Detect rings/circuits of connected voxels

Question

I have a skeletonized voxel structure that looks like this:

The actual structure is significantly larger than this exampleIs there any way to find the closed rings

Answer 1

First I reduce the size of the problem considerably by contracting neighbouring nodes of degree 2 into hypernodes: each simple chain in the graph is substituted with a single node.

Then I find the cycle basis, for which the maximum cost of the cycles in the basis set is minimal.

For the central part of the network, the solution can easily be plotted as it is planar:

For some reason, I fail to correctly identify the cycle basis but I think the following should definitely get you started and maybe somebody else can chime in.

Recover data from posted image (as OP wouldn't provide some real data)

import numpy as np
import matplotlib.pyplot as plt
from skimage.morphology import medial_axis, binary_closing
from matplotlib.patches import Path, PathPatch
import itertools
import networkx as nx

img = plt.imread("tissue_skeleton_crop.jpg")
# plt.hist(np.mean(img, axis=-1).ravel(), bins=255) # find a good cutoff
bw = np.mean(img, axis=-1) < 200
# plt.imshow(bw, cmap='gray')
closed = binary_closing(bw, selem=np.ones((50,50))) # connect disconnected segments
# plt.imshow(closed, cmap='gray')
skeleton = medial_axis(closed)

fig, ax = plt.subplots(1,1)
ax.imshow(skeleton, cmap='gray')
ax.set_xticks([])
ax.set_yticks([])

def img_to_graph(binary_img, allowed_steps):
    """
    Arguments:
    ----------
    binary_img    -- 2D boolean array marking the position of nodes
    allowed_steps -- list of allowed steps; e.g. [(0, 1), (1, 1)] signifies that
                     from node with position (i, j) nodes at position (i, j+1)
                     and (i+1, j+1) are accessible,

    Returns:
    --------
    g             -- networkx.Graph() instance
    pos_to_idx    -- dict mapping (i, j) position to node idx (for testing if path exists)
    idx_to_pos    -- dict mapping node idx to (i, j) position (for plotting)
    """

    # map array indices to node indices and vice versa
    node_idx = range(np.sum(binary_img))
    node_pos = zip(*np.where(np.rot90(binary_img, 3)))
    pos_to_idx = dict(zip(node_pos, node_idx))

    # create graph
    g = nx.Graph()
    for (i, j) in node_pos:
        for (delta_i, delta_j) in allowed_steps: # try to step in all allowed directions
            if (i+delta_i, j+delta_j) in pos_to_idx: # i.e. target node also exists
                g.add_edge(pos_to_idx[(i,j)], pos_to_idx[(i+delta_i, j+delta_j)])

    idx_to_pos = dict(zip(node_idx, node_pos))

    return g, idx_to_pos, pos_to_idx

allowed_steps = set(itertools.product((-1, 0, 1), repeat=2)) - set([(0,0)])
g, idx_to_pos, pos_to_idx = img_to_graph(skeleton, allowed_steps)

fig, ax = plt.subplots(1,1)
nx.draw(g, pos=idx_to_pos, node_size=1, ax=ax)

NB: These are not red lines, these are lots of red dots corresponding to nodes in the graph.

Contract Graph

def contract(g):
    """
    Contract chains of neighbouring vertices with degree 2 into one hypernode.

    Arguments:
    ----------
    g -- networkx.Graph or networkx.DiGraph instance

    Returns:
    --------
    h -- networkx.Graph or networkx.DiGraph instance
        the contracted graph

    hypernode_to_nodes -- dict: int hypernode -> [v1, v2, ..., vn]
        dictionary mapping hypernodes to nodes

    """

    # create subgraph of all nodes with degree 2
    is_chain = [node for node, degree in g.degree() if degree == 2]
    chains = g.subgraph(is_chain)

    # contract connected components (which should be chains of variable length) into single node
    components = list(nx.components.connected_component_subgraphs(chains))
    hypernode = g.number_of_nodes()
    hypernodes = []
    hyperedges = []
    hypernode_to_nodes = dict()
    false_alarms = []
    for component in components:
        if component.number_of_nodes() > 1:

            hypernodes.append(hypernode)
            vs = [node for node in component.nodes()]
            hypernode_to_nodes[hypernode] = vs

            # create new edges from the neighbours of the chain ends to the hypernode
            component_edges = [e for e in component.edges()]
            for v, w in [e for e in g.edges(vs) if not ((e in component_edges) or (e[::-1] in component_edges))]:
                if v in component:
                    hyperedges.append([hypernode, w])
                else:
                    hyperedges.append([v, hypernode])

            hypernode += 1

        else: # nothing to collapse as there is only a single node in component:
            false_alarms.extend([node for node in component.nodes()])

    # initialise new graph with all other nodes
    not_chain = [node for node in g.nodes() if not node in is_chain]
    h = g.subgraph(not_chain + false_alarms)
    h.add_nodes_from(hypernodes)
    h.add_edges_from(hyperedges)

    return h, hypernode_to_nodes

h, hypernode_to_nodes = contract(g)

# set position of hypernode to position of centre of chain
for hypernode, nodes in hypernode_to_nodes.items():
    chain = g.subgraph(nodes)
    first, last = [node for node, degree in chain.degree() if degree==1]
    path = nx.shortest_path(chain, first, last)
    centre = path[len(path)/2]
    idx_to_pos[hypernode] = idx_to_pos[centre]

fig, ax = plt.subplots(1,1)
nx.draw(h, pos=idx_to_pos, node_size=20, ax=ax)

Find cycle basis

cycle_basis = nx.cycle_basis(h)

fig, ax = plt.subplots(1,1)
nx.draw(h, pos=idx_to_pos, node_size=10, ax=ax)
for cycle in cycle_basis:
    vertices = [idx_to_pos[idx] for idx in cycle]
    path = Path(vertices)
    ax.add_artist(PathPatch(path, facecolor=np.random.rand(3)))

TODO:

Find the correct cycle basis (I might be confused what the cycle basis is or networkx might have a bug).

EDIT

Holy crap, this was a tour-de-force. I should have never delved into this rabbit hole.

So the idea is now that we want to find the cycle basis for which the maximum cost for the cycles in the basis is minimal. We set the cost of a cycle to its length in edges, but one could imagine other cost functions. To do so, we find an initial cycle basis, and then we combine cycles in the basis until we find the set of cycles with the desired property.

def find_holes(graph, cost_function):
    """
    Find the cycle basis, that minimises the maximum individual cost of the cycles in the basis set.
    """

    # get cycle basis
    cycles = nx.cycle_basis(graph)

    # find new basis set that minimises maximum cost
    old_basis = set()
    new_basis = set(frozenset(cycle) for cycle in cycles) # only frozensets are hashable
    while new_basis != old_basis:
        old_basis = new_basis
        for cycle_a, cycle_b in itertools.combinations(old_basis, 2):
            if len(frozenset.union(cycle_a, cycle_b)) >= 2: # maybe should check if they share an edge instead
                cycle_c = _symmetric_difference(graph, cycle_a, cycle_b)
                new_basis = new_basis.union([cycle_c])
        new_basis = _select_cycles(new_basis, cost_function)

    ordered_cycles = [order_nodes_in_cycle(graph, nodes) for nodes in new_basis]
    return ordered_cycles

def _symmetric_difference(graph, cycle_a, cycle_b):
    # get edges
    edges_a = list(graph.subgraph(cycle_a).edges())
    edges_b = list(graph.subgraph(cycle_b).edges())

    # also get reverse edges as graph undirected
    edges_a += [e[::-1] for e in edges_a]
    edges_b += [e[::-1] for e in edges_b]

    # find edges that are in either but not in both
    edges_c = set(edges_a) ^ set(edges_b)

    cycle_c = frozenset(nx.Graph(list(edges_c)).nodes())
    return cycle_c

def _select_cycles(cycles, cost_function):
    """
    Select cover of nodes with cycles that minimises the maximum cost
    associated with all cycles in the cover.
    """
    cycles = list(cycles)
    costs = [cost_function(cycle) for cycle in cycles]
    order = np.argsort(costs)

    nodes = frozenset.union(*cycles)
    covered = set()
    basis = []

    # greedy; start with lowest cost
    for ii in order:
        cycle = cycles[ii]
        if cycle <= covered:
            pass
        else:
            basis.append(cycle)
            covered |= cycle
            if covered == nodes:
                break

    return set(basis)

def _get_cost(cycle, hypernode_to_nodes):
    cost = 0
    for node in cycle:
        if node in hypernode_to_nodes:
            cost += len(hypernode_to_nodes[node])
        else:
            cost += 1
    return cost

def _order_nodes_in_cycle(graph, nodes):
    order, = nx.cycle_basis(graph.subgraph(nodes))
    return order

holes = find_holes(h, cost_function=partial(_get_cost, hypernode_to_nodes=hypernode_to_nodes))

fig, ax = plt.subplots(1,1)
nx.draw(h, pos=idx_to_pos, node_size=10, ax=ax)
for ii, hole in enumerate(holes):
    if (len(hole) > 3):
        vertices = np.array([idx_to_pos[idx] for idx in hole])
        path = Path(vertices)
        ax.add_artist(PathPatch(path, facecolor=np.random.rand(3)))
        xmin, ymin = np.min(vertices, axis=0)
        xmax, ymax = np.max(vertices, axis=0)
        x = xmin + (xmax-xmin) / 2.
        y = ymin + (ymax-ymin) / 2.
        # ax.text(x, y, str(ii))