MATLAB: How could I create stl file based on the ellipsoid command

alphashapeellipsoidstl

I try several methods to write an stl file based on the ellipsoid geometry.

But I haven't find way to successfully do it.

Following is my code

clear 
clc
close all
[x, y, z] = ellipsoid(0,0,0,5.9,3.25,3.25,30);
s=surf(x,y,z)
p=surf2patch(s)
%Extract faces and vertices from structure p
pf=p.faces
pv=p.vertices
tr=triangulation(pf,pv) 
stlwrite(tr,'ell.stl')

The message ''Tetrahedron triangulation is not supported.'' appears, I know I could use ''boundaryFacets'' command to solve this problem, but I don't know how to create an alphaShape of ellipsoid.

Could anyone gives me a hint?

Best Answer

stlwrite only support triangles.

You can get the alphaShape of ellipsoid using the alphaShape function.

shp = alphaShape(tr.Points)

There is also the freeBoundary function.

[F,P] = freeBoundary(tr)
trNew = triangulation(F,P)
stlwrite(trNew, "freeboundary.stl")

Related Solutions

MATLAB: Stlwrite error: Tetrahedron triangulation is not supported.

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');

MATLAB: How to convert a triangulation object into volume and create a BITMAP image

Below code is a good example for creating a STL file for 3D geometry, read the STL as traingulation object, save it as bitmap image and calculate the volume:

Script to create a STL file for 3D geometry. eg: cylinder

[X,Y,Z] = cylinder(r,100);
DT = delaunayTriangulation(X(:),Y(:),Z(:));
TR = triangulation(DT.ConnectivityList,DT.Points);
[F,P]=freeBoundary(TR);
TR = triangulation(F,P);
stlwrite(TR,'stl_test000.stl');

Method 1: may not be accurate for complicated shapes

%read the stl as a triangulation
gm = stlread('stl_test000.stl');
% use alphashape to create a bounding volume
shp=alphaShape(gm.Points);
plot(shp)
% If the plot does not look like the shape that you are expecting, change
% the critical radius for the alphashape
shp.Alpha = 5;
plot(shp); % this should plot the cylinder. the plot can be saved as a bitmap
% Get the volume value
shp.volume

Method 2: more accurate. Needs PDE Toolbox

pdem = createpde;
importGeometry(pdem,'stl_test000.stl');
% plot the geometry (3D)
pdegplot(pdem); % the plot can be saved as a bitmap
% mesh the geometry. For more accuracy, adjust the Hmax
msh = generateMesh(pdem,'Hmax',0.1);
% calculate the volume
msh.volume()

Best Answer

Related Solutions

MATLAB: Stlwrite error: Tetrahedron triangulation is not supported.

MATLAB: How to convert a triangulation object into volume and create a BITMAP image

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