Question about computing constraint forces using XPBD integrator

[Shaoxiong Yao]

Hi,

I am using the XPBD integrator to simulate a tetrahedron FEM model. I am reading the original paper which mentioned the constraint forces can be computed https://matthias-research.github.io/pages/publications/XPBD.pdf. Equation (6) provides a way to compute elastic forces.

However, I realize these values are not saved in the body_f or particle_f attributes. Especially in the solve_tetrahedron function, these values are never computed. If possible, I wonder if there these values can be saved in the integrator. Or, if you may provide a recommended way to create these values. The current implementation seems to have a different notation compared to the original paper. I am looking forward to your reply.

[Miles Macklin]

The current implementation is based on this paper: https://mmacklin.com/neohookean.pdf

It is relatively straightforward to compute the forces given the pose of the tetrahedron, you first compute C, then multiply by inverse stiffness and gradient (as in the equation above).

Alternatively, you can look at the SemiImplicitIntegrator() which computes the forces from the tet pose directly using the same model. https://github.com/NVIDIA/warp/blob/main/warp/sim/integrator_euler.py#L608

Hope that helps.

Cheers,
Miles