Exporting OBJ files : wrong normals

Hello, I don’t know why but the images cannot be displayed, even though I’ve used the proper tool and copied the links in my previous post…

So basically the normals displayed in SOFA are properly perpendicular to my planar surface, but they are not anymore after I’ve exported the mesh from SOFA and re-imported it either on Unity or on Blender.

However I’m getting on holidays, so I might be slower to answer too.

Thank you in advance for your help 🙂

Thank you @jnbrunet 🙂

Hi @younessss , thank you a lot for your help 🙂

I’m gonna try it as soon as possible.

Have a great day 🙂

Okie guys, we learn something every day, right ?
So today I’ve learnt that according to the model considered by SOFA (mechanical or visual, for instance), the normals aren’t the same at all !
You can compare by your own eyes the normals calculated from the FEM models (in red, green and blue) and the normals computed from the visual models (in white). The last ones are wrong while the others are good.

PS : I have an issue to send the image, so here’s the link

image

As an example, my scene :

<?xml version="1.0" ?>

<Node name="root" dt="0.1" gravity="0 0 0">
    <RequiredPlugin pluginName="SofaSparseSolver"/>
    <RequiredPlugin pluginName="SofaOpenglVisual"/>
    <RequiredPlugin pluginName="SofaMiscCollision"/>
    <RequiredPlugin pluginName='SofaBoundaryCondition'/>
    <RequiredPlugin pluginName='SofaGeneralDeformable'/>
    <RequiredPlugin pluginName='SofaImplicitOdeSolver'/>
    <RequiredPlugin pluginName='SofaLoader'/>
    <RequiredPlugin pluginName='SofaMiscFem'/>

	<VisualStyle displayFlags="showBehavior hideVisual hideCollision" />

	<EulerImplicit name="cg_odesolver" printLog="false" />
	<CGLinearSolver iterations="25" name="linear solver" tolerance="1.0e-9" threshold="1.0e-9" />
	
	<!-- define variables to reuse several times -->
	<MeshGmshLoader name="export_filename"  filename="C:/Users/jducr/Documents/Developpement/Sofa_ws/scenes/exports/membrane_7x7_quads"/> 
	
	<!-- The object to export -->
	<Node name="MyLittleObject">

		<MechanicalObject name="mo"/> 
		<UniformMass vertexMass="0.2" />
		<RegularGrid name="hexaGrid" nx="8" ny="8" nz="4" xmin="-10.0" xmax="10.0" ymin="-10.0" ymax="10.0" zmin="0" zmax="0.50"/>
		<HexahedronSetTopologyContainer name="Topo" src="@hexaGrid" />		
		<HexahedralFEMForceField name="FEM" youngModulus="1e13" poissonRatio="0.3" method="large" />
		
		<!-- Visual node to allow the exportation -->
		<Node name="VisualNode" >
			<OglModel name="Visual" color="green" />
			<IdentityMapping input="@.." output="@Visual" /> 
		</Node>
		
		<!-- <OBJExporter name="export_OBJ" filename="@../export_filename.filename"  exportAtBegin="true" exportEveryNumberOfSteps="0" /> -->
		<MeshExporter name="export_GMSH" filename="../exports/membrane_7x7_quads" position="@Visual.position" edges="@Visual.edges" triangles="@Visual.triangles" exportAtBegin="true" exportEveryNumberOfSteps="0" format="gmsh"/>
		
	</Node>
	

</Node>

Hello @Hugo, I

I’ll try to do one of those options, but the issue is probably more on the mathematical definition of the faces normals of a regularGridTopology than a code issue.
It’s more a thing like “do we want the faces normals to point inwards or outwards” ?
In my case, i would have liked them to be computed outwards, but perhaps some others would prefer them inwards so I don’t know if this is really an issue.

However, thanks to the awesome explanations of Damien I’ve understood that the vertices normals were poorly calculated, in my case, because some faces normals of my regularGridTopology were pointing inwards and others were pointing outwards.

Best wishes,

Julien

Hey @Hugo

Yes I think you’re right, you can close this topic.

Thank you very much for your help and advices,

Best whishes,

Julien