Unable to create alpha shapes from point cloud

[schFuelPC]

Hi all,

Very new user of Open3D and hoping to get some assistance on the function "create_from_point_cloud_alpha_shape". Using version 15.2 of open3d for windows.

I have a point cloud (see below) of a tree canopy that I am looking to create a 3D alpha shape around to get an estimate of the volume of the canopy.
WC1_Cloud_canopy.txt

I am using the following code to test different size alphas and then will examine the resultant meshes in CloudCompare.

alpha_value_list = np.arange(0, 10, 0.01)

for i, alpha in enumerate(alpha_value_list):
    pcd = o3d.io.read_point_cloud(file_name, 'xyz')
    mesh = o3d.geometry.TriangleMesh.create_from_point_cloud_alpha_shape(pcd, alpha)
    print(f'Alpha: {alpha}. Mesh: {mesh}')
    o3d.io.write_triangle_mesh(filename=f'{outpath}/{alpha}_mesh.obj',
                               mesh=mesh,
                               write_ascii=True,
                               compressed=False,
                               write_vertex_normals=True,
                               write_vertex_colors=True,
                               write_triangle_uvs=True,
                               print_progress=False)

On all values of alpha, the output of create_from_point_cloud_alpha_shape is TriangleMesh with 0 points and 0 triangles.

Any thoughts or suggestions on what I might be doing wrong?

Thanks

[adam-grant-hendry]

@schFuelPC The shape of your data is very odd. Loading it into a numpy array reveals it is (2318, 11), which is not a set of x, y, z points (otherwise axis 1 would be 3 instead of 11). What is this data of?

The error TriangleMesh with 0 points and 0 triangles usually happens when alpha is not set big enough, but it may also be in your case that the dataset does not actually appear to be real points.

NOTE: The open3d examples use very small alpha values, which erroneously leads many users to think their alpha values should be small too. Alpha must be at least a little bigger than the the smallest distance between three points in your mesh. The choice of alpha depends on how spread out points are in your dataset.

If I use a large alpha like 60000, I get a triangle mesh (the smallest number that yields some triangles is about 1000). But, as I suspected, the output doesn't make much sense:

I would suggest reexamining your input data set and first try to organize it into an N x 3 shape of 3d data points.