# Problem using G4MultiUnion

Hi,
I’m trying to build a mesh to put it in front of a target. Looking at the forum I found a very useful instrument G4MultiUnion. I built an object done by circle and then I subtracted it from a cylinder, to obtain the mesh. In the following the code:

``````    auto meshTemplate
= new G4Tubs("meshTemplate",     // its name
0, meshRadius, meshThickness/2., 0.0*deg, 360.0*deg); // its size

G4MultiUnion* munion_solid = new G4MultiUnion("UnitedBoxes");

G4cout << "Number of holes " << nHole << G4endl;

for (G4int i = 0; i < nHole; i++) {
G4Tubs *holeTemplate
= new G4Tubs("holeTemplate",     // its name
0, holeRadius, meshThickness/2., 0.0*deg, 360.0*deg); // its size
G4RotationMatrix rot = G4RotationMatrix(0, 0, 0);
G4Transform3D tr = G4Transform3D(rot, pos);
munion_solid -> AddNode( *holeTemplate, tr );
}

munion_solid -> Voxelize();

G4SubtractionSolid *meshS = new G4SubtractionSolid("mesh", meshTemplate, munion_solid);//munion_solid);

auto meshLV
= new G4LogicalVolume(
meshS,        // its solid
meshMaterial, // its material
"mesh");        // its name

new G4PVPlacement(
0,                // no rotation
G4ThreeVector(0, 0, 0),  // at (0,0,0)
meshLV,          // its logical volume
"mesh",    // its name
worldLV,          // its mother  volume
false,            // no boolean operation
0,                // copy number
fCheckOverlaps);  // checking overlaps
``````

But when I run the simulation what I find is:

The circles, which constitute the mesh’s holes are in the right place, but now the base of the mesh that should be continuous it’s a cloud of points.

Do you have an idea of about it is happening?

Yes. As it says (you have to read the message), the polyhedron (the usual way of getting a drawing) has failed for some reason so it is drawing the solid as a cloud of points.

The solid itself may be perfectly OK for tracking - it’s probably just the visualisation that’s having trouble making something it can draw from such complicated solid.

Thank you for the reply, my concern was about if the effect is only on visualization or on tracking too.