Game Development Reference
InDepth Information
F
IGURE
15.2
The longdistance dependence of reaction forces.
Each of these situations leads to problems in solving the mathematics to get a set
of reaction forces. Specialcase code or tailored solving algorithms are needed to de
tect impossibilities and react differently to them (typically by introducing an impulse
of some kind).
If this sounds complex, it's because it is. Fortunately there is a much simpler
(though slightly less accurate) solution. Rather than resolving all contacts using forces
(i.e., converting collision impulses into forces), we can do the opposite and treat rest
ing contacts as if they were collisions.
15.2
M
ICRO

COLLISIONS
Microcollisions replace reaction forces by a series of impulses: one per update. In the
same way that an impulse can be thought of as a force applied in a single moment of
time, a force can be thought of as a whole series of impulses. Applying a force of 10
f
N
to an object over 10 updates is equivalent to applying impulses of
f
Ns at each update.
Rather than calculate a set of reaction forces at each time step, we allow our im
pulse resolution system to apply impulses. Figure 15.3 shows this in practice. The
block should be resting on the ground. At each frame (ignoring interpenetration for