Controlling Creatures with Linux
The Jim Henson Company is well known for creating characters. Low-tech characters like the Muppets don't need much technology, but animatronics, from gerbils to dinosaurs, do need it, not to mention our 3-D computer graphic puppets. Performing live, in real time, so they can interact with human actors and be captured on film, these characters have a curious set of needs from a technology perspective.
One of Jim Henson's original performance goals was that one person should be in command of each character, bringing a spontaneity and personality harder to achieve in a “performance by committee” (where several people perform a puppet together). The fascinating thing about a creature that achieves this goal is that people forget who or what is controlling it and simply interact with it. Actors and audience alike start conversing with a dog or a frog or a snowman as if it were human.
With the proliferation of servo motor technology in animatronic puppets in the early 1980s, managing increasing numbers of servos became a challenge, so computerized control systems were designed. During the last 15 years, several generations of control systems have been developed at the Jim Henson Creature Shop, including a version that won a Technical Achievement Academy Award in 1992. The latest Henson Performance Control System (HPCS) encompasses the best features of previous systems, while adding new technology available only with today's hardware and computing environments such as Linux.
This system was begun under the guidance of Computer/Electronics Supervisor Jeff Forbes in early 1998. We had a vision that one system on a standard architecture could service all the company's needs. Steve Rosenbluth joined the project at that point as the control system designer, and Michael Babcock as the multimedia programmer. Our needs turned out to be rather expansive, and Linux seemed to be the only thing that could do it all without flinching.
The system has to support two back ends: one animatronic and the other computer graphic. So, our puppets are either real-world robots, or virtual puppets made of polygons and pixels. We can handle either separately or both at the same time.
Once the software “set up” of a puppet is in the system, even puppeteers new to the technology can jump in and perform well within hours. Using the input devices is akin to performing a musical instrument. At a certain point, the puppeteer, like the musician, no longer has to think about what he's doing—he just performs.
Henson input devices are not motion capture technologies. Motion capture is both directly analogous to the performer and largely is nonprogrammable. In motion capture, a performer's arm simply corresponds to the creature arm, a knee corresponds to a knee, etc. The performer cannot enhance or reprogram these relationships. The Henson input scheme is both non-analogous and user-programmable. Our input devices are abstractions. For example, a puppeteer's index finger might proportionally control the sincerity or sarcasm of a creature's entire face. And a puppeteer can reprogram puppet movement easily between and even during performances. A person in a motion capture suit would be hard pressed to perform an octopus. A person operating our control system could take it in stride.
At the core of the system is a Control Computer, running RTLinux, which processes and distributes motion data to puppets. The process that runs motion-mixing algorithms on the Control Computer is called the Motion Engine. Steve Rosenbluth wrote it in C++. Performer movement, coming through input transducers from the outside world, passes through the Motion Engine on its way to networked puppets. Inside the Motion Engine, various algorithms are performed on the live data, resolving final actuator positions. An actuator is like a muscle of the puppet; it could be an electromechanical or hydraulic servo in an animatronic or a “virtual servo” (a mesh deformation) in a computer graphic puppet. Motion-mixing relationships are configurable in software to be one-to-many or many-to-one, and compound mixes can be performed on top of those. Physics, which add effects like weight or smoothing, can be added to performance data as it passes through the Motion Engine.