Question about the difference between "skel->getVelocities() to get root linear velocity" and "skel->getBodyNode(0)->getCOMLinearVelocity()""

[ColinZhou1993]

Hi, I build a skeleton(marked as "skel" below) with "Torso of FreeJoint" and 17 other Joints of "BallJoint", but I am confused by the difference in the definition of "root velocity", can someone help me figure that out:I can get the root velocity using "skel->getRootBodyNode()->getCOMLinearVelocity() ", but the value is different from the function"skel->getCOMLinearVelocity()" or the value extracted from the function "skel->getVelocities()" . So, can someone help me with that, which one is the correct "root velocity"? What are the differences among the three function :"skel->getCOMLinearVelocity()", "skel->getVelocities()" and "skel->getRootBodyNode()->getCOMLinearVelocity() ". Thank you very much!

[mxgrey]

The root velocity refers to the velocity of the BodyNode (a.k.a. rigid body or link) that represents the "root" of the skeleton's tree. A root is simply a BodyNode that does not have a parent BodyNode.

When you do skel->getRootBodyNode()->getCOMLinearVelocity() you are getting the linear velocity of the center of mass of just the root body node. This will ignore the velocities of any other bodies in the skeleton.

When you do skel->getCOMLinearVelocity() you are getting the linear velocity of the center of mass of the overall skeleton, taking all BodyNodes into account.

When you do skel->getVelocities() you are getting the generalized velocities of the skeleton. That's a vector of length N where N is the number of degrees of freedom in the skeleton. You can think of this as a vector of joint velocities.

[ColinZhou1993]

The root velocity refers to the velocity of the BodyNode (a.k.a. rigid body or link) that represents the "root" of the skeleton's tree. A root is simply a BodyNode that does not have a parent BodyNode.

When you do skel->getRootBodyNode()->getCOMLinearVelocity() you are getting the linear velocity of the center of mass of just the root body node. This will ignore the velocities of any other bodies in the skeleton.

When you do skel->getCOMLinearVelocity() you are getting the linear velocity of the center of mass of the overall skeleton, taking all BodyNodes into account.

When you do skel->getVelocities() you are getting the generalized velocities of the skeleton. That's a vector of length N where N is the number of degrees of freedom in the skeleton. You can think of this as a vector of joint velocities. @mxgrey Thank you for your answer. There are still some points I want to make clear: the function "skel->getVelocities()" gives out the joint velocities, and they are different from "velocities of the center of mass of the nodes", is it right? I set the position vector of the skeleton using the "root global positions(3D vector) and all the body node joint Axis-angle(3D)", when I get the vector skel->getVelocities(), the first three elements do not equal to skel->getRootBodyNode()->getCOMLinearVelocity(), for the reason of the difference between root joint velocities and the velocities of the center of mass of the root node, am I right? If I misunderstood, the value of "skel->getRootBodyNode()->getCOMLinearVelocity()" should be part of the vector from the function skel->getVelocities(). Can you explain more details about that? Thank you very much.

[mxgrey]

Generalized velocities (which are returned by getVelocities()) are the time derivatives of generalized coordinates. Generalized velocities will not necessarily be the same as linear and angular velocities that are used when describing the velocity of a single rigid body.

In particular, I believe DART uses Lie algebra representations for the generalized velocities of FreeJoint and BallJoint.