Motion and Collision - Beginning Android 3D Game Development - page 156

Center of Mass Body 1

CollisionDistance

Center of Mass Body 2

Figure 5-22. The collision distance

The CollisionNormal is calculated by normalizing the distance vector between the two centers
of mass calculated in the previous step. The relative velocity between the two objects is also
calculated. By taking the dot product between the relative velocity vector and the collision normal
vector, the magnitude of the relative velocity along the collision normal is found. (See Figure 5-23 .)

Body 1

m_RelativeVelocity

m_CollisionNormal

(Unit Vector of Length 1)

Body 2

RelativeVelocityNormal

Figure 5-23. Calculating the collision direction

Now that you know the collision distance and the relative velocity along the collision normal, you
have the all the information you need to find out if the objects are colliding. You know if they are
touching each other, based on the collision distance, and if they are moving toward each other,
based on the relative velocity of the objects along the collision normal. See Listing 5-18 to see the
actual code.

Listing 5-18. Collision Detection Function

CollisionStatus CheckForCollisionSphereBounding(Object3d body1, Object3d body2)
{
Float ImpactRadiusSum = 0;
float RelativeVelocityNormal = 0;
float CollisionDistance = 0;
Vector3 Body1Velocity;
Vector3 Body2Velocity;
CollisionStatus retval;