Problems while specifying root link

[llliuxiao]

My robot structure is: "etc -> torso_link2 -> torso_joint3 -> torso_link3 -> left_arm_base_joint -> left_arm_base_link -> left_arm_joint1 -> left_arm_link1 -> etc".

pk.build_serial_chain_from_urdf(urdf, end_link_name="left_arm_link1", root_link_name="torso_link3")

Now I want to calculate the relative transformation between left_arm_lin1 and torso_link3, but I was asked to provide the joint position of torso_joint3.

What is the correct usage for my requirement?

Thanks~