ArticulatedSystemMapping: Breaking changes or misconception?

[Scheikl]

Dear Sofa Community,
in v20.06 I used the ArticulatedSystemMapping to build robot kinematics.
I heavily leaned on the Tripod tutorial from the SoftRobots plugin to understand how to build nice rotational joints with the SofaPython3 plugin.

Last week I tried to updated my code base to commit fdb0dcaad4a4cea3214d43a589db7f9f206bb58b. The code that ran smoothly before now throws an error during the creation of the ArticulatedSystemMapping.

As a “minimal” reproducible example I implemented the Documentation example in Python:

base_frame = rootNode.addChild("base")
base_frame.addObject("MechanicalObject", name="dofs", template="Rigid3d", position=[0, 0, 0, 0, 0, 0, 1])

articulation = base_frame.addChild("articulation")
articulation.addObject("MechanicalObject", name="Articulations", template="Vec1d", position=[0, 0, 0, 0])

mechanics = articulation.addChild("Mechanics")
mechanics.addObject("MechanicalObject", template="Rigid3d", name="dofs", position=[                                                                                      
    [0, 0, 0, 0, 0, 0, 1],                                                                                       
    [1, 0, 0, 0, 0, 0, 1],                                                                                        
    [3, 0, 0, 0, 0, 0, 1],                                                                                      
    [5, 0, 0, 0, 0, 0, 1],                                                                                      
    [7, 0, 0, 0, 0, 0, 1]                                                                                      ])
mechanics.addObject("BeamFEMForceField", name="FEM", radius=0.1, youngModulus=1e8, poissonRatio=0.45)
mechanics.addObject("MeshTopology", name="lines", lines="0 1 1 2 2 3 3 4 ")
mechanics.addObject("UniformMass", template="Rigid3d", name="mass", vertexMass="0.1 0.1 [1 0 0,0 1 0,0 0 1]")
mechanics.addObject("FixedConstraint", template="Rigid3d", name="fixOrigin", indices=0)
mechanics.addObject("ArticulatedSystemMapping", 
    input1=articulation.Articulations.getLinkPath(), 
    input2=base_frame.dofs.getLinkPath(), output=mechanics.DOFs.getLinkPath())

articulation.addObject("ArticulatedHierarchyContainer")

centers = articulation.addChild("articulationCenters")

center1 = centers.addChild("center1")
center1.addObject("ArticulationCenter", parentIndex=0, childIndex=1, posOnParent=[0, 0, 0], posOnChild=[-1, 0, 0])
rotation1 = center1.addChild("articulations")
rotation1.addObject("Articulation", translation=False, rotation=True, rotationAxis=[0, 0, 1], articulationIndex=0)

center2 = centers.addChild("center2")
center2.addObject("ArticulationCenter", parentIndex=1, childIndex=2, posOnParent=[1, 0, 0], posOnChild=[-1, 0, 0], articulationProcess=2)
rotation2 = center2.addChild("articulations")
rotation2.addObject("Articulation", translation=False, rotation=True, rotationAxis=[0, 0, 1], articulationIndex=1)

center3 = centers.addChild("center3")
center3.addObject("ArticulationCenter", parentIndex=2, childIndex=3, posOnParent=[1, 0, 0], posOnChild=[-1, 0, 0], articulationProcess=0)
rotation3 = center3.addChild("articulations")
rotation3.addObject("Articulation", translation=False, rotation=True, rotationAxis=[0, 0, 1], articulationIndex=2)

center4 = centers.addChild("center4")
center4.addObject("ArticulationCenter", parentIndex=3, childIndex=4, posOnParent=[1, 0, 0], posOnChild=[-1, 0, 0], articulationProcess=1)
rotation4 = center4.addChild("articulations")
rotation4.addObject("Articulation", translation=False, rotation=True, rotationAxis=[0, 0, 1], articulationIndex=3)

rootNode.init()

On creation, this results in the following error:
[ERROR] [ArticulatedSystemMapping(ArticulatedSystemMapping)] ArticulationCenters 'ArticulatedHierarchyContainer' size: 8 is bigger than the size of input model 'Articulations' position vector: 4

The same code runs fine in v20.06.

From what I understand, the 4 is perfectly fine, as we actually want to implement 4 rotational joints. The 8, however, is a bit cryptic to me. With only one center and one value in Articulations the error is still there, but with size: 2 and vector: 1 respectively.

My understanding was, that for n serial joints, I need n+1 positions in mechanics.dofs.

So my questions are:
– Do I have a fundamental misconception about the topic?
– What changed between v20.06 and the soon to be v20.12?
– Is there a way to fix that? I kind of hacked my way around it by dirtily duplicating the entries in Articulations and only using every second entry.

Cheers,
Paul

[Hugo]

Hi @scheikl

Did you already solve this topic?
This comes from a recent PR in master: #1448

@epernod do you have any idea why the default scene now returns this error?

Best

Hugo

[Scheikl]

Hey @hugo,
no, the topic is still unsolved. 🙁

If anyone has an idea how to solve/avoid this issue, I would be happy to try it out and report on the results.

Cheers,
Paul

[Author]

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