Abnormal bounce effect with container physical body in Havok

Questions
1

see line 57 ~ 79

Expectations:

Current :

After setting the physical body of the ground to Container, the strength of the bounce doesn’t seem to change no matter how I change the restitution parameter.

And as long as the physical body type of the ground is not a container, everything works normally. Is this a bug or have I made a mistake somewhere?

2

cc @Cedric

3

Interesting, I see that the only difference between both PG’s is Line 65: BABYLON.PhysicsShapeType.BOX vs. BABYLON.PhysicsShapeType.CONTAINER

Have you tried creating a BABYLON.PhysicsShapeContainer like in the example: https://doc.babylonjs.com/features/featuresDeepDive/physics/compounds

4

Yes, I’ve tried, here’s an example.

I also tested modifying the restitutionCombine property (which determines how the restitutions of two PhysicsShapes are mixed together), with no luck. This is because I noticed that the default value of containerShape’s restitutionCombine is different from the others.

But to keep things simple, I’ve commented out the relevant section

Also, I logged the material of all the shapes in the scene to check if I’m missing something.

5

I tried to modify the restitution in the documentation example, and he seems to have the same problem.

see line 65

6

In the Babylon.js havokPlugin.ts source: setMaterial() function: (which sets restitution and friction):

this._hknp.HP_Shape_SetMaterial(shape._pluginData, hpMaterial);

the Result returned by HP_Shape_SetMaterial() is not checked. My hunch is that this function is silently failing for BABYLON.PhysicsShapeType.CONTAINER

I just tried implementing a container shape in my project where I check Result for every Havok function call. HP_Shape_SetMaterial() is succeeding for individual shapes, but failing for container shapes

Havok Container Fail
Havok Container Fail1066×360 10.6 KB

Let me look further into this tomorrow

1 Like
7

Indeed, it works in my project if HP_Shape_SetMaterial() is run for each child shape instead of the container shape

@eoin @Cedric Is it possible to expose a Havok function to return the children shape IDs (Array<HP_ShapeId>) of a container shape? Currently, we only have a function that returns the number of children:

HavokPhysics.d.ts

/** Get the number of children of the container. */
HP_Shape_GetNumChildren(container : HP_ShapeId): [Result, number];

If we have a function that returns the children shape IDs, it would be much easier to implement a fix for this in the Babylon.js source. Otherwise, we may have to keep track of a container and all of its children in havokPlugin.ts. Thank you for your help!

8

Hi,

I’m currently on the road, so can’t give a super detailed answer right now, but it’s expected that you can’t set a material on a compound shape - the compoudn doesn’t have any geometry of it’s own, so there’s nothing to set a material on; we decided that the alternative behaviour where a compound would set the materials of the child shapes could be confusing, since those shapes might be used in multiple different places, so we wanted to avoid implicit side-effects like this.

Totally agree that we should have an interface to access the child shapes of a compound. If you want to log a bug in GitHub - BabylonJS/havok: The Havok Physics plugin runtime files (wasm and js), I’ll look into it once I’m back in the office :slight_smile:

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9

You need to set the material on the child shapes; so if you change line 65 to read:

cubeShape.material = {restitution: 1};

it should have the effect you’re after.

10

I’ve tried setting the material for each childShape before, but it looks like there’s something wrong with the way I set it up.

  // ❌ The material will not be updated.
  childShape.material.restitution = 1

  // 👍 The material will be refreshed
  childShape.material = { restitution: 1 };

  // 👍 or Manually trigger the set function
  childShape.material.restitution = 1
  childShape.material = childShape.material

perhaps we should clarify in the documentation that ContainerShape is not affected by materials, and also provide more details about the small considerations when setting materials.

1 Like
11

Thank you so much, @eoin :slight_smile: I’ve submitted an enhancement request as you recommended

I really like your and the team’s approach of not using the alternative behaviour. It seems to give us more flexibility in that we can have a container with children each with different friction, restitution, and collision filter masks

12

Interesting find. It looks like this is because of the setter:

public set material(material: PhysicsMaterial) {
    this._physicsPlugin.setMaterial(this, material);
    this._material = material;
}
1 Like
13

There’s a similar issue (where the setter triggers an important side effect) in assigning shape to a physicsBody. The .shape setter recalculates mass properties whereas assigning .density directly to shape does not trigger the recalculation.

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