What does act and actInv do?

[BolunDai0216]

I see a lot of code that has lines like

data.oMf[bl_id].actInv(data.oMf[id].act(l_sp__F))

and

data.oMi[joint.id].actInv(data.oMf[bl_id].act(fext__BL))

I was trying to find more information about actInv and act in the documentation, but all I found for act is ay = aXb.act(by) and for actInv is by = aXb.actInv(ay). I am not quite sure what those mean. Is it possible to help me better understand it. Is it trying to say $^ay =\ ^aX^b\texttt{.act(}^by\texttt{)}$ and $^by =\ ^aX^b\texttt{.actInv(}^ay\texttt{)}$ where $^ay = {^aX^b}{^by}$?

Thanks in advance!

[MegMll]

Hi,

I think you understood correctly what those functions do.

Using aXb.act() will transform coordinates of point p in frame B into coordinates of point p in frame A.
${}^ay = {}^aM_b{}^bp$

And aXb.actInv() will transform coordinates of point p in frame A into coordinates of point p in frame B.
${}^by = {}^bM_a{}^ap$

[fabinsch]

If you are not sure about the usage of a concept like act in your case, you find most of the time some explanations in the python docstring.

In [1]: import pinocchio as pin

In [2]: pin.SE3.act?
Docstring:
act( (SE3)self, (numpy.ndarray)point) -> numpy.ndarray :
    Returns a point which is the result of the entry point transforms by *this.

act( (SE3)self, (SE3)M) -> SE3 :
    Returns the result of *this * M.

act( (SE3)self, (Motion)motion) -> Motion :
    Returns the result action of *this onto a Motion.

act( (SE3)self, (Force)force) -> Force :
    Returns the result of *this onto a Force.

act( (SE3)self, (Inertia)inertia) -> Inertia :
    Returns the result of *this onto a Force.
Type:      function

I hope this can additionally help you.