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getObjectOrientation of a joint
Posted: 28 Jul 2022, 06:11
by kercos
Hi,
according to the documentation, sim.getObjectOrientation applied to a joint "returns the orientation relative to the joint's moving frame". However, if I invoke this function on a joint before/after changing the joint position (angle) it returns the same result. So it seems the function returns the orientation relative to the joint's base frame by default.
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> sim.getJointPosition(79)
0.000155210495
> sim.getObjectOrientation(79, sim.handle_world)
{-4.565654308e-05, 0.0006102334009, -4.285298928e-05}
> sim.setJointPosition(79, 1.5)
> sim.getJointPosition(79)
1.5
> sim.getObjectOrientation(79, sim.handle_world)
{-4.565654308e-05, 0.0006102334009, -4.285298928e-05}
In addition, using sim.handleflag_reljointbaseframe doesn't alter the result, confirming the base frame is already used by default:
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> sim.getObjectOrientation(79+sim.handleflag_reljointbaseframe, sim.handle_world)
{-4.565654308e-05, 0.0006102334009, -4.285298928e-05}
Re: getObjectOrientation of a joint
Posted: 28 Jul 2022, 09:17
by coppelia
hello,
consider following 2 cases:
calling sim.getObjectOrientation (or sim.getObjectPosition and similar) relative to the parent's frame, i.e. sim.getObjectOrientation(objHandle,sim.handle_parent), will return the orientation of the item
relative to the parent's moving frame. In the first situation, the parent's moving frame is not existent (the shape doesn't have a moving frame). In the second situation, the parent's moving frame is the moving frame of joint2.
Above two cases can be discribed as:
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shape --> joint1FixedFrame --> joint1MovingFrame
joint2FixedFrame --> joint2MovingFrame --> shape
Getting the orientation/position of object1 relative to object2 will always get the orientation/position of the fixed frame of object1 relative to the moving frame of object2.
Cheers
Re: getObjectOrientation of a joint
Posted: 28 Jul 2022, 09:30
by fferri
The documentation says that when the
second argument (i.e.:
relativeToObjectHandle) is a joint, then you can combine the
first argument (i.e.:
objectHandle) with sim.handleflag_reljointbaseframe.
Everything works as expected for me.
Setup:
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> j=sim.createJoint(sim.joint_revolute_subtype,sim.jointmode_kinematic,0)
> d=sim.createDummy(0.1)
> sim.setObjectParent(d,j)
Test:
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> sim.getObjectOrientation(d+sim.handleflag_reljointbaseframe,j)
{0, 0, 0}
> sim.setJointPosition(j,0.3)
1
> sim.getObjectOrientation(d+sim.handleflag_reljointbaseframe,j)
{0, 0, 0.3000000119}
> sim.getObjectOrientation(d,j) -- relative to moving frame of j
{0, 0, 0}
Re: getObjectOrientation of a joint
Posted: 30 Jul 2022, 08:40
by kercos
Thank you very much @fferri and @coppelia for the clarification.
I misunderstood the documentation, as I thought that the handle in question was the one of the first argument and not the second one.
Would there be a simple way to obtain the orientation of a joint moving frame wrt the world (without dummy elements)?
Thank you!
Re: getObjectOrientation of a joint
Posted: 01 Aug 2022, 12:37
by coppelia
yes, simply do following for the orientation of the moving frame of a joint, relative to the world:
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sim.getObjectOrientation(jointHandle,sim.handle_world)
and following for the orientation of the fixed frame of a joint, relative to the world:
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sim.getObjectOrientation(jointHandle+sim.handleflag_reljointbaseframe,sim.handle_world)
Cheers
Re: getObjectOrientation of a joint
Posted: 01 Aug 2022, 12:51
by kercos
Now I'm very confused because this is exactly what I tried to do and documented in the very first post of this thread.
There I tried to show that after changing the joint position (and therefore the moving frame of the joint), the orientation wouldn't change (with or without the sim.handleflag_reljointbaseframe flag).
Re: getObjectOrientation of a joint
Posted: 03 Aug 2022, 12:16
by coppelia
Did you try
the suggestion of fferri? This works fine here too.
Cheers
Re: getObjectOrientation of a joint
Posted: 03 Aug 2022, 19:26
by kercos
I think @fferri suggestion describes how to get the orientation of a dummy node with respect to a joint base/moving frame. I'm looking for the way around, i.e., to get the orientation of the joint moving frame with respect to another object (or the world).
Re: getObjectOrientation of a joint
Posted: 08 Aug 2022, 07:52
by coppelia
Ah, ok, now I understand, please excuse the confusion. If you want to get the intrinsic position of an object, use sim.getObjectChildPose. This will return the transformation of the joint's moving frame relative to the joint's fixed frame. This also works for other objects, e.g. in a dynamic simulation, depending on the selected physics engine, other objects such as
force/torque sensors can see an internal error, which can be measured with that function.
You can also use sim.getJointMatrix, that would do the same.
So in order to get the matrix of the moving frame of a joint, relative to the world, you would do:
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local relMatrixOfMovingFrame=sim.getJointMatrix(jointHandle)
local absMatrixOfFixedFrame=sim.getObjectMatrix(jointHandle,sim.handle_world)
local absMatrixOfMovingFrame=sim.multiplyMatrices(absMatrixOfFixedFrame,relMatrixOfMovingFrame)
Cheers