Game Development Reference
In-Depth Information
// Reset aggregate impact force
p->vImpactForces.x = 0;
p->vImpactForces.y = 0;
// check for collisions with ground plane
if(p->vPosition.y <= (_GROUND_PLANE+p->fRadius)) {
n.x = 0;
n.y = 1;
vr = p->vVelocity;
vrn = vr * n;
// check to see if the particle is moving toward the ground
if(vrn < 0.0) {
J = -(vr*n) * (_RESTITUTION + 1) * p->fMass;
Fi = n;
Fi *= J/_TIMESTEP;
p->vImpactForces += Fi;
p->vPosition.y = _GROUND_PLANE + p->fRadius;
p->vPosition.x = ((_GROUND_PLANE + p->fRadius −
p->vPreviousPosition.y) /
(p->vPosition.y - p->vPreviousPosition.y) *
(p->vPosition.x - p->vPreviousPosition.x)) +
p->vPreviousPosition.x;
hasCollision = true;
}
}
return hasCollision;
}
CheckForCollisions makes two checks: 1) it checks to see whether or not the particle
is making contact or passing through the ground plane; and 2) it checks to make sure
the particle is actually moving toward the ground plane. Keep in mind a particle could
be in contact with the ground plane right after a collision has been handled with the
particle moving away from the ground. In this case, we don't want to register another
collision.
Let's consider the details of this function, starting with the local variables. n is a vector
that represents the unit normal vector pointing from the ground plane to the particle
colliding with it. For ground collisions, in this example, the unit normal vector is always
straight up since the ground plane is flat. This means the unit normal vector will always
have an x component of 0 and its y component will be 1.
The Vector vr is the relative velocity vector between the particle and the ground. Since
the ground isn't moving, the relative velocity is simply the velocity of the particle. vrn
is a scalar that's used to store the component of the relative velocity in the direction of
the collision unit normal vector. We compute vrn by taking the dot product of the
relative velocity with the unit normal vector. J is a scalar that stores the impulse resulting
from the collision. Fi is a vector that stores the impact force as derived from the impulse
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