GImpactCollisionShape causing strange bouncing behavior

Troubleshooting physics
1

Hi @sgold ,
I hope you can help me please. I am adding a GImpactCollision shape on this plant model to get some accurate collisions going, however I noticed that it keeps bouncing around.
I can confirm that the Restitution is set to 0.

Maybe this could help show the problem. Thanks.

1 Like
2

Hmm. I wonder where the center (of gravity) is on that plant model.
You can visualize centers (in Minie) using bulletAppState.setAxisLength(1f);.

I also wonder if the bottom of the plant’s collision shape is flat, and what types of shapes it is colliding with.

Also, if you’re not using Minie, you should be. And let me know which version.

3

Thanks for responding.
Firstly, I am using Minie 8.2.0 on my Linux machine.

The model I use has a flat bottom from what you can see here.

image
image893Ă—820 88.5 KB

However the scale of the model is very very tiny.
image

And then some screenshot of the debug axis.

image
image658Ă—631 111 KB

1 Like
4

That’s not a plausible location for the plant’s center of gravity.
The center of gravity should be about 10 cm (4 inches) higher.
CollisionShapeFactory puts the center of gravity at the model’s (local) origin.

5

Interesting, so what should one do when I want to have a physics model where the center of gravity is at the bottom?

1 Like
6

I have now changed the center of gravity on the plant, however it is still bouncing around like a MAD pot.

image
image630Ă—676 132 KB

1 Like
7

So now I have tried it with a very simple scene. Look at the donut bounce.

jiggle

2 Likes
8

Reminds me of Fallout 3… and sometimes Skyrim. :slight_smile:

3 Likes
9

I’m guessing the supporting bodies in these cases have MeshCollisionShape, so these are “mesh-versus-mesh” type collisions. I suspect these are problematic because mesh and GImpact shapes don’t always have well-defined insides and outsides. That makes it difficult to simulate contact forces. As I recall, mesh-versus-mesh is prone to snagging and instability.

I ran some experiments using Minie’s DropTest app, dropping GImpact shapes (such as #64 “teapotGi”) onto a MeshCollisionShape (such as #3 “candyDish”). I observed some “bouncing” (instability) but nowhere near as bad as in your video captures.

To me, this seems like yet another reason to avoid GImpact shapes. Please re-read the tutorial section on choosing collision shapes:

As it says, “GImpactCollisionShape should be your last resort!”

I see no reason a doughnut or a flowerpot can’t be adequately approximated using compound-of-convex shapes.

I realize that CollisionShapeFactory tends to steer people in the direction of GImpact. And I’m not trying to downplay or excuse GImpact’s failings, just hoping to steer you in a more fruitful direction.

1 Like
10

I did further experiments in DropTest, varying the “damping fraction” parameters of the rigid bodies:

  • Reducing the damping ratios from 0.6 to 0 made the snagging and instability much worse.
  • Increasing the damping to 0.9 seemed to help.

If you’re not ready to abandon GImpactCollisionShape, you might try tuning the damping (using the rigidBody.setDamping(float, float) method) in your game.

2 Likes
11
  1. You can use insert a parent com.jme3.scene.Node (into the visual model) with the required translation and then use CollisionShapeFactory.
  2. You can edit the original model (or create a new one) in Blender 3D or another modeling tool.
  3. You can transform the model’s meshes using MyMesh.translate() method in the Heart library.

[edit:]
4. If the shape is a compound, you can edit the child translations. If not, you can apply translation by making it a child in a compound shape.

1 Like
12

What do you mean here?

Should I use this code?

CompoundCollisionShape shape = new CompoundCollisionShape();                
                physicsRigidBody.setCollisionShape(shape);
                physicsRigidBody.setMass(mass);
                physicsRigidBody.setKinematic(kinematic);
1 Like
13

There are several approaches to approximating shapes, which are outlined in the tutorial.

For the flowerpot, I’d start by trying V-HACD, either the classic version:

            VHACDParameters parameters = new VHACDParameters();
            CompoundCollisionShape shape = CollisionShapeFactory.createVhacdShape(
                modelRoot, parameters, null);

or the newer Version 4:

        Vhacd4Parameters parameters = new Vhacd4Parameters();
        CompoundCollisionShape shape = CollisionShapeFactory.createVhacdShape(
                modelRoot, parameters, null);

If those default parameters don’t produce a satisfactory collision shape, I’d tune them using the VhacdTuner application.

For donuts, there’s a simple algorithm to approximate them using a compound of 20 capsules. It’s found here:

1 Like