Empty Graph returned for circular skeletons without branches

[Quasimondo]

I am not sure if this is on purpose, but it looks like there is no graph returned for skeletons that result in a single chain that connects back to itself - I tried both multi=False and True but the result is aways an empty graph:

img = loadImg(path)>127
ske = skeletonize(~img).astype(np.uint16)
graph = sknw.build_sknw(ske)
print("\n".join(nx.generate_gml(graph)))`

Result:
graph [ ]

[yxdragon]

sknw follow these steps:

1. detect the node end junction (which has 1 neighbour or more than 2neighbours)
2. trace from one node to another through the path (which has 2 neighbours)

so a pure ring without any branch would not be detected now.
If you need detect pure ring or isolate point, we can try to add some features.

[Quasimondo]

Thanks for the quick response! I am looking at the source right now and will see if I can patch it to my needs. I am thinking of adding one random start node if there are 0 nodes, but the skeleton image has non-zero entries.

[Quasimondo]

I think this might do for my purposes:

def build_sknw(ske, multi=False):
    buf = buffer(ske)
    nbs = neighbors(buf.shape)
    acc = np.cumprod((1,)+buf.shape[::-1][:-1])[::-1]
    mark(buf, nbs)
    pts = np.array(np.where(buf.ravel()==2))[0]
    
    if len(pts)==0 and np.sum(ske)>0:
        print("ring detected")
        pts = np.where(ske.ravel())[0]
        buf[np.array(np.unravel_index(pts, ske.shape))+1] = 2
    
    nodes, edges = parse_struc(buf, pts, nbs, acc)
    return build_graph(nodes, edges, multi)

[yxdragon]

1. np.where did not make sure the ring series. (it is in row and columns series)
2. it did not treat when (both pure ring and branch exist)

I learn np.unravel_index from you, thanks.

[yxdragon]

i would give a update soon

[Quasimondo]

Oh yes - thanks for checking the code and finding the issues with it - I just needed it for a very simple example so I did miss the other possible cases.

[yxdragon]

add isolate and ring parameter, have a try.
ps: if you only need the ring's coordinates, you can try skimage.regionprops

[Quasimondo]

Perfect - looks like it works as intended. Thanks for the quick fix!