How to get 3D Points of separate regions obtained from alphaShape?

Taking example from MATLAB docs regarding alphaShape:

 [x1, y1, z1] = sphere(24);
 x1 = x1(:); y1 = y1(:); z1 = z1(:);
 x2 = x1+5; P = [x1 y1 z1; x2 y1 z1]; 
 % if you plot 'P' you'll see two spheres 
 % plot3(P(:,1),P(:,2),P(:,3),'.')
 P = unique(P,'rows');
 shp = alphaShape(P(:,1),P(:,2),P(:,3),1);
 N = numRegions(shp)

Output:

 N = 2

I am interested in finding which points of P belong to which region. In other words labeling points in P. This can be done using different clustering methods, but I would like to know if it is possible with alphaShape.

Answer 1

[Sean de Wolski]

You can pass RegionID into alphaTriangulation. Loop over the region ids to grab the triangulations.

http://www.mathworks.com/help/releases/R2016a/matlab/ref/alphashape.alphatriangulation.html

[Dadhichi]

If I understand you correct then is it the following what you are suggesting?

 tri1 = alphaTriangulation(shp,1);
 tri2 = alphaTriangulation(shp,2);
 idx1 = unique(tri1(:));
 idx2 = unique(tri2(:))
 points1 = P(idx1,:);
 points2 = P(idx2,:);

Is there a better way? Or could you put the content of the link in the answer. Thanks!

[Sean de Wolski]

That looks right to me and matches the expectations for set intersection, i.e. no points in both but all points represented:

>>intersect(idx1,idx2)
ans =
 Empty matrix: 0-by-1
>> all(ismember(alphaTriangulation(shp),union(idx1,idx2)))
ans =
   1     1     1     1