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))
