Problems with exporting in STL format

Question

I have been searching for the answer pretty much everywhere. The forum holds some similar questions, but I cannot derive a solution though. The problem is the following.

When I create one sphere and then use Graphics3D, I can export it in STL format without any problems. When I create a sequence of spheres from a tool path that is loaded, I can correctly visualize it with Graphics3D, but when I try to convert it into STL format, I receive the error

Export::nodta: Graphics3D contains no data that can be exported to the STL format.

as already reported in other questions. What is wrong with my idea?

My code is the following:

path = Import["C:\\Program Files\\MATLAB\\R2012a\\bin\\pos.mat"]

{{{1., 1., 2.}, {2., 1., 2.}, {3., 1., 2.}, {4., 1., 
   2.}, {5., 1., 2.}, {6., 1., 2.}, {7., 1., 2.}, {8., 1., 2.}, {9., 
   1., 2.}, {10., 1., 2.}, {11., 1., 2.}, {12., 1., 2.}, {13., 1., 
   2.}, {14., 1., 2.}, {15., 1., 2.}, {16., 1., 2.}, {17., 1., 
   2.}, {18., 1., 2.}, {19., 1., 2.}, {20., 1., 2.}, {21., 1., 
   2.}, {22., 1., 2.}, {23., 1., 2.}, {24., 1., 2.}, {25., 1., 
   2.}, {26., 1., 2.}, {27., 1., 2.}, {28., 1., 2.}, {29., 1., 
   2.}, {30., 1., 2.}, {30., 1., 2.}, {30., 2., 2.}, {30., 3., 
   2.}, {30., 4., 2.}, {30., 5., 2.}, {30., 6., 2.}, {30., 7., 
   2.}, {30., 8., 2.}, {30., 9., 2.}, {30., 10., 2.}, {30., 11., 
   2.}, {30., 12., 2.}, {30., 13., 2.}, {30., 14., 2.}, {30., 15., 
   2.}, {30., 16., 2.}, {30., 17., 2.}, {30., 18., 2.}, {30., 19., 
   2.}, {30., 20., 2.}, {30., 21., 2.}, {30., 22., 2.}, {30., 23., 
   2.}, {30., 24., 2.}, {30., 25., 2.}, {30., 26., 2.}, {30., 27., 
   2.}, {30., 28., 2.}, {30., 29., 2.}, {30., 30., 2.}, {30., 30., 
   2.}, {29., 30., 2.}, {28., 30., 2.}, {27., 30., 2.}, {26., 30., 
   2.}, {25., 30., 2.}, {24., 30., 2.}, {23., 30., 2.}, {22., 30., 
   2.}, {21., 30., 2.}, {20., 30., 2.}, {19., 30., 2.}, {18., 30., 
   2.}, {17., 30., 2.}, {16., 30., 2.}, {15., 30., 2.}, {14., 30., 
   2.}, {13., 30., 2.}, {12., 30., 2.}, {11., 30., 2.}, {10., 30., 
   2.}, {9., 30., 2.}, {8., 30., 2.}, {7., 30., 2.}, {6., 30., 
   2.}, {5., 30., 2.}, {4., 30., 2.}, {3., 30., 2.}, {2., 30., 
   2.}, {1., 30., 2.}, {1., 30., 2.}, {1., 29., 2.}, {1., 28., 
   2.}, {1., 27., 2.}, {1., 26., 2.}, {1., 25., 2.}, {1., 24., 
   2.}, {1., 23., 2.}, {1., 22., 2.}, {1., 21., 2.}, {1., 20., 
   2.}, {1., 19., 2.}, {1., 18., 2.}, {1., 17., 2.}, {1., 16., 
   2.}, {1., 15., 2.}, {1., 14., 2.}, {1., 13., 2.}, {1., 12., 
   2.}, {1., 11., 2.}, {1., 10., 2.}, {1., 9., 2.}, {1., 8., 2.}, {1.,
    7., 2.}, {1., 6., 2.}, {1., 5., 2.}, {1., 4., 2.}, {1., 3., 
   2.}, {1., 2., 2.}, {1., 1., 2.}}}

a = Ball[path, 3];
b = Graphics3D[a]

volume = Export["contorno.stl", b]

Export::nodta: Graphics3D contains no data that can be exported to the STL format.
$Failed

Do you have some useful suggestions? I would be very grateful since this problem is an obstacle to my thesis work.

Answer 1

Exporting in STL appears to require BoundaryDiscretizeGraphics, which is not implemented for the composition of Graphics3D elements you are trying to produce. A workaround is to discretize each ball individually prior to combining them into one object and then exporting. This works for me:

o = BoundaryDiscretizeGraphics[Ball[#, 3], MaxCellMeasure -> 0.5] & /@ 
   path[[1]] // Show
Export["test.stl", o]

Adjust MaxCellMeasure to something useful for you, being mindful that the rendering time becomes quite significant.

Does it 3D print?

I don't know, because I haven't actually printed it. However, the object loads without error into my slicing program and it appears to generate a printable object.

Answer 2

this "almost" works but unfortunately has put some strange artifacts at the inside corner. I'll leave it, maybe playing with mesh refinement options can fix that.

Graphics3D[
 MeshPrimitives[
  BoundaryDiscretizeRegion[RegionUnion[Ball[#, 3] & /@ path[[1]]]], 2]]
Export["test.stl", %];
Import["test.stl"]

Answer 3

Thanks for the replies. I solved the problem throught two actions mainly: - i used sphere instead of ball - instead of giving "path" as location to the graphics3d, i gave directly the output of the path, that is the list of all positions. Now no more errors, i can export it in STL successfully. I need to have STL object of this geometry and other more complex geometries later on, to use a collision detection library. I won't Print them, it was just one of the easiest way to get a feasible format.