What does the data in a collision event mean?

[a-type]

The shape of the event provided for a collision is documented, but the meanings of properties are not:

type Event = {
  op: string
  type: string
  body: THREE.Object3D
  target: THREE.Object3D
  contact: {
    ni: number[]
    ri: number[]
    rj: number[]
    impactVelocity: number
  }
  collisionFilters: {
    bodyFilterGroup: number
    bodyFilterMask: number
    targetFilterGroup: number
    targetFilterMask: number
  }
}

Some things I care about when processing a collision:

• Contact point, in world space
• Normal of contact point
• Identifying data of the colliding object (i.e. a unique ID which I can reference back to the other entity, or some sort of arbitrary userData I can assign and retrieve on contact to reason about the other object)

Are these things provided? Is ni related to the normal, for example? Is ri or rj the contact point?

[a-type]

For anyone else looking, I think I found the answer in cannon-es source:

https://github.com/pmndrs/cannon-es/blob/master/src/equations/ContactEquation.ts#L16

  ri: Vec3 // World-oriented vector that goes from the center of bi to the contact point.
  rj: Vec3 // World-oriented vector that starts in body j position and goes to the contact point.
  ni: Vec3 // Contact normal, pointing out of body I.

Will update once I understand which body is i and which is j in relation to the contact event.

Update: maybe I'm reading this wrong, but it... looks like there's no way to really tell which body is i vs j for any particular contact event. The contact itself would normally have bi and bj fields, and I suppose you could compare them to the current body vs the colliding body to tell what ri/rj/ni mean. But the worker event doesn't provide those identities, so for one of the bodies the ni will be a normal from the target body, and for the other the ni will be for the colliding body, and I'm not sure there's a way with the current API to tell which is which.

[jak2030]

Thanks for figuring this out, helps a lot! Curious if anyone can verify this to be the case -- I am also hoping to distinguish between different objects in order to determine different collision behaviors.