Are joints in systems like this typically allowed to twist in place? I would think that would be necessary for certain movements (a person rotating their hand around their wrist, for example), but it also could make sense if they do something else to achieve this.
All of the joints we consider in A4 are simple ball joints, but if we were to consider a constrained joint, would it be sufficient to just clamp its rotation into the legal range (relative to its parent)? Or are there other considerations we would need to take into account?
@Kuragama As far as I can tell, riggers do exactly that and clamp out certain axes/trajectories for rigs, i.e. elbows which should really only have one degree of freedom. But I don't know if there's other stuff to consider from a graphics perspective.
@silentQ You can of course design whatever kind of joints you like, and (as you point out) different joints may be suitable for different "rigs." @merc has it right: typically, there is a standard set (ball joint, axial joint, etc.) available in any given piece of software. But when developing your own software, the possibilities are limited only by you! :-)
@Kuragama Clamping would be fine for keyframe animation; for simulation or IK you may find that simply clamping can lead to poor physical or numerical behavior. Instead, you probably want to incorporate these constraints into your optimization problem, and use an optimization algorithm that correctly handles these constraints (i.e., finds a solution that satisfies the KKT conditions).