bullet-featherstone: Non-base links return zero velocities

[iche033]

Environment

• OS Version: Ubuntu 22.04
• Source or binary build?
  source build, gz-physics7 and main branches

Description

• Expected behavior: A moving non-base link should return correct linear and angular velocity values.
• Actual behavior: Zero velocities are returned

In the bullet-featherstone plugin, non-base links currently returns zero linear and angular linear velocities. The relevant code for generating velocities for non-base links are located here:

gz-physics/bullet-featherstone/src/KinematicsFeatures.cc

Lines 43 to 44 in 2bc19ad

	data.linearVelocity = convert(link.m_absFrameTotVelocity.getLinear());
	data.angularVelocity = convert(link.m_absFrameTotVelocity.getAngular());

A closer look at the bullet source code reveals that the btMultiBodyLink's m_absFrameTotVelocity variables are never filled unless btMultiBody::useGlobalVelcoties() is called and set to true