600 12. Collision and Rigid Body Dynamics
z Rag doll physics. Rag dolls require a lot of fi ne-tuning and oft en suff er
from instability in the simulation. An animation may drive parts of a
character’s body into penetration with other collision volumes—when
the character turns into a rag doll, these interpenetrations can cause
enormous instability. Steps must be taken to avoid this.
z Graphics. Physics-driven motion can have an eff ect on renderable ob-
jects’ bounding volumes (where they would otherwise be static or more
predictable). The presence of destructible buildings and objects can in-
validate some kinds of precomputed lighting and shadow methods.
z Networking and multiplayer. Physics eff ects that do not aff ect gameplay
may be simulated exclusively (and independently) on each client ma-
chine. However, physics that has an eff ect on gameplay (such as the
trajectory that a grenade follows) must be simulated on the server and
accurately replicated on all clients.
z Record and playback. The ability to record gameplay and play it back at
a later time is very useful as a debugging/testing aid, and it can also
serve as a fun game feature. This feature is much more diffi cult to imple-
ment in the presence of simulated dynamics because chaotic behavior
(in which the simulation takes a very diff erent path as a result of small
changes in initial conditions) and diff erences in the timing of the phys-
ics updates can cause playbacks to fail to match the recorded original.12.1.3.3. Art Impacts
z Additional tool and workfl ow complexity. The need to rig up objects with
mass, friction, constraints, and other att ributes for consumption by the
dynamics simulation makes the art department’s job more diffi cult as
well.
z More-complex content. We may need multiple visually identical versions of
an object with diff erent collision and dynamics confi gurations for diff er-
ent purposes—for example, a pristine version and a destructible version.
z Loss of control. The unpredictability of physics-driven objects can make
it diffi cult to control the artistic composition of a scene.12.1.3.4. Other Impacts
z Interdisciplinary impacts. The introduction of a dynamics simulation into
your game requires close cooperation between engineering, art, and de-
sign.
z Production impacts. Physics can add to a project’s development costs,
technical and organizational complexity, and risk.