MATLAB: Stlwrite error: Tetrahedron triangulation is not supported.

alphashapeMATLABstltriangulation

Hello,
I am attempting to convert my alphashape object into an STL file by using stlwrite. This is the code I have so far, and an image of the alphaShape object:
            shp = alphaShape(app.Schunk.getVolCoordinates(), 100000);
            [T, P] = alphaTriangulation(shp);
            TR = triangulation(T, P)
            
            stlwrite(TR, 'STLfileFromApp.stl');
Screen Shot 2019-07-25 at 2.13.59 PM.png
I keep getting the error
"Error using stlwrite (line 33)
Tetrahedron triangulation is not supported."
and I quite simply dont know how to fix it. Any help would be hugely appreciated. Thanks.
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Best Answer

A 3D alphashape is a volume, therefore its triangulation is made of tetrahedrons not triangles. stlwrite only supports triangulations made of triangles, i.e. surfaces triangulations in 2D or 3D, not volumes triangulations. I don't know enough about the stl format to tell if it's a restriction of matlab or a restriction of the format itself. But, as it is now, it is impossible to write an alpha shape triangulation to an stl file with stlwrite. There may be some stlwiters on the FileExchange that may work (I haven't looked).
Alternatively, you may just want to write the surface of your alpha shape. You can get the triangles making that surface with boundaryFacets. Maybe that's what you're after, in which case:
 shp = alphaShape(app.Schunk.getVolCoordinates(), 100000);
 [bf, P] = boundaryFacets(shp);
 stlwrite(triangulation(bf, P), 'STLfileFromApp.stl');
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