Adding an ellipsoid joint to the joint collection

[Ipuch]

Thanks for taking the time to make and fill out this pull request! I'll be open to any enhancement. Thanks to @MegMll and @LucasJoseph for helping out on this one !

Description

In this PR, we introduce the new ellipsoid joint to pinocchio. This implementation is based on the paper:

Seth A, Sherman M, Eastman P, Delp S. Minimal formulation of joint motion for biomechanisms. Nonlinear Dyn. 2010 Oct 1;62(1):291-303. doi: 10.1007/s11071-010-9717-3. PMID: 21170173; PMCID: PMC3002261.

available here. It has been adapted to the formalism of Pinocchio. It allows defining an ellipsoid on which a solid can glide and rotate with respect to the "normal" of the ellipsoid. The joint is driven as function of angles XYZ, meaningful for biomechanists and thoraco-scapular joint.

Capture.video.du.2025-11-06.23-35-48.webm

Checklist

• I have run pre-commit run --all-files or pixi run lint
• I have performed a self-review of my own code
• I have commented my code where necessary
• I have made corresponding changes to the doxygen documentation
• I have added tests that prove my fix or feature works
• I have updated the CHANGELOG or added the "no changelog" label if it's a CI-related issue
• I have updated the README credits section
• We have run PINNOCHIO_EIGEN_NO_MALLOC

Extra To-dos

• no reverse mode with ellipsoid joint for model-graph
• Finite differences for motion subspace derivative Sdot, as we have some non-zero terms in Sdot, we (with @MegMll) feel like it would be a good idea to have a test to check Sdot with finite differences as for S.

We have tested that the ellipsoid joint behave as a ZYX spherical joint when radii are zeros. We have to test the translation part as well with a composite joint.

• Rotation part - The ellipsoid joint behaves as a ZYX spherical joint when radii are zeros.
• Translation part - Test the ellipsoid joint with a composite joint to check the translation part.
• Add doc on how to implement a joint in doc/e-development

Linting

• pre-commit run --all-files

Question

• Should we add references in the code for what we have implemented ? Doc added in `joint-ellipsoid.hxx

• To make it pass I had to modify second Dim and second MaxDim to 1 in this line :
  

  pinocchio/include/pinocchio/multibody/joint-motion-subspace-generic.hpp

  Line 52 in 468acc3

  typedef Eigen::Matrix<Scalar, Dim, Dim, Options, MaxDim, MaxDim> ReturnType;

• Model-graphs, need to make sure the build of the graph happens the expected way for ellipsoid joint -> we can't reverse the joint ellipsoid because the translations and the motion subspace are tight to the parent frame, and it would necessitate to have a new joint definition to get the transform in the child frame. To say it simply, we can't just modify q to express the spatial transform in the child frame.

[github-actions]

👋 Hi,
This is a reminder message to assign an extra build label to this Pull Request if needed.
By default, this PR will be build with minimal build options (URDF support and Python bindings)
The possible extra labels are:

• build_collision (build Pinocchio with coal support)
• build_casadi (build Pinocchio with CasADi support)
• build_autodiff (build Pinocchio with CppAD support)
• build_codegen (build Pinocchio with CppADCodeGen support)
• build_extra (build Pinocchio with extra algorithms)
• build_mpfr (build Pinocchio with Boost.Multiprecision support)
• build_sdf (build Pinocchio with SDF parser)
• build_accelerate (build Pinocchio with APPLE Accelerate framework support)
• build_all (build Pinocchio with ALL the options stated above)

Thanks.
The Pinocchio development team.

[jorisv]

You can specialize JointMotionSubspace to take advantage of S sparsity

[Ipuch]

We will leave it for now.