Game Development Reference
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if(s <= 0.0)
n = d;
vr = p->vVelocity - Obstacles[i].vVelocity;
vrn = vr*n;
if(vrn < 0.0)
J = -(vr*n) * (_RESTITUTION + 1) /
(1/p->fMass + 1/Obstacles[i].fMass);
Fi = n;
p->vImpactForces += Fi;
p->vPosition -= n*s;
hasCollision = true;
The new code is nearly the same as the code that checks for and handles particle-to-
ground collisions. The only major differences are in how we compute the distance be‐
tween the particle and the obstacle and how we adjust the colliding particle's position
to prevent it from penetrating an obstacle, since the unit normal vector may not be
straight up as it was before. The rest of the code is the same, so let's focus on the differ‐
As explained earlier and illustrated in Figure 8-6 , we need to compute the separation,
s , between the particle and the obstacle. So to get s , we declare a variable r and equate
it to the sum of radii of the particle and the obstacle against which we're checking for a
collision. We define d , a Vector , as the difference between the positions of the particle
and obstacle. The magnitude of d minus r yields s .
If s is less than 0, then we make the relative velocity check. Now, in this case the collision
normal vector is along the line connecting the centers of the two circles representing
the particle and the obstacle. Well, that's just the vector d we already calculated. To get
the unit normal vector, we simply normalize d . The relative velocity vector is simply the
difference in velocities of the particle and the obstacle. Since the obstacles are static, the
relative velocity is just the particle's velocity. But we calculated the relative velocity by
taking the vector difference vr = p->vVelocity - Obstacles[i].vVelocity , because
in a more advanced scenario, you might have moving obstacles.
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