How to achieve Connecting Tissue in SOFA？

[drakelzh]

Hello everyone！I am currently trying to use SOFA to simulate cholecystectomy. I hope to achieve the ability to use instruments to pull tissues into thin filaments.
I have personally made some attempts so far. I divided a flat model into multiple sections, assigned line topology to each section, and mapped them onto a visual model. Then, I bound it to the two end organizations using AttachProjectiveConstraint (For some unknown reason, using bilateral constraints cannot achieve the correct binding effect). However, AttachProjectiveConstraint will affect the initial position of each line model, so they cannot be assembled into a complete plane. And doing so will bring serious performance issues. Currently, I have configured about 40 lines in the scene, and the frame rate has already dropped to about 20+FPS.
I hope experienced SOFA developers can give me some advice or other possible implementation ideas. Thank you very much!
Here I attach screenshots of the model I designed in 3Dmax and the simulation results in SOFA.

[hugtalbot]

Hi @drakelzh

Thank you for your post and this interesting question.
I am just unsure about what you mean by "a flat model into multiple sections, assigned line topology to each section, and mapped them onto a visual model."

Could you detail more please?
Could you share a complete scene repo (even privately if necessary) ?

Thanks in advance.

[drakelzh]

Thank you for your reply! Currently, I would like to try using SOFA to simulate the connecting tissue near the gallbladder and use instruments to hook and separate the connecting tissue. When hooked, I hope to simulate the feeling of fibrous tissue being pulled up and drawn. Therefore, I divided a complete plane into multiple sections and generated a corresponding line topology for each section after segmentation. I used BarycentricMapping to map each line topology to a corresponding section model. To this end, I completed the class BarycentricMapperEdgeSetTopology in SOFA BarycentricMapping as SOFA currently does not implement the function of mapping corresponding edges to planes.
The effect is shown in the following figure:

My current XML configuration code for each connecting tissue section is as follows, which includes some C++content that I have implemented myself:
`

	<StringMeshCreator name="loader" resolution="20" startPoint="" endPoint="" printlog="false"/>	
	<TransformEngine name="translate" input_position="@loader.position" translation="0 0 0"/>
	
	<EdgeSetTopologyContainer name="EdgeTopology" position="@translate.output_position" edges="@loader.edges" />
	<EdgeSetTopologyModifier name="EdgeModifier" />		
	<EdgeSetGeometryAlgorithms drawEdges="1" showIndicesScale="0.04" showEdgeIndices="0"/>	
	<MechanicalObject name="AdhensionLineMec" template="Vec3d" />
	
	<UniformVelocityDampingForceField template="Vec3" name="uniformVelocityDampingFF0" 
	implicit="true" dampingCoefficient="3.0"/>
						
	<Node name="extensionsNode" >
		<MechanicalObject template="Vec1" name="extensionsDOF" />
		<DistanceMapping name="distanceMapping" topology="@../EdgeTopology" input="@../AdhensionLineMec" output="@extensionsDOF" geometricStiffness="0" applyRestPosition="true" computeDistance="true"/>
		<RestShapeSpringsForceField name="RestShapeSpringsFF" template="Vec1" stiffness="3000" drawSpring="0" rayleighStiffness="0.1"/>
	</Node>
	
	<DiagonalMass massDensity="0.1" />
	<FixedProjectiveConstraint indices="19" />

	<UncoupledConstraintCorrection name="correction" defaultCompliance="60" />

	<Node name="VM">
		<MeshOBJLoader name="VisualObj" filename="" /> 
	
		<TriangleSetTopologyContainer name="VisualContainer" src="@VisualObj"/>		
		<TriangleSetTopologyModifier name="VisualModifier"/>
		<TriangleSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" drawEdges="0" drawTriangles="0" showIndicesScale="0.01" showTriangleIndices="0" showPointIndices="0"/>
		<MechanicalObject name="VisualMech" template="Vec3d" src="@VisualContainer"/>
		
		<BarycentricMapping name="BM" input="@../AdhensionLineMec" output="@VisualMech" printLog="false"/>

		<Node name="VisuChild">
			<OglModel template="Vec3" name="VisualModel" color="0.5736 0.2224 0.0792 1" material="Default Diffuse 1 0 0 0 0.5 Ambient 1 0.0000 0.0000 0.0000 1 Specular 1 0.5000 0.5000 0.5000 1 Emissive 1 0.0000 0.0000 0.0000 1 Shininess 1 26.5805" />		
			<IdentityMapping input="@../VisualMech" output="@VisualModel" />
		</Node>						
	</Node>
	
	<Node name="LineCM">	
		<EdgeSetTopologyContainer name="LineCollTopology" position="@../translate.output_position" edges="@../loader.edges" />
		<EdgeSetTopologyModifier name="LineCollModifier" />	
		<EdgeSetGeometryAlgorithms drawEdges="0" showIndicesScale="0.04" showEdgeIndices="0"/>	
		<MechanicalObject template="Vec3d" name="colDOF" src="@LineCollTopology"/>
		<LineCollisionModel  name="LineCollisionModel" contactStiffness="5000" simulated="1" moving="1" group="2" />
		<BarycentricMapping name="BMLineColl" input="@../AdhensionLineMec" output="@colDOF" printLog="false"/>
	</Node>
	
	<Node name="PointCM">			
		<PointSetTopologyContainer name="pointContainer" position="@../translate.output_position"  />
		<PointSetTopologyModifier name="pointModifier" />
		<MechanicalObject template="Vec3d" name="colDOF" src="@pointContainer"/>
		<PointCollisionModel name="adhesionP" topology="@pointContainer" contactStiffness="50" simulated="1" moving="1" group="2" displayFreePosition="1"/>
		<BarycentricMapping name="BMPointColl" input="@../AdhensionLineMec" output="@colDOF" printLog="false"/>
	</Node>
		
	<NearestPointROI template="Vec3d" name="np2" object1="@../Cholecyst/dofsB" object2="@AdhensionLineMec" radius="" drawPairs="1" printLog="0"/>
	<AttachProjectiveConstraint object1="@../Cholecyst/dofsB" object2="@AdhensionLineMec" twoWay="false" indices1="@np2.indices1" indices2="0"/>
				
	<AdhesiveBodyCuttingEdge2Mesh name="AdhesiveBodyCuttingEdge2Mesh" printlog="false" autoLoadFeedbackDevice="false" CollisionTopoContainer="@EdgeTopology" CollisionTopoModifier="@EdgeModifier" CollisionMech="@AdhensionLineMec"
	VisualTopoContainer="@VM/VisualContainer" VisualTopoModifier="@VM/VisualModifier" VisualMech="@VM/VisualMech" UncoupleCC="@correction" RestShapeSpringsFF="@extensionsNode/RestShapeSpringsFF" 
	BarycentricMapping="@VM/BM" DistanceMapping="@extensionsNode/distanceMapping" PointTopoModifier="@PointCM/pointModifier" 
	PointCollMech="@PointCM/colDOF" PointCollBarycentricMapping="@PointCM/BMPointColl" PointCollModel="@PointCM/adhesionP"
	LineCollContainer="@LineCM/LineCollTopology" LineCollModifier="@LineCM/LineCollModifier" LineCollBM="@LineCM/BMLineColl"/>

`

My current problem is:
① I am unable to effectively associate the connecting tissue with the gallbladder and nearby tissues, as using AttachProjectiveConstraint would project and pull points from the model onto another model, which would alter the shape of the model itself designed in 3Dmax. May I ask if there is a more suitable solution in SOFA?
② Once the number of connecting tissue sections in this implementation mode is too large, it will seriously affect the running performance and cannot achieve the effect I described above very well. But I don't know if there are any other implementation methods. If you have good ideas or suggestions, I look forward to them.
③ I also tried using a complete plane as the connecting tissue and wanted to use the SOFA Tearing plugin to separate the plane, but I still encountered the same situation as problem ① when binding the plane to the nearby tissues, and I don't know if there is a good way to perfectly bind them together.
④ How to achieve realistic connecting tissue effects in cholecystectomy in SOFA? If there are some helpful examples, they will be of great help to me.
This is my current confusion, thank you.

[hugtalbot]

Hi @drakelzh

In that case, why not simply use springs with a LineCollisionModel on it. It would mimic the behavior of thin connecting tissues, right?
Then, you would only need to manage the cut of these connecting tissues.

Such simulations have already been widely tackled by companies like InfinyTech3D (@epernod) or InSimo