Valid combinations of ConstraintCorrection and LinearSolver

[ScheiklP]

Hi,
I just read through the SOFA documentation and was confused by the following.

If I am using the FreeMotionAnimationLoop, I need ConstraintCorrection for the Lagrange Constraints.
If I want to model a very deformable, cuttable object, I should not use the

• UncoupledConstraintCorrection (because the deformation is not uncoupled between nodes) or the
• PrecomputedConstraintCorrection (because of topology changes. And maybe also because of large deformations?)

This leaves the

• GenericConstraintCorrection and
• LinearSolverConstraintCorrection.

If I understand the documentation correctly, they do basically the same thing, but LinearSolverConstraintCorrection builds W for each object while GenericConstraintCorrection should only be added at the root node because it builds a global W.
Please correct me if I am already wrong here. :D

The documentation further indicates that they require a DIRECT linear solver to work.
But the snippet

node.addObject("EulerImplicitSolver")
node.addObject("CGLinearSolver")
node.addObject("EdgeSetTopologyContainer", position=[[x, 0, 0] for x in range(5)], edges=[[x, x + 1] for x in range(4)])
node.addObject("MechanicalObject", template="Rigid3d", position=[[x, 0, 0, 0, 0, 0, 1] for x in range(5)])
node.addObject("UniformMass", totalMass=1.0)
node.addObject("BeamFEMForceField", radius=0.1, youngModulus=5e6, poissonRatio=0.49)
node.addObject("LinearSolverConstraintCorrection")

works just fine, even though I am using an ITERATIVE linear solver.

If I replace the LinearSolverConstraintCorrection with a GenericConstraintCorrection, I get

[WARNING] [GenericConstraintCorrection(GenericConstraintCorrection)] Can not use the solver CGLinearSolver because it is templated on GraphScatteredType
[ERROR]   [GenericConstraintCorrection(GenericConstraintCorrection)] No LinearSolver found.

which I am also not quite sure how to interpret.

Cheers,
Paul

[alxbilger]

From what I can see, GenericConstraintCorrection calls applyConstraintForce on the solver, and it is empty for GraphScattered template.
Other functions are used in LinearSolverConstraintCorrection.

Even if a CG works with LinearSolverConstraintCorrection, I am not sure that the compliance matrix is built accurately due to the iterative nature of the solver.

[hugtalbot]

I guess the code snippet you gave is just a part of the scene (since there, there is not Lagrangian constraint).

works just fine, even though I am using an ITERATIVE linear solver.

Does it work even when constraints appear? I would expect a crash looking at the code : a pointer on a LinearSolver is saved internally in LinearSolverConstraintCorrection, but this might fail when calling addJMInvJt

If I replace the LinearSolverConstraintCorrection with a GenericConstraintCorrection, I get

indeed the GenericConstraintCorrection checks this correctly at init.

[ScheiklP]

Ah, I see. If there is an actual collision, I get a warning, but no crash:

[WARNING] [GraphScatterredType] set an element is not supported in MultiVector

And is the following correct?

If I understand the documentation correctly, they do basically the same thing, but LinearSolverConstraintCorrection builds W for each object while GenericConstraintCorrection should only be added at the root node because it builds a global W.

[hugtalbot]

yes at my best knowledge, this is my understanding (therefore what I wrote in the doc!)
I will take another close look in the code ..

[ScheiklP]

Perfect, thank you! :)

[hugtalbot]

do you have any more question in this topic? Else feel free to set it as answered

[hugtalbot]

I complete the discussion with the PR #3055 remark:

further to short mind refreshment, indeed, the LinearSolverConstraintCorrection (as the GenericConstraintCorrection) calls:

linearsolvers[i]->addJMInvJt(W, &J, factor);

which belongs to MatrixLinearSolver. In this function addJMInvJt, there is calls to the function element which is templated on the matrix type, e.g.:

linearSystem.systemRHVector->element(i)

AND ...
This function is not implemented for GraphScattered matrix type. Same for the set function by the way.