Game Development Reference
void detectContacts(const Primitive &firstPrimitive,
const Primitive &secondPrimitive,
where the Primitive type holds the data for the collision geometry. The Primitive
class holds data that any contact generator will need to know, such as the rigid body
corresponding to the geometry and the offset of the primitive from the coordinate
origin of the rigid body.
Throughout this chapter I will assume that the offset matrix represents transla-
tion and rotation only: it has no scaling or skewing effect. We could make this much
simpler and assume that the primitive is aligned perfectly with the center of the rigid
body with no rotation (as would be the case if we had a cylinder representing a bar-
rel, for example). Unfortunately this would not work for assemblies of objects or for
rigid bodies with centers of mass that aren't at their geometric center. For flexibility it
is best to allow primitives to be offset from their rigid bodies.
Each implemented contact generation function will use a subtype of Primitive
with some additional data (such as Sphere and Plane ). I'll introduce these types as we
C OLLIDING T WO S PHERES
Colliding two spheres is as simple as it gets. Two spheres are in contact if the distance
between their centers is less than the sum of their radii.
If they are in contact, then there will be precisely one contact point: each sphere
consists of one surface, so it will be a face-face contact (see figure 13.8).
The point of deepest contact is located along the line between the sphere cen-
ters. This is exactly the same algorithm we saw in chapter 7 when looking at particle
To implement this we need a data structure for a sphere. Spheres are completely
defined by their center point and radius: