Update documentation of JointModel.shortname in python bindings

[Danfoa]

Following the discussion of issue #802, this PR presents an update to the documentation of the shortname function for the python class JointModel exposed by Boost.python.

This function is the only way to determine the joint type in python and was previously left undocumented.

The new function documentation describes the following possible cases:

            - JointModelR[*]: Revolute Joint, with rotation axis [*] ∈ [X,Y,Z]
            - JointModelRevoluteUnaligned: Revolute Joint, with rotation axis not aligned with X, Y, nor Z
            - JointModelRUB[*]: Unbounded revolute Joint (without position limits), with rotation axis [*] ∈ [X,Y,Z]
            - JointModelRevoluteUnboundedUnaligned: Unbounded revolute Joint, with rotation axis not aligned with X, Y, nor Z
            - JointModelP[*]: Prismatic Joint, with rotation axis [*] ∈ [X,Y,Z]
            - JointModelPlanar: Planar joint
            - JointModelPrismaticUnaligned: Prismatic joint, with translation axis not aligned with X, Y, nor Z
            - JointModelSphericalZYX: Spherical joint (3D rotation)
            - JointModelTranslation: Translation joint (3D translation)
            - JointModelFreeFlyer: Joint enabling 3D rotation and translations.

If there are any other possible outputs of JointModel.shortname please let me know to update the docstring.

Some questions emerge after understanding the shortname method: After a robot is loaded in Pinocchio, all of its joints are exposed to python as JointModel class instances. However, there are also specific joint-type classes exposed to python (e.g., JointModelFreeFlyer, JointModelRUBY) which led me to think that a more elegant and pythonic way of identifying the joint type is by use of isinstanceof method (e.g., isinstanceof(joint, JointModelRUBY)). This if, the robot joints were instantiated as specific joint type classes instead of the generic JointModel.

• Why are joints instantiated after robot load as JointModel and not as instances of the joint-type specific class?

[jcarpent]

Nice initiative @Danfoa.
Thanks a lot. It will help others for sure.

[jcarpent]

Some questions emerge after understanding the shortname method: After a robot is loaded in Pinocchio, all of its joints are exposed to python as JointModel class instances. However, there are also specific joint-type classes exposed to python (e.g., JointModelFreeFlyer, JointModelRUBY) which led me to think that a more elegant and pythonic way of identifying the joint type is by use of isinstanceof method (e.g., isinstanceof(joint, JointModelRUBY)). This if, the robot joints were instantiated as specific joint type classes instead of the generic JointModel.

*

This is what is done in Pinocchio 3x. SHould arrive soon.

[jcarpent]

Some questions emerge after understanding the shortname method: After a robot is loaded in Pinocchio, all of its joints are exposed to python as JointModel class instances. However, there are also specific joint-type classes exposed to python (e.g., JointModelFreeFlyer, JointModelRUBY) which led me to think that a more elegant and pythonic way of identifying the joint type is by use of isinstanceof method (e.g., isinstanceof(joint, JointModelRUBY)). This if, the robot joints were instantiated as specific joint type classes instead of the generic JointModel.

*

This is what is done in Pinocchio 3x. SHould arrive soon.

[thanhndv212]

Hello. I would like to ask a question here, because I encounter issues related to this PR when I try to retrieve the joint axis in python. I have used a robot with urdf model in which some revolute joints have joint axis defined as <axis xyz="0 0 -1"/> . Meanwhile it is quite clear that this revolute joint has joint axis along Z, but in pinocchio, it is classified as JointModelRevoluteUnaligned.
I have 2 questions:

• 1st, is there a way to retrieve the joint axis, even in the case of JointModelRevoluteUnaligned such as getting the values of <axis xyz=" ..."/> in python?
• 2nd, (more like suggestion): shouldn't the case of <axis xyz="..."/> that has value "-1 0 0", or "0 -1 0", or "0 0 -1" be also classified as JointModelR[*]?
  Thank you very much.

[jcarpent]

The feature to retrieve the joint axis is available in the preview of Pinocchio 3x, but not yet incorporated within Pinocchio 2x.

For the second point, we will add this feature soon to account properly for negative values within Pinocchio 3x.