Physics Initialization - Learning Game Physics with Bullet Physics and OpenGL - page 40

Understanding what's happening, and how to optimize these kinds of bulk pro-
cesses is a key component in becoming an effective game developer.
But, imagine if only two of those 100 objects are even remotely close together and
the rest are spread too far apart to matter; why would we waste the time doing pre-
cise collision checks on the other 9,899 pairs? This is the purpose of broad phase
collision detection . It is the process of quickly culling away object pairs, which have
little or no chance of collision in the current step, and then creating a shortlist of those
that could collide. This is an important point because the process merely provides a
rough estimate, in order to keep the mathematics computationally cheap. It does not
miss any legitimate collision pairs, but it will return some that aren't actually colliding.
Once we have shortlisted the potential collisions, we pass them on to another com-
ponent of the physics simulation called narrow phase collision detection, which
checks the shortlist for legitimate collisions using more intense, but accurate math-
ematical techniques.
For our project we will use a broad phase technique based on dynamic bounding
volumes. This algorithm create volumes of space which envelop pairs of objects in
a tree hierarchy using Axis-aligned bounding boxes ( AABBs ). These AABBs sur-
round the object with the smallest box shaped volume possible, that is aligned with
each axis, as we can see here: