Ghost Objects/Triggers

[austkast]

I'm wondering if there is anyway through physijs if it's possible to add Collision event listeners that only register that a collision has happened, and not apply an impulse? If not, any ideas on where I should look in the source to add that?

[chandlerprall]

At the moment there is not. I don't know if Ammo.js includes this functionality from Bullet Physics, but the starting point would be to locate it in Ammo.js.

[yousefamar]

This would indeed be quite a useful feature. Apparently in bullet physics the only thing you change are the collision flags: http://www.bulletphysics.org/mediawiki-1.5.8/index.php/Collision_Callbacks_and_Triggers#Trigger

I'm not quite sure how Ammo.js works but this might be the relevant part of the code:
https://github.com/kripken/ammo.js/blob/master/bullet/src/BulletCollision/CollisionDispatch/btCollisionObject.h#L388

[chandlerprall]

I haven't had good luck with the callbacks or collision flags with Ammo. It's somewhat probable I'm just not doing it right; unless someone else figures those out, ghost objects will be included starting in version 2

[yousefamar]

Until this gets implemented, here's the workaround I've been using in case anybody is interested. After you've created a new mesh (but before you add it to the scene) just manually set the collision flags:

myPhysijsMesh._physijs.collision_flags = 4;

where myPhysijsMesh is your mesh. 4 (0b100) is the bullet collision flag CF_NO_CONTACT_RESPONSE; ideally it would be some kind of constant in Physijs or something. Your mesh then acts as a trigger meaning this:

myPhysijsMesh.addEventListener('collision', function(other_object, relative_velocity, relative_rotation) {
});

would run just one time when the object enters (or re-enters) the space inside your mesh but travels through it like air. There might be a flag for continuous triggering while it's still inside.

The reason this works is because adding an object to your scene runs this in the web worker:

body = new Ammo.btRigidBody( rbInfo );

if ( typeof description.collision_flags !== 'undefined' ) {
    body.setCollisionFlags( description.collision_flags );
}

world.addRigidBody( body );

and _myPhysijsMesh.physijs ends up becoming description. I checked the ammo.js source code and sure enough there's also a getCollisionFlags which would allow the normal bit setting when you change the collision flags (i.e. setCollisionFlags(getCollisionFlags() | 4)).

I'm not sure how difficult it would be to add collision flag getters and setters to the Physijs.Mesh prototype (or even if they can be changed on the fly like that; not sure what happens to body once it's added to the world) or if it would make more sense to pass them as a parameter like mass. Either way if I figure it out before anybody else I'll be sure to request a pull or something.

[schteppe]

@chandlerprall Did you find any way of using triggers and callbacks yet? I've been experimenting a bit with the callbacks, I can make Ammo execute my callback, but then I can't get the participating collision objects :(

[chandlerprall]

@schteppe I never had any luck with Ammo's callbacks

[schteppe]

@chandlerprall :( Do you know if there is any other way of implementing working triggers/sensor objects then?

[chandlerprall]

I haven't looked into it for at least a year. It's very possible one of the recent Ammo builds enabled these.

[schteppe]

Ok, thanks! Will let you know if I succeed with this.

[chandlerprall]

@schteppe take a look at the code in this PR which adds a callback https://github.com/chandlerprall/Physijs/pull/150/files