Speaking of Rigs

June 19th, 2007
Daffy Duck
A bit too Daffy

In the world of 3D animation, “rigging” is the process wherein virtual control mechanisms are connected to a model to enable its manipulation. There are a few general techniques which may be employed, but the methods of their execution vary between 3D packages and must be adjusted for individual models, design goals, and motion styles.

The large number of controls necessary for basic manipulations — compounded by inevitable structural problems, application glitches, and the difficulty of applied kinesthetics — makes rigging even a stiff-looking biped a tricky proposition. Making a 3D rig that can move like Daffy Duck is downright Quixotic at best.

Rigs I have known

Basic character rigs are the equivalent of an artist’s wooden figure model: rigid pieces connected by ball joints. Though stiff, these rigs are reliable and easy to build, and are thus the most common sort of rig. Consequently, 3D animation has a reputation for looking stiff, especially compared to traditional 2D animation. It’s difficult to make 3D rigs look more like a Tex Avery cartoon, but desirable, for the same reason that 2D animators use exaggeration: emphasis.

wooden artist's model
Get the oil can

Though the human frame is also made of rigid pieces, it’s covered in a lot of squishy, stretchy stuff. Muscles, fat, and clothing can accentuate or disguise the motion of the structure underneath. Additionally, our experience of kinesthetics trains us to anticipate certain kinds of motion and not others, and the degree to which those expectations are met or subverted is another factor which may be exploited by an animator for emphasis.

A kind of emphasis commonly used by animators is described by the phrase “squash and stretch”, referring to the way malleable forms respond to force. Chewing gum and chicken necks stretch when pulled; rubber balls and duck heads squash out when bonked with mallets; muscles bulge when contracting. When these motions are emphasized, the emotional impact of the motion is likewise emphasized. In fact, if a cartoony figure does not move in cartoony ways, that too is a kind of emphasis, and the figure may look unnaturally stiff.

Man the rigging

With hand-drawn animation, motion and form can easily be controlled for emphasis, assuming the artist knows where to put the lines. With a 3D rig, all such functionality must be built into the rig itself, which involves prep work, the natural enemy of the artist.

Building a single rig to do this, for example, would be tricky:

Much of the problem is computational: working with a 3D rig requires a workstation that can easily process the rig’s position and motion, so that the animator can adjust the rig’s position without undue delay, receive timely feedback from the interface, and play back animated sequences in real time. Complex models with complex rigs can be a drain on cpu and gpu power, and are more likely to exhibit unintended behavior such as glitches or free will.

Elastigirl from the Incredibles
A bit of a stretch

Most current solutions to the problem of complexity involve the division of labor: multiple models (low- and high-resolution), or multiple rigs. Computerarts.co.uk describes an approach Pixar took on The Incredibles involving two separate hot-swappable rigs for Elastigirl — one for standard motions and another for her silly-putty rubber-hose moves (“Inside the Incredibles”, under the heading “Loop-the-loops”).

Isaac Kerlow, director of Digital Production at Disney, has an article at vfxworld.com with even more detail on Pixar’s process, which included automatic baking of skin deformations to speed up processing.

Breaking the rig

AWN‘s 7th article in their Open Season Diary series, “Animating the Animals”, makes an excellent point: 3D rigs typically attempt to control models from the inside, with bones and other devices indirectly determining the overall shape, or “silhouette”, of a character. They expect a certain range of motion, and attempting to go beyond this range “breaks” the rig.

The Open Season rigs included deformers which allowed animators to adjust the silhouette directly, pushing and pulling the character in ways not explicitly allowed by the rig. This afforded much more control over form, allowing old-school squash and stretch capabilities.

Rigging outside the box

Pixar and its lesser cousins suffer from a certain amount of wheel-reinventing due to proprietary in-house systems, intellectual property hoarding, and non-compete agreements. But it’s also because rigging is so young, and changing so quickly, that there’s no advantage to standardizing. New techniques for plastic rigs are developed with every new piece of hardware and software.

Outside of the upper echelon, independents labor under similar constraints. Online tutorials read like an alchemist’s codex: wing of bat, eye of newt, and it won’t work if you look at it sideways. Until technological limits no longer impede the basics, advanced rigging techniques will continue to resemble rumbles between rival gangs of insane Rube Goldberg machines.

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