Game Development Reference
In-Depth Information
high-speed collisions will be used to resolve resting contact. This is a significant as-
sumption that needs justifying, and I'll return to it later in the chapter and at various
points further into the topic. To avoid changing terminology later, I'll use the terms
collision and contact interchangeably during this chapter.
When two objects collide, their movement after the collision can be calculated
from their movement before the collision: this is collision resolution. We resolve the
collision by making sure the two objects have the correct motion that would result
from the collision. Because collision happens in such a small instant of time (for most
objects we can't see the process of collision happening; it appears to be instant), we
go in and directly manipulate the motion of each object.
The laws governing the motion of colliding bodies depend on their closing velocity.
The closing velocity is the total speed at which the two objects are moving together.
Note also that this is a closing velocity, rather than a speed, even though it is a
scalar quantity. Speeds have no direction; they can only have positive (or zero) values.
Velocities can have direction. If we have vectors as velocities, then the direction is the
direction of the vector; but if we have a scalar value, then the direction is given by
the sign of the value. Two objects that are moving apart from each other will have a
closing velocity that is less than zero, for example.
We calculate the closing velocity of two objects by finding the component of their
velocity in the direction from one object to another:
v c = ˙
( p b
+ ˙
( p a
p a ·
p a )
p b ·
p b )
where v c is the closing velocity (a scalar quantity), p a
and p b
are the positions of
objects a and b , the dot (
) is the scalar product, and
p is the unit-length vector in the
same direction as p . This can be simplified to give
( p a p b )
( p a
v c =−
p b )
Although it is just a convention, it is more common to change the sign of this
quantity. Rather than a closing velocity, we have a separating velocity. The closing
velocity is the velocity of one object relative to another, in the direction between the
two objects.
In this case two objects that are closing in on each other will have a negative rela-
tive velocity, and objects that are separating will have a positive velocity. Mathemati-
cally this is simply a matter of changing the sign of equation 7.1 to give
( p a
v s =
p a − ˙
p b )
p b )
where v s is the separating velocity, which is the format we'll use in the rest of this topic.
You can stick with closing velocities if you like: it is simply a matter of preference,
although you'll have to flip the sign of various quantities in the engine to compensate.
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