Hello, I have been looking into crb and cinert variable in the mujoco model. The documentation shows that crb is returning the mass and composite rigid body inertia. According to CRBA the inertia matrix is 6x6 but the crb returns a vector is of length 10 for each subtree. How does they represent crb inertia? Also is cinert just returning inertia of each body in CoM frame? or Am I missing something here?
crb and cinert are intermediate results that were not meant to be documented and exposed to the user. they express the mass and inertia of a single (cinert) or composite (crb) rigid body relative to the com of the subtree to which the body belongs. the 6x6 matrix you are referring to has special structure where only 9 numbers can vary, and MuJoCo represents it in compressed form. the 10th number is the mass.
I would like to know how MuJoco calculates inertia of a single rigid body w.r.t. CoM. I set the diagonal inertia in the xml but i can't get the diagonalized inertia of the body at CoM from reading data->cinert. Thank you.