How to create a distance joint constraint?

[Hugo_D]

Hi there,

I’m having trouble trying to create a distance joint between two entities.

Basically I want EntityA (with a dynamic rigid body) and EntityB (with a static rigid body) to always be the same distance from each other.

My script:

var Hooked = pc.createScript('hooked');

Hooked.attributes.add("hook", { type: "entity" });

// initialize code called once per entity
Hooked.prototype.initialize = function() {
    this.on('attr:hook', function (value, prev) {
        this.onHookChange(value, prev);
    });
    if(this.hook) this.onHookChange(this.hook, null);
};

var center = new Ammo.btVector3( 0, 0, 0 );

Hooked.prototype.onHookChange = function(value, prev) {
    if(this.joint) {
        console.log("destroy join");
    }
    if(value) {
        console.log("create joint");
        this.joint = new Ammo.btPoint2PointConstraint( this.entity.rigidbody.body , this.hook.rigidbody.body, center, center);
        console.log("joint:", this.joint);
        this.app.systems.rigidbody.dynamicsWorld.addConstraint(this.joint, false);
    }
};

// update code called every frame
Hooked.prototype.update = function(dt) {
    if (this.joint) {
        // render a line between the 2 entities
    }
};

Currently this.entity (EntityA) is attracted by this.hook (EntityB) until they collide: how can I add a fixed distance? I tried playing with pivots but couldn’t make it work.

Thank you!

[yaustar]

When you create joint, IIRC there is an offset that can be applied of where pivot point of the joint is.

[Hugo_D]

Hey, I’m not sure what you mean. Ammo.btPoint2PointConstraint accepts at most 4 arguments, no length or offset.

By the way, I couldn’t find Ammo documentation, does PlayCanvas exposes it somewhere?

[will]

I recommend reading this thread:

Trying to build a robot arm grabber using Ammo.js

So I am working on a project where the user can pick up objects using a grabber of sorts. I want the grabber to be able to pinch an object, such as a cube, and carry it around. Here is my project: https://playcanvas.com/editor/scene/757743 You can hold ‘A’ to grip and ‘Q’ to release. The cube can be moved with forward and backward arrow keys. I have two problems I need to solve; I want the gripper to be able to move around and not just be stationary, it does not seem to respond to forces…

[yaustar]

Ammo is a complied version of Bullet so I had to look up documentation for Bullet to apply Ammo.

You can see the functions/API/classes that are exposed for Ammo here: https://github.com/playcanvas/ammo.js/blob/master/ammo.idl

There’s also Will’s ragdoll project here which can help: https://playcanvas.com/editor/scene/468930

[will]

And be sure to read this page in the User Guide:

https://developer.playcanvas.com/en/user-manual/physics/calling-ammo/

[Hugo_D]

Thank you for your answers.

I’ve already read the thread about the robot arm but it seemed very complicated for a simple distance joint.

If I understand correctly, in order to achieve what I want, I will need 3rd body which will have 2 point2pointConstraints : one with the dynamic body and one with the static body.

[yaustar]

If entity A needs to be freely rotating around its pivot point then yes, it does sound like you need 2 joints.

[Hugo_D]

I have modified my script and it’s now working as intended.

Here’s the code:

var Hooked = pc.createScript('hooked');

Hooked.attributes.add("hook", { type: "entity" });

// initialize code called once per entity
Hooked.prototype.initialize = function() {
    this.on('attr:hook', function (value, prev) {
        this.onHookChange(value, prev);
    });
    if(this.hook) this.onHookChange(this.hook, null);
};

var center = new Ammo.btVector3( 0, 0, 0 );

Hooked.prototype.onHookChange = function(value, prev) {
    if(this.rope) {
        console.log("destroy join");
    }
    if(value) {
        console.log("create joint");
        
        // create rope body
        this.rope = new pc.Entity("Rope");
        this.rope.addComponent("rigidbody", {
            type: "dynamic",
        });
        this.rope.addComponent("collision", {
            type:"box",
        });
        
        // add rope to world
        this.app.systems.rigidbody.dynamicsWorld.addRigidBody(this.rope.rigidbody.body);
        
        var pos = this.entity.getPosition();
        var to = this.hook.getPosition();
        var dist = new Ammo.btVector3( pos.x - to.x, pos.y - to.y, pos.z - to.z );
        // create joint between entity and rope
        this.joint1 = new Ammo.btPoint2PointConstraint( this.entity.rigidbody.body , this.rope.rigidbody.body, center, dist);
        this.app.systems.rigidbody.dynamicsWorld.addConstraint(this.joint1, true);
        
        // create joint between hook and rope
        this.joint2 = new Ammo.btPoint2PointConstraint( this.hook.rigidbody.body , this.rope.rigidbody.body, center, center);
        this.app.systems.rigidbody.dynamicsWorld.addConstraint(this.joint2, true);
    }
};

// update code called every frame
Hooked.prototype.update = function(dt) {
    if (this.joint) {
        // this.app.renderLine(this.entity.getPosition(), this.hook.getPosition(), [1,0,0]);
    }
};

Thanks again for your replies

[will]

I don’t get why you needed to edit the script. Did you look at this project:

https://playcanvas.com/project/618829/overview/physics-constraints

In particular, this part of the scene:

That’s essentially a rope attached to a fixed point. You could keep chaining point to point constraints from the bottom to add more articulations.

[Hugo_D]

Hi,

I needed to create a script because I need to create the constraint programmatically, when user clicks for example.

I’m very new to playcanvas so maybe I’m doing it wrong.

[will]

You can create the structures in my project programmatically too. You should be able to use my script without making any changes.

[Hugo_D]

Hi again,

I forked your project to try your physics scripts.

Please take a look at this project:
https://playcanvas.com/project/630736/overview/phys-fork

I have set up 2 entities : “working2d” and “broken2d”.
The only difference is the Ball’s collision sphere radius (Ball1 radius=0.5, Ball2 radius=0.14).

I don’t get why “broken2” behaves like this ?

Thank you for your time