CarCollision

CarCollision

Cars that hit like cars, and a world they can crash into.

What it does

Every frame, after CarDrive has moved everyone, CarCollision finds each pair of cars that overlap and each car touching the world, and works out the shove each one gets. It moves nothing itself. Each car's fix goes back to CarDrive as one carDrive.nudge (position, velocity and a yaw kick), and the damage goes out as health.damage.

Use it in a scene

{
  "Car": {
    "components": {
      "CarCollision": {
        "mass": 1.6,
        "halfWidth": 0.95,
        "halfLength": 2.05,
        "armor": 0.15,
        "ramDamage": 1.6
      }
    }
  }
}

The car also needs CarDrive and a MeshPrimitive.

Oriented boxes

A car is about twice as long as it is wide. As a circle, two cars in side-by-side lanes would touch, and a tap on a rear corner would push the whole car sideways. As boxes:

1. Separating axes find the overlap and the contact normal.
2. The corner that dug in is the contact point.
3. One push along the normal (the bounce) and one along the surface (the scrape, capped by friction) are solved for the speed of the two contact points, which includes how fast each car is turning.

Corner hits

The turn a hit gives is the push times its lever arm, the distance from the car's middle to the contact point:

  // Perpendicular of the lever arm, in the yaw convention above.
  const raPx = c.pz - a.z, raPz = -(c.px - a.x);
  // ...
  out.dwA = -(raPx * Jx + raPz * Jz) * invIA;

Hit a car dead centre and the arm is zero, so it only slides. Hit the rear corner of a 4.1 m car and the arm is about 2 m, so the same push turns it. spinGain then scales that turn for arcade feel.

Damage from a knock

export function impactDamage(dv: number, threshold: number, perUnit: number): number {
  return dv > threshold ? (dv - threshold) * perUnit : 0;
}

// With the defaults (a worked call, not from the source):
impactDamage(10, 4, 2.2) // → 13.2
impactDamage(3, 4, 2.2)  // → 0

Against another car, that is scaled by this car's armor and the other car's ramDamage. A tree or a wall hurts 1.35 times as much, because it does not give.

damageTaken scales every hit from anything on the road: rams, gunfire, bombs and craters. AutoRunnerProjectile and RoadHazard read it through damageTo() in CarCollision.defs. A car that drives into a river is lost whatever it is set to.

Spin-outs

A hard hit, or carCollision.spin from an oil slick, sets the car spinning and takes its grip away for a moment. It slides on its old line while it turns, and its own grip comes back as the spin dies down. Set recoverDamping low and the nose keeps swinging after the slide, fighting the steering.

A grip change from anything else (a tuning panel, a refit) is kept as the car's own grip. A pooled car that leaves play mid-slide gets its grip back first, so it never comes back stuck with the slide grip.

The world

With a HighwayRoad and RoadsideScenery in the scene, roadside props are solid and a bridge's rails are walls. Grass drags the car down and jolts it. The river drowns it. fieldEdge is a hedge at the far edge of the fields, so no car drives off the generated ground into empty fog.

A kinematic car (a trailer on its hitch) pushes and is never pushed back. A car tagged ghost is ignored, like a car inside a truck or between lives.

Settings

Events

closing is in m/s. A touch under 2 m/s sends nothing, and one car sends at most one crash every 0.35 s.

Needs

• CarDrive moves every car. This reads its heading, speed and grip, and sends fixes back through carDrive.nudge.
• MeshPrimitive gives each car's position, and its size when the half sizes are 0.
• HighwayRoad says what is under each car, where the bridge rails are, and where the river runs.
• RoadsideScenery lists the props beside the road.