Rasterize nodes. The flares in particular were further broken down into core, shock diamonds, and edge.

“It’s great that we weren’t locked into a particular way of doing things, and if the directors asked for something the setup didn’t account for, we were confident we could find a way to make it work,” says Blair. “It was a very organic process.”

Guide curves were once again utilized to define flare properties per vertex, properties such as shape, color, and turbulence. Each flare had a guide curve that defined its length and alignment. The length of the flares were linked to Baymax’s speed, but each flare could be adjusted individually if needed. Alignment could be weighted towards different parts of Baymax’s body, such as torso, legs, feet, or an average. Guide curves could also be bent to better define Baymax’s line of action. CHOPs were used to modulate length and alignment to make the flare appear more controlled or random.

“This workflow provided us with an incredibly fast turnover because nothing we were creating was simulation based,” remarks Jesse Erickson, Effects Artist at Disney Animation. “We could go to specific frame and modify our settings until we got the shape we were after and everything else would fall into place. We couldn’t have used this approach without VDB because the smoke trails were visible so far into the distance.”

For the smoke trails, thruster origin points were fed into CHOPS and pulled back out as curves with an age attribute. This kept the geometry live while front-loading the cook, creating faster than realtime playback without caching to disk. Age was used to remove points that hadn’t been “born” yet and to drive noise offsets, rotations, density & scale ramps, and to push points along their birth vector. These curves were then populated with spheres for visualization and pyroclastic puffs for output to VDB sequences.

“The thruster setup was more or less developed in 2 weeks and was updated throughout production” says Blair. “The smoke trails were also setup in less than a week and were also revised during production. It’s an intuitive process to prototype an effect in one shot and then package it up for artists.”

Into The Portal

For the climactic sequence at the end of Big Hero 6, the filmmakers needed to create a wormhole-like environment that would suggest a gap between the folds of spacetime. The importance of these shots cannot be overstated. The “Into the Portal” sequence was the climactic sequence of Big Hero 6, and the directors were relying on the effects department to create a truly unique environment for this critical sequence.

“We imagined a realm of fractal forms inspired partly by theories of spacetime structure found in an approach to quantum gravity known as causal dynamical triangulation” says David Hutchins, Senior Effects Lead at Disney Animation.

Houdini was an integral tool in the Disney Animation process of planning and modeling an environment that was constructed entirely of animated, three dimensional volumetric fractals using an extension of the classic Mandelbrot fractal expressed in polar coordinates, a fractal referred to as the “Mandelbulb”. Their variation on this algorithm featured parameters which made possible a wide variety of forms and animation options.


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