Using SparseGridTopology with a .msh file

[itsmebonny]

Hi everyone,

I'm trying to add a regular grid on top of an irregular shape and, from what I understood, SparseGridTopology could help me in doing that.

I tried to run the example from the repository without problems. I tried to do the same inside my simulation, but instead of loading a .obj file, I tried to load my .msh file, obtaining this error:

[SparseGridTopology(coarseGrid)] Loading mesh mesh/liver_389.msh failed.

If I load the dragon.obj mesh inside my simulation, it runs flawlessly, so I imagine the problem lies within the file generated by Gmsh.

My question is: is this behaviour intended, meaning that SparseGridTopology is built only for .obj files? I do not really know the differences between .obj and .msh files to be honest.

I can provide the code for the simulation, but I think that what I'm trying to do is simply not possible, so even just a confirmation would be great. In that case I'll try to build a workaround.

Thanks in advance for the help!

[bakpaul]

Hello,

This is because you are using the MeshOBJLoader and it only loads .obj files. To load .msh files you need to use the MeshGmshLoader.

Hope that worked for you !

[itsmebonny]

Hi,

thanks for the quick reply.

I already changed the loader with the correct one, I even tried to change it in the example given in the repository. breaking the example. Trying to load my mesh in that example, it didn't work. It just load the mesh once but doesn't create the three child nodes.

Here's my code

        filename_high = 'mesh/liver_3718.msh'

        # Here I load the mesh
        self.exactSolution = rootNode.addChild('HighResSolution2D', activated=True)
        self.exactSolution.addObject('MeshGmshLoader', name='grid', filename=filename_high)

        self.exactSolution.addObject('TriangleSetTopologyContainer', name='triangleTopo', src='@grid')
        self.MO1 = self.exactSolution.addObject('MechanicalObject', name='DOFs', template='Vec3d', src='@grid')
        self.exactSolution.addObject('StaticSolver', name='ODE', newton_iterations="20", printLog=True)
        self.exactSolution.addObject('CGLinearSolver', iterations=250, name="linear solver", tolerance="1.0e-6", threshold="1.0e-6") 
        self.exactSolution.addObject('TriangularFEMForceField', name="FEM", youngModulus=5000, poissonRatio=0.4, method="large")
        self.exactSolution.addObject('BoxROI', name='ROI', box="-0.1 -0.1 -0.1 2.1 0.1 0.1")
        self.exactSolution.addObject('FixedConstraint', indices='@ROI.indices')
        self.exactSolution.addObject('BoxROI', name='ROI2', box="-8.1 5.9 -0.1 -6.9 8.1 0.1")

        self.cff = self.exactSolution.addObject('ConstantForceField', indices="@ROI2.indices", totalForce=self.externalForce, showArrowSize=0.1, showColor="0.2 0.2 0.8 1")

        # Here I create the SparseGridTopology
        self.coarse = self.exactSolution.addChild('CoarseMesh')
        self.coarse.addObject('SparseGridTopology', n="10 10 1", fileTopology=filename_low, name='coarseGrid')

        self.coarse.addObject('TriangleSetTopologyContainer', name='triangleTopo', src='@coarseGrid')
        self.MO1_LR = self.coarse.addObject('MechanicalObject', name='coarseDOFs', template='Vec3d', src='@coarseGrid')
        self.coarse.addObject('SphereCollisionModel', radius=sphereRadius, group=1, color='0 1 0')
        self.coarse.addObject('BarycentricMapping', name="mapping", input='@../DOFs', input_topology='@../triangleTopo', output='@coarseDOFs', output_topology='@triangleTopo')

In this example, if I change the filename_high variable to be the .obj mesh and the loader with the MeshOBJLoader it works without problem. Using it in this way instead is giving me the error I wrote in the first post.

I suspect I may be doing something incorrectly, maybe I need to add a MeshGmshLoader to the child node where the SparseGridTopology is created?

[hugtalbot]

Hi @itsmebonny

In the SparseGridTopology, instead of using the data field fileTopology you may try to fill directly the input fields : position (coming from another mesh, MeshLoader) could you try this and keep us posted?

[itsmebonny]

Hi @hugtalbot,

Thanks for the tip, now it is working exactly as I needed.

The only problem is that it is not working on a 2D mesh, but once I gave as input a 3D mesh created in Gmsh it worked.

For future reference here is the line that must be changed in the example I used before

self.coarse.addObject('SparseGridTopology', n="10 10 1", position='@grid.position', name='coarseGrid')

It would be great to find a way to have it work on 2D mesh, but for the moment it solves my problem.

Thanks again!

[hugtalbot]

Indeed, the SparseGridTopology does not seem to work in 2D cases (it's actually only used in 3D cases usually)
We could investigate this 👍 thanks for the report