Game Development Reference
the animation of a bone model (for an overview in this field, see, e.g., [Jacka
et al. 07]). These techniques work very well in practice, even for challenging
regions such as shoulders or heels. They are of a purely kinematic nature and
there is no time dependence, so it does not matter if a limb is moved slowly or
quickly—the calculated surface vertices are the same.
On the other hand, physics-based simulation has entered computer games,
for example, in form of the simulation of ragdolls (a collection of multiple rigid
bodies, where each of the bodies is linked to a bone of the skeletal animation
system). A famous example is the game Hitman: Codename 47 by IO Interactive
[Jakobsen 01]. Such simulations can be used to model cloth, plants, waving flags,
or dying characters.
More advanced physics simulations quickly become computationally inten-
sive and thus not suitable for real-time processing. This is a pity because there are
a lot of physical effects that get completely lost even in ragdoll physics—effects
that would be stunning if achieved in a real-time simulation. It would be great
to realistically simulate the properties of solid materials—watch how they react
and deform when applying pressure to the surface—or when under the influence
of gravity. Or concerning character animation: animating a character in its low
frequency motion using its bone model, defining some material properties, and
letting the physics system take care of the small and high-frequency motion—
think of the jiggling of fat tissue when an ogre starts to move.
It is this tiny motion that adds most to the realism in a simulation.
Of course, this animation system would have to take care of maintaining sur-
face details, such as the layout of veins on an arm or the wrinkles on an old man's
In computer games, performance is very important, and only a small percent-
age of computation time can be spent on the physics subsystem, but more and
more realistic simulations can enter our homes as processors get faster and graph-
ics hardware more programmable.
In this chapter, a physics simulation is developed that can add secondary
deformation to a mesh, while the primary deformation can still be driven by a
skeleton—the comfort of animating a character by some simple bones will be
One thing that we have to bear in mind is that for simulation in computer
games, we are not ultimately striving for accuracy, as we would in a scientific
simulation—but rather, we strive for believability—the programmer is in the po-
sition to trick the player into thinking that what the player sees is real.
Such a simulation can dramatically improve the realism of an animation and
still be economic in computational effort. In fact, the techniques presented in this