feat!: Body::TriMesh implementation

[Dmajster]

Opening this as a draft for #41 and as a place to ask for help with potential issues regarding it.

In it's current state i'm faced with the following errors.

thread '<unnamed>' panicked at 'index out of bounds: the len is 128 but the index is 128', C:\Users\domen\.cargo\registry\src\github.com-1ecc6299db9ec823\rapier3d-0.5.0\src\dynamics\solver\interaction_groups.rs:89:21

I noticed that this feature is under the 'parallel' feature and tried to disable it. Once I did that I got the following error.

thread 'main' panicked at 'assertion failed: proxy.aabb.mins[dim] <= self.max_bound', C:\Users\domen\.cargo\registry\src\github.com-1ecc6299db9ec823\rapier3d-0.5.0\src\geometry\broad_phase_multi_sap.rs:171:13

As of now all the code required to produce the Vec and Vec<[u32; 3]> pair from Asset lives in my project https://github.com/Dmajster/radwars. So if you wish to reproduce this errors or help out I think this would be the fastest path.

I also think this would be a great feature to move this into Heron down the road.

[jcornaz]

Thanks @Dmajster for adding the TriMesh, that will be a great addition.

I noticed that this feature is under the 'parallel' feature and tried to disable it. Once I did that I got the following error.

thread 'main' panicked at 'assertion failed: proxy.aabb.mins[dim] <= self.max_bound', C:\Users\domen\.cargo\registry\src\github.com-1ecc6299db9ec823\rapier3d-0.5.0\src\geometry\broad_phase_multi_sap.rs:171:13

Is your trimesh convex? AFAIK, Rapier (like many other physics engine) do not support concave shapes.

[jcornaz]

Note, you may prefer work against the main branch rather than next-bevy. The next-bevy branch can be broken at anytime by stuff commited in the bevy repository or in bevy_prototype_lyon. And, you may find unpleasant to have to continuously catch up the state of this two repos, instead of being able to focus on the implementation.

But I say that mostly for your confort. If you prefer to work agains the next-bevy branch, it is fine for me ;-)

[Dmajster]

Thanks for reminding me about the convex limitations. What is weird about this is that when I called the same trimesh builder in bevy_rapier directly it didn't crash and from the little I tested it seemed to have accurate physics meshes. I have now played around with the physics mesh generation crash and reduced it to this:

it is just 2 cubes where the upper one slightly intersects with the bottom one. I tried using convex and concave objects and that didn't seem to affect the outcome, nor did the transform of the object. It seems that the intersections are what cause the crash

As to why I chose to work against next-bevy is because my main use case was to load a .gltf file that represented all the static geometry in my game and create mesh colliders for that: Bevy only supports loading whole GLTF assets like that using their new path#Scene0 syntax for gltf loaders.

[jcornaz]

Thanks for your investiguations @Dmajster 👍

it is just 2 cubes where the upper one slightly intersects with the bottom one.

In a single trimesh? Or two trimeshes?

If it is a single trimesh, I think it is simply a case not supported by rapier. And I would personally not expect this case to be supported.

If it is two trimeshes, that would be a bug of rapier wouldn't it? In this case, I think it should be reported in the rapier repo.

[Dmajster]

In a single trimesh? Or two trimeshes?

It is two trimeshes.

I will go and file an issue with rapier for this.

[Dmajster]

[jcornaz]

Ah ok, that makes sense.

I wanted to introduce #45 anyway. I'll try to push it (maybe tonight if I have time). It should be fairly easy to implement.

As part of #45, I'll define a "default" mass, so that every body (except sensors) always have a mass. I think a wrong mass is better than none.

If I understood well, that should basically solve your problem.

[jcornaz]

Finally I don't think #45 will help much. I think I mostly want do deal with densities rather than absolute mass and angular inerta. That should provide more natural defaults and should be quite natural to use in general. (the mass & angular inertia would automatically be proportional to the volume of the shape).

But density won't work out of the box for TriMesh, because it is hard to compute its the volume and unit angular inertia.

I propose to explicitly set the body MassProperties in case of a TriMesh, and compute it from the axis-aligned bounding box. It won't be perfect, but is better than a panic.

[jcornaz]

Also, we should add a center_of_mass: Vec3 field to the Body::TriMesh variant. It is certain that any trimesh will need to have the center of mass to be user-defined. Because, AFAIK, there is no easy way to compute a good center of mass automaticaly.

EDIT: Forget it, I'll make possible to define the center of mass as part of #45

EDIT2: Finally #45 doesn't make possible to define center of mass, but only allow to set density

[Dmajster]

I will close this pull request as I have opened a new one that works with the main branch.