Non uniform mass distribution for RigidBody3D

[baracil]

Motivation

I want to develop a space simulation game (something like KSP) in which you could create rockets, spaceships by assembling different parts (engine, tank, cockpit). Each part has its own mass that might change over time (mostly for tanks).

In Godot, I use a RigidBody3D for the spaceship and for the parts a pair of (MeshInstance3D,CollisionShape3D) that you can choose from a list of parts.

As far as I can tell (I gather these information by browsing the source code) the CollisionShape3D is used to detect collisions (obviously) and to compute the mass_properties (i.e. the center of mass and the inertia tensor of the body).

The issue I face, is that the computation of the mass properties assumes that the mass is uniformly distributed over the CollisionShape3D in the RigidBody3D (the effect of a CollisionShape3D during the computation is based on its volume only).

I would like to be able to define a non uniformly distributed mass over the CollisionShape3D in a given RigidBody3D.

Implications

This implies a modification on how the mass of a RigidBody3D is defined and how the mass properties are computed.

1. Add a Mass Mode with 2 choices for the RigidBody3D

   1. Uniform (the default choice)
   2. Non Uniform

   In Uniform mode, we have the current behavior of Godot and the field Mass is visible and editable for the RigidBody3D in the editor. In Non Uniform mode the Mass is not editable (and "not visible" or "visible and filled automatically with the computed mass" ? )

2. On each collision shape, add a field Mass only visible and editable if the mass mode is Non Uniform.

3. On the core engine, the computation of the mass_properties must be updated in the case of the Non Uniform mode.

   1. The total mass must be computed
   2. The computation of center of mass and the inertia tensor must use the mass of the CollisionShape3D instead of its volume.

Although, the starting point is a specific need for my game, I feel like this a more general request to improve the 3D physic in Godot with not that much effort.

If this discussion is successful and requires development, I am ready to ensure it.

[theraot]

Be aware that you can already set the center_of_mass of the RigidBody3D (set center_of_mass_mode to CENTER_OF_MASS_MODE_CUSTOM), which is a way to represent a not uniform mass distribution. The center_of_mass is used for the torque component of off-center impulses, among other things. So, I believe that setting the center_of_mass would be sufficient for the feature you want.

[baracil]

Yes the center of mass can be set manually, but not the inertia tensor : only a fraction of the tensor can be set (i.e. the principal components which are relative to the local frame) so no asymmetric mass distribution can be represented with the custom values for the inertia tensor (well you could but that would imply a lot of side effects to deal with in your program).

[AThousandShips]

• Duplicate of: Expose the full inertia tensor to scripting #7033

Thank you for opening, consolidating there

[baracil]

Hi @AThousandShips, not sure what is the process here. Should I consider this feature will be implemented indirectly by with #7033, or should I comment the issue #7033 with my suggestions event though this is not exactly the same feature ?

[AThousandShips]

Add the details I'd say, what more would be required than inertia and the center of mass? The physics engine wouldn't simulate any more detailed mass properties than that