Game Development Reference
W HAT I S G AME P HYSICS ?
Physics is a huge discipline, and academic physics has hundreds of subfields. Each
describes some aspect of the physical world, whether it is the way light works or the
nuclear reactions inside a star.
Some bits of physics might be useful in games. We could use optics, for exam-
ple, to simulate the way light travels and bounces and use it to make great-looking
graphics. This is the way ray-tracing works, and (although it is still very slow) it had
been used in several titles. This isn't what we mean when we talk about game physics.
Although they are part of academic physics, they are not part of game physics, and I
won't consider them in this topic.
Other bits of physics have a more tenuous connection: I can't think of a use for
nuclear physics simulation in a game, unless the nuclear reactions were the whole
point of the gameplay.
When we talk about physics in a game, we really mean classical mechanics: the
laws that govern how large objects move under the influence of gravity and other
forces. In academic physics these laws have largely been superseded by new theories:
relativity and quantum mechanics. In games they are used to give objects the feel of
being solid things, with mass, inertia, bounce, and buoyancy.
Game physics has been around almost since the first games were written. It was
first seen in the way particles move: sparks, fireworks, the ballistics of bullets, smoke,
and explosions. Physics simulation has also been used to create flight simulators for
nearly three decades. Next came car physics, with ever increasing sophistication of
tire, suspension, and engine models.
As processing power became available, we saw crates that could be moved around
or stacked, walls that could be destroyed and crumble into their constituent blocks.
This is rigid body physics, which rapidly expanded to include softer objects like
clothes, flags, and rope. Most recently we have seen the rise of the ragdoll: a physi-
cal simulation of the human skeleton that allows more realistic trips, falls, and death
In this topic we'll cover the full gamut of physics tasks. With a gradually more
comprehensive technology suite our physics engine will support particle effects, flight
simulation, car physics, crates, destructible objects, cloth and ragdolls, along with
many other effects.
W HAT I SA P HYSICS E NGINE ?
Although physics in games is more than thirty years old, there has been a distinct
change in recent years in the way that physics is implemented. Originally each effect
was programmed for its own sake, creating a game with only the physics needed for
that title. If a game needed arrows to follow trajectories, then the equation of the
trajectory could be programmed into the game. It would be useless for simulating
anything but the trajectory of arrows, but it would be perfect for that.