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Collision System

Overview

This project currently use a simple collision group / mask based collision reporting system. In its nature, each rigid object (or rigid actor) can be assigned a collision group (32-bit integer) and a collisionMask (also 32-bit integer). These two set of values together will decide what kind of collisions will be reported and what kind of collisions will be ignored.

Collision Group / Mask

Intuition

Intuitively, collision group can be think of the Id of an object while collision mask specifies a list of Id of objects each of which is allowed to be collided with this object. Here allow colliding simply means that such collisions will be reported. We cannot prevent any collision from happening in current implementation, instead, we just choose which pairs of objects in a collision that we care about.

Details

If you're familiar with bit operations then it's easy to come up with a simple scheme to implement this idea, in an efficient manner. The trick is that we can use binary bits to encode this concept of Id. More specifically, since we are using 32-bit integers, we say that the Id of this object is i iff the ith bit of the collision group is set to 1. All other bits in the collision group must be set to 0 to avoid confusions. Then we can define collision mask naturally. If this object is allowed to collide with object with Id j, then the jth bit of the collision mask must be set to 1. To be more specific, collisions involving object with Id i and j will be reported iff the jth bit of object i 's collision mask is set to 1, and, ith bit of object j 's collision mask is set to 1. I.e, they are mutually allowed to collide with each other. If we express the idea in code, we'll get the following:

if ((collisionMask1 & collisionGroup2) && (collisionMask2 & collisionGroup1)) {
    reportCollision();
}
Set up collision groups and masks in pyPhysX

To enable collision filtering for a rigid actor, we can call the following:

# set collision group to 1, and report collisions with object 0 and 2
actor.SetupCollisionFiltering(1 << 1, (1 << 0) | (1 << 2))

This is an interface call to RigidActor::SetupCollisionFiltering

Fetch collision pairs

After one step of simulation, we can fetch all collision pairs through the following: (python)

contactList = scene.GetAllContactPairs()
for contact in contactList:
    print(contact[0], contact[1])

This function will return a list of tuples of integers. Each tuple represents a contact pair (two integers). The two integers represent the collision groups of the two objects involving in a contact. Please note that if contact information is not retrieved after one scene stepping, it will be lost after the next scene step.

This is an interface call to Scene::GetAllContactPairs

Limitations

  • Since we are using a 32-bit integer for collision group, there are at most 32 possible collision groups. Namely from 1 << 0 to 1 << 31.
  • We only report collisions when it is first detected. Even if the colliding two objects remain touched with each other, no future reports will be issued.