A powerful CAD engine

Why?

GMSH’s engine is great but cannot manage boolean operations, which is a classical tool in CAD (Computer Aided Design). The open-source and powerful OpenCascade engine can be used in GMSH which provides numerous advantages, for example:

  • “Ready-to-use” geometries:
    • 2D : rectangle, disk, …
    • 3D : box, sphere, tor, cone, cylinder, …
  • Splines
  • Boolean operations: Union, Difference, Intersection, Fragmentation

The major drawback is that the index of created entities are managed by OpenCascade and hence are no longer known a priori, especially:

  • For an anisotropic mesh (different mesh size per region): Point with different mesh size must be sough a posteriori
  • During the design part, indices of the elementaty entities can be found via the GUI, leading to switching between code and GUI

It’s however highly recommended to use this CAD engine instead of the native one. The few drawbacks listed above can moreover be easily circumvent.

A short example

Numerous example using boolean operations can be found in the demos/boolean folder in the source code of GMSH.

An example of CAD object is provided on the dediacted wikipedia page:

Constructive solid Geometry (crédit: Wikipédia)

In GMSH and using OpenCascade, the file demos/boolean.geo reproduce it in a few lines only:

SetFactory("OpenCASCADE");
// from http://en.wikipedia.org/wiki/Constructive_solid_geometry
Mesh.Algorithm = 6;
Mesh.CharacteristicLengthMin = 0.4;
Mesh.CharacteristicLengthMax = 0.4;

R = DefineNumber[ 1.4 , Min 0.1, Max 2, Step 0.01,
  Name "Parameters/Box dimension" ];
Rs = DefineNumber[ R*.7 , Min 0.1, Max 2, Step 0.01,
  Name "Parameters/Cylinder radius" ];
Rt = DefineNumber[ R*1.25, Min 0.1, Max 2, Step 0.01,
  Name "Parameters/Sphere radius" ];

Box(1) = {-R,-R,-R, 2*R,2*R,2*R};
Sphere(2) = {0,0,0,Rt};

BooleanIntersection(3) = { Volume{1}; Delete; }{ Volume{2}; Delete; };

Cylinder(4) = {-2*R,0,0, 4*R,0,0, Rs};
Cylinder(5) = {0,-2*R,0, 0,4*R,0, Rs};
Cylinder(6) = {0,0,-2*R, 0,0,4*R, Rs};

BooleanUnion(7) = { Volume{4}; Delete; }{ Volume{5,6}; Delete; };
BooleanDifference(8) = { Volume{3}; Delete; }{ Volume{7}; Delete; };
Try it! Mesh in 2D and you should see the wanted geometry.