[Gmsh] Boolean operations with stl files (OpenCASCADE - Gmsh )

[Christophe Geuzaine]

An STL is a just a mesh - so no, there are no boolean operations available.

CG

> On 4 Feb 2019, at 17:47, Octavio Castillo Reyes <ocastilloreyes at gmail.com> wrote:
> 
> Dear gmsh team, 
> 
> I am trying to create meshes with topography (including bathymetry and other complex embedding bodies). I have made several experiments:
> 
> * Boolean operations with meshes (not supported by gmsh)
> * Create flat surfaces and then interpolate them to a given topography (works well but is only valid for a physical material/group).  
> 
> The above led me to try the OpenCASCADE kernel. I am learning to use it and I have modified one of its examples:
> 
>          SetFactory("OpenCASCADE");
>          Mesh.CharacteristicLengthMin = 0.5;
>          Mesh.CharacteristicLengthMax = 0.2;
>          Box(1) = {0, -1, -1, 2, 2, 2};
>          Sphere(2) = {0, 0, 0, 1.};
>          BooleanDifference(3) = { Volume{1}; Delete; }{ Volume{2}; Delete; };
> 
> However, my input topography is given as a STL file. Therefore,I would like to know if it is possible to carry out operations such as the following:
> 
>          SetFactory("OpenCASCADE");
>          Merge "Box.stl" ;        // Volume 1
>          Merge "Sphere.stl";     // Volume 2
>          Mesh.CharacteristicLengthMin = 0.5;
>          Mesh.CharacteristicLengthMax = 0.2;
>          BooleanDifference(3) = { Volume{1}; Delete; }{ Volume{2}; Delete; };
> 
> Or something like:
> 
>         SetFactory("OpenCASCADE");
>          Merge "Volume_Topo.stl" ;         // Volume 1
>          Merge "Volume_No_Topo.stl";     // Volume 2
>          Mesh.CharacteristicLengthMin = 0.5;
>          Mesh.CharacteristicLengthMax = 0.2;
>          BooleanDifference(3) = { Volume{1}; Delete; }{ Volume{2}; Delete; }; 
> 
> Is it possible to import STL files and perform boolean operations? How to define a volume from Boolean operations between two STL files (previously created)? Actually, the gmsh error is the following: 
> 
> Unknown OpenCASCADE entity of dimension 3 with tag 1
> .... Could not apply boolean operator
> 
> I am very interested in knowing about the best strategy to perform this task. Unfortunately, until now it has not been clear to me how to solve my problem.
> 
> Thank you for your time and support
> 
> Best regards!
> 
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— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine