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F IGURE 14.6
Nonlinear projection is more believable.
F IGURE 14.7
Nonlinear projection does not add friction.
impact situation: the nonlinear projection method doesn't introduce any additional
friction. In fact it slightly diminishes the friction by allowing the object to slide farther
than it otherwise would. I don't know why, but in practice this is far less noticeable
than extra friction.
I will return to the details of implementing this algorithm later in the section.
Relaxation
Relaxation isn't, strictly speaking, a new resolution method. Relaxation resolves only
a proportion of the interpenetration at one go, and can be used in combination with
any other method. It is most commonly used with nonlinear projection, however.
Relaxation is useful when there are lots of contacts on one object. As each contact
is considered and resolved, it may move the object in such a way that other objects
are now interpenetrated. For a brick in a wall, any movement in any direction will
cause it to interpenetrate with another brick. This can cause problems with the order
in which interpenetration resolution is carried out, and can leave the simulation with
contacts that still have noticeable penetration.
By performing more interpenetration resolution steps, but having each one only
resolve a proportion of the interpenetration, a set of contacts can have a more
equitable say over where an object ends up. Each gets to resolve a little, and then the
others take their turns. This typically is repeated several times. In situations where
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