How to Create a Non-Stretchable Rope with Havok

Questions
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1

Hello everyone,

I am currently using the Babylon.js game engine, and I understand that the recommended physics engine is Havok. However, I am having trouble creating a rope using Havok physics.

I came across a discussion on the official forum (Rope physics with Havok) which suggested using a 6DOF constraint to create a rope. The implementation can be seen in the following playground example: https://playground.babylonjs.com/#XTCZ07#22.

While this approach works to some extent, I am facing an issue where the rope appears to stretch significantly when a weight is applied to it. The rope behaves more like a spring, which is not the desired effect. Specifically, when I increase the weight of the attached ball, the rope stretches too much, losing its realistic appearance.

Could someone please guide me on how to create a rope that does not stretch, maintaining a more realistic behavior without the spring-like effect?

Thank you in advance for your help!

2

Does the same thing happen with distance constraints?

Edit: it might be the nature of the physics solver in Havok: Stretchy Distance Constraint - #4 by eoin

3

Hello @YongKang_Yang and welcome !

I would say : yes it is ! :slight_smile:

Havok is solving the physics iteratively, and by definition the constraint will never be “respected” because it’s basically waiting for it to be “out of law” (ex: distance constraint out of bound) to “solve” this by “slightly pushing” the joints toward the right place. Then it adds a damping effect so that things don’t go crazy.

In order to better understand you can have a look at my project with Custom wire physics using GreasedLines and have a look a the custom (no Havok) physics :

// Gravity
this.speed.y -= GRAVITY;
// Apply forces for all linked joints
for (const [joint, dist] of this.links) {
    const d = this.pos.subtract(joint.pos).length();
    if (d !== dist) {
        const dir = joint.pos.subtract(this.pos).normalize();
        const pos_ = this.pos.add(dir.scale(d-dist));
        this.speed.addInPlace(pos_.subtract(this.pos).scale(ELASTICITY));
    }
}
// Damping effect
this.speed.scaleInPlace(Math.max(0.5, 1.0-DAMPING*Math.pow(this.speed.length(), 0.2)));

It’s not exactly Havok, but the principle is very close :

  • Gravity
  • if (d !== dist) → act on joint if constraint is not OK
  • Damping

Now, about solving your issue :

  • Gravity acts the same on any mass (heavier object won’t fall faster)
  • Theorically only reason why you would need the ball to be very heavy, is because you want it “much heavier” than other objects (for interaction purpose)
  • Then, instead of having the ball crazy heavy, try to have the other objects lighter.

There is an equilibrium to be found, a bit like for render params where everything should remain into some bounds : a mesh should not be of size 0.0001 as well as should not be of size 10000.0, etc…

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