Prerequisites for using FreeMotionAnimationLoop and LDLConstraintSolver for animation

[zbounik]

Hi.
What is the prerequisite of using FreeMotionAnimationLoop and LDLConstraintSolver?
When I use these two components, there is no animation. Below you can find a scene example:

<VisualStyle name="visualStyle1"  displayFlags="showVisual showBehaviorModels" />

<!--my change in visualStyle2-->
<VisualStyle name="visualStyle2"  displayFlags="showBehaviorModels" />
<DefaultVisualManagerLoop name="defaultVisualManagerLoop1" />
<FreeMotionAnimationLoop name="FreeMotionAnimationLoop" />
<DefaultPipeline name="CollisionPipeline" />
<MinProximityIntersection name="Proximity"  alarmDistance="0.5"  contactDistance="1" />
<DefaultContactManager name="Response"  response="default" />
<LCPConstraintSolver name="lCPConstraintSolver1"  build_lcp="0"  tolerance="0.0001"  maxIt="1000"  group="0" />
<BruteForceBroadPhase/>
<BVHNarrowPhase/>
<Node name="test">
<EulerImplicitSolver name="eulerImplicitSolver2" />
    <CGLinearSolver iterations="25" name="linear solver" tolerance="1.0e-9" threshold="1.0e-9" />
    <MeshObjLoader name="meshLoader_0"  filename="mesh/liver.obj" />
    <MechanicalObject src="@meshLoader_0" name="DOFs" scale="10"/>
    <MeshTopology src="@meshLoader_0" />
    <TriangleSetTopologyContainer src="@meshLoader_0" name="Container" />
    <TriangleSetTopologyModifier name="Modifier" />
    <TriangleSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" />
    <UniformMass vertexMass="1" />
    <FixedConstraint indices="38 39 40 41 42 43 123 124 137" />
    <TriangleFEMForceField name="FEM" youngModulus="5000" poissonRatio="0.3" method="large" />
    <TriangleCollisionModel />
    <Node name="VisuA">
    	<MeshOBJLoader name="meshLoader_000" filename="mesh/liver.obj" handleSeams="1" />
        <OglModel src="@meshLoader_000" template="Vec3d" name="Visual" position="@meshLoader_000.position"  edges="@meshLoader_000.edges"  triangles="@meshLoader_000.triangles"  quads="@meshLoader_000.quads"  material="Default Diffuse 1 0.2 1 0.2 0.5 Ambient 1 0.04 0.2 0.04 0.5 Specular 0 0.2 1 0.2 0.5 Emissive 0 0.2 1 0.2 0.5 Shininess 0 45 "  primitiveType="DEFAULT"  blendEquation="GL_FUNC_ADD"  sfactor="GL_SRC_ALPHA"  dfactor="GL_ONE_MINUS_SRC_ALPHA" />
        <IdentityMapping input="@DOFs" output="@Visual" />
    </Node>
</Node>

[hugtalbot]

Hey @zbounik

The FreeMotionAnimationLoop is used to be able to solve constraints (e.g. contacts, attachments formulated as constraints) using the method of the Lagrange Multipliers. The documentation of the method is here : https://www.sofa-framework.org/community/doc/simulation-principles/constraint/lagrange-constraint/

The FreeMotionAnimationLoop basically works in four steps:
1 - free motion : physics is solved without considering any collision / contact / constraint
2 - collision detection (using the CollisionPipeline)
3 - constraint resolution : the ConstraintSolver builds and solves using a GaussSeidel the constraint problem
4 - constraint correction : correct the free motion with the forces computed from the constraint resolution

The fact that using the FreeMotionAnimationLoop your object is not moving is characteristic from a missing "ConstraintCorrection" (see documentation).

[zbounik]

Hi @hugtalbot
Right now I Understood your point and come to say thanks for your notation. So thank you again for this message. Is it possible with SOFA to make a volumetric model from an Obj mesh? I found RegularGridTopology.scn similar to what I am trying to do, but it is a regular grid and not a 3D volumetric object.

[hugtalbot]

You could create a 3D mesh from your obj using the MeshTetraStuffing but better would be to use a real mesher
Else using only surface meshes in a grid to get an accurate physical simulation is a complex topic ongoing in the community : it's related to immersed boundary method (IBM)

[zbounik]

Great! Thank you very very much.

[hugtalbot]

you're very welcome @zbounik, glad to see you on this new SOFA forum!

[zbounik]

Thanks :)