Game Development Reference
In-Depth Information
J . Finally, hasCollision is a flag that's set based on whether or not a collision has been
detected.
Now we'll look at the details within CheckForCollisions . The first task is to initialize
the impact force vector, vImpactForces , to 0. Next, we make the first collision check by
determining if the y-position of the particle is less than the ground plane height plus
the particles radius. If it is, then we know a collision may have occurred. (_GROUND_PLANE
represents the y coordinate of the ground plane, which we have set to 100.) If a collision
may have occurred, then we make the next check—to determine if the particle is moving
toward the ground plane.
To perform this second check, we compute the unit normal vector, relative velocity, and
relative velocity component in the collision normal direct as described earlier. If the
relative velocity in the normal direction is negative (i.e., if vrn < 0), then a collision has
occurred. If either of these checks is false , then a collision has not occurred and the
function exits, returning false .
The interesting stuff happens if the second check passes. This is where we have to de‐
termine the impact force that will cause the particle to bounce off the ground plane.
Here's the specific code that computes the impact force:
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;
We compute the impulse using the formulas presented in Chapter 5 . J is a scalar equal
to the negative of the relative velocity in the normal direction times the coefficient of
restitution plus 1 times the particle mass. Recall that the coefficient of restitution, _RES
TITUTION , governs how elastic or inelastic the collision is, or in other words, how much
energy is transferred back to the particle during the impact. We have this value set to
0.6, but it is tunable depending on what effect you're trying to achieve. A value of 1
makes the particles very bouncy, while a value of, say, 0.1 makes them sort of stick to
the ground upon impact.
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