# Source code for skan.csr

```
from __future__ import annotations
import networkx as nx
import numpy as np
import pandas as pd
from scipy import sparse, ndimage as ndi
from scipy.sparse import csgraph
from scipy.spatial import distance_matrix
from skimage import morphology
from skimage.graph import central_pixel
from skimage.util._map_array import map_array, ArrayMap
import numpy.typing as npt
import numba
import warnings
from typing import Tuple, Callable
from .nputil import _raveled_offsets_and_distances
from .summary_utils import find_main_branches
def _weighted_abs_diff(
values0: np.ndarray, values1: np.ndarray, distances: np.ndarray
) -> np.ndarray:
"""A default edge function for complete image graphs.
A pixel graph on an image with no edge values and no mask is a very
boring regular lattice, so we define a default edge weight to be the
absolute difference between values *weighted* by the distance
between them.
Parameters
----------
values0 : array
The pixel values for each node.
values1 : array
The pixel values for each neighbor.
distances : array
The distance between each node and its neighbor.
Returns
-------
edge_values : array of float
The computed values: abs(values0 - values1) * distances.
"""
return np.abs(values0 - values1) * distances
[docs]
def pixel_graph(
image, *, mask=None, edge_function=None, connectivity=1, spacing=None
):
"""Create an adjacency graph of pixels in an image.
Pixels where the mask is True are nodes in the returned graph, and they are
connected by edges to their neighbors according to the connectivity
parameter. By default, the *value* of an edge when a mask is given, or when
the image is itself the mask, is the euclidean distance betwene the pixels.
However, if an int- or float-valued image is given with no mask, the value
of the edges is the absolute difference in intensity between adjacent
pixels, weighted by the euclidean distance.
Parameters
----------
image : array
The input image. If the image is of type bool, it will be used as the
mask as well.
mask : array of bool
Which pixels to use. If None, the graph for the whole image is used.
edge_function : callable
A function taking an array of pixel values, and an array of neighbor
pixel values, and an array of distances, and returning a value for the
edge. If no function is given, the value of an edge is just the
distance.
connectivity : int
The square connectivity of the pixel neighborhood: the number of
orthogonal steps allowed to consider a pixel a neigbor. See
`scipy.ndimage.generate_binary_structure` for details.
spacing : tuple of float
The spacing between pixels along each axis.
Returns
-------
graph : scipy.sparse.csr_matrix
A sparse adjacency matrix in which entry (i, j) is 1 if nodes i and j
are neighbors, 0 otherwise.
nodes : array of int
The nodes of the graph. These correspond to the raveled indices of the
nonzero pixels in the mask.
"""
if image.dtype == bool and mask is None:
mask = image
if mask is None and edge_function is None:
mask = np.ones_like(image, dtype=bool)
edge_function = _weighted_abs_diff
# Strategy: we are going to build the (i, j, data) arrays of a scipy
# sparse COO matrix, then convert to CSR (which is fast).
# - grab the raveled IDs of the foreground (mask == True) parts of the
# image **in the padded space**.
# - broadcast them together with the raveled offsets to their neighbors.
# This gives us for each foreground pixel a list of neighbors (that
# may or may not be selected by the mask.) (We also track the *distance*
# to each neighbor.)
# - select "valid" entries in the neighbors and distance arrays by indexing
# into the mask, which we can do since these are raveled indices.
# - use np.repeat() to repeat each source index according to the number
# of neighbors selected by the mask it has. Each of these repeated
# indices will be lined up with its neighbor, i.e. **this is the i
# array** of the COO format matrix.
# - use the mask as a boolean index to get a 1D view of the selected
# neighbors. **This is the j array.**
# - by default, the same boolean indexing can be applied to the distances
# to each neighbor, to give the **data array.** Optionally, a
# provided edge function can be computed on the pixel values and the
# distances to give a different value for the edges.
# Note, we use map_array to map the raveled coordinates in the padded
# image to the ones in the original image, and those are the returned
# nodes.
padded = np.pad(mask, 1, mode='constant', constant_values=False)
nodes_padded = np.flatnonzero(padded)
neighbor_offsets_padded, distances_padded = _raveled_offsets_and_distances(
padded.shape, connectivity=connectivity, spacing=spacing
)
neighbors_padded = nodes_padded[:, np.newaxis] + neighbor_offsets_padded
neighbor_distances_full = np.broadcast_to(
distances_padded, neighbors_padded.shape
)
nodes = np.flatnonzero(mask)
nodes_sequential = np.arange(nodes.size)
# neighbors outside the mask get mapped to 0, which is a valid index,
# BUT, they will be masked out in the next step.
neighbors = map_array(neighbors_padded, nodes_padded, nodes)
neighbors_mask = padded.reshape(-1)[neighbors_padded]
num_neighbors = np.sum(neighbors_mask, axis=1)
indices = np.repeat(nodes, num_neighbors)
indices_sequential = np.repeat(nodes_sequential, num_neighbors)
neighbor_indices = neighbors[neighbors_mask]
neighbor_distances = neighbor_distances_full[neighbors_mask]
neighbor_indices_sequential = map_array(
neighbor_indices, nodes, nodes_sequential
)
if edge_function is None:
data = neighbor_distances
else:
image_r = image.reshape(-1)
data = edge_function(
image_r[indices], image_r[neighbor_indices], neighbor_distances
)
m = nodes_sequential.size
mat = sparse.coo_matrix(
(data, (indices_sequential, neighbor_indices_sequential)),
shape=(m, m)
)
graph = mat.tocsr()
return graph, nodes
csr_spec_float = [
('indptr', numba.int32[:]),
('indices', numba.int32[:]),
('data', numba.float64[:]),
('shape', numba.int32[:]),
('node_properties', numba.float64[:]),
] # yapf: disable
csr_spec_bool = [
('indptr', numba.int32[:]),
('indices', numba.int32[:]),
('data', numba.bool_[:]),
('shape', numba.int32[:]),
('node_properties', numba.float64[:]),
] # yapf: disable
class NBGraphBase:
def __init__(self, indptr, indices, data, shape, node_props):
self.indptr = indptr
self.indices = indices
self.data = data
self.shape = shape
self.node_properties = node_props
def edge(self, i, j):
return _csrget(self.indices, self.indptr, self.data, i, j)
def set_edge(self, i, j, value):
return _csrset(self.indices, self.indptr, self.data, i, j, value)
def neighbors(self, row):
loc, stop = self.indptr[row], self.indptr[row + 1]
return self.indices[loc:stop]
@property
def has_node_props(self):
return self.node_properties.strides != (0,)
NBGraph = numba.experimental.jitclass(NBGraphBase, csr_spec_float)
NBGraphBool = numba.experimental.jitclass(NBGraphBase, csr_spec_bool)
def csr_to_nbgraph(csr, node_props=None):
if node_props is None:
node_props = np.broadcast_to(1., csr.shape[0])
node_props.flags.writeable = True
return NBGraph(
csr.indptr,
csr.indices,
csr.data,
np.array(csr.shape, dtype=np.int32),
node_props,
)
def _pixel_graph(image, steps, distances, num_edges, height=None):
row = np.empty(num_edges, dtype=int)
col = np.empty(num_edges, dtype=int)
data = np.empty(num_edges, dtype=float)
if height is None:
k = _write_pixel_graph(image, steps, distances, row, col, data)
else:
k = _write_pixel_graph_height(
image, height, steps, distances, row, col, data
)
graph = sparse.coo_matrix((data[:k], (row[:k], col[:k]))).tocsr()
return graph
@numba.jit(nopython=True, cache=True, nogil=True)
def _write_pixel_graph(image, steps, distances, row, col, data):
"""Step over `image` to build a graph of nonzero pixel neighbors.
Parameters
----------
image : int array
The input image.
steps : int array, shape (N,)
The raveled index steps to find a pixel's neighbors in `image`.
distances : float array, shape (N,)
The euclidean distance from a pixel to its corresponding
neighbor in `steps`.
row : int array
Output array to be filled with the "center" pixel IDs.
col : int array
Output array to be filled with the "neighbor" pixel IDs.
data : float array
Output array to be filled with the distances from center to
neighbor pixels.
Returns
-------
k : int
The number of entries written to row, col, and data.
Notes
-----
No size or bounds checking is performed. Users should ensure that
- No index in `indices` falls on any edge of `image` (or the
neighbor computation will fail or segfault).
- The `steps` and `distances` arrays have the same shape.
- The `row`, `col`, `data` are long enough to hold all of the
edges.
"""
image = image.ravel()
n_neighbors = steps.size
start_idx = np.max(steps)
end_idx = image.size + np.min(steps)
k = 0
for i in range(start_idx, end_idx + 1):
if image[i] != 0:
for j in range(n_neighbors):
n = steps[j] + i
if image[n] != 0 and image[n] != image[i]:
row[k] = image[i]
col[k] = image[n]
data[k] = distances[j]
k += 1
return k
@numba.jit(nopython=True, cache=True, nogil=True)
def _write_pixel_graph_height(image, height, steps, distances, row, col, data):
"""Step over `image` to build a graph of nonzero pixel neighbors.
Parameters
----------
image : int array
The input image.
height : float array, same shape as `image`
This is taken to be a height map along an additional
dimension (in addition to the image dimensions), so the distance
between two neighbors `i` and `n` separated by `j` is given by:
`np.sqrt(distances[j]**2 + (height[i] - height[n])**2)`
steps : int array, shape (N,)
The raveled index steps to find a pixel's neighbors in `image`.
distances : float array, shape (N,)
The euclidean distance from a pixel to its corresponding
neighbor in `steps`.
row : int array
Output array to be filled with the "center" pixel IDs.
col : int array
Output array to be filled with the "neighbor" pixel IDs.
data : float array
Output array to be filled with the distances from center to
neighbor pixels.
Returns
-------
k : int
The number of entries written to row, col, and data.
Notes
-----
No size or bounds checking is performed. Users should ensure that
- No index in `indices` falls on any edge of `image` (or the
neighbor computation will fail or segfault).
- The `steps` and `distances` arrays have the same shape.
- The `row`, `col`, `data` are long enough to hold all of the
edges.
"""
image = image.ravel()
height = height.ravel()
n_neighbors = steps.size
start_idx = np.max(steps)
end_idx = image.size + np.min(steps)
k = 0
for i in range(start_idx, end_idx + 1):
if image[i] != 0:
for j in range(n_neighbors):
n = steps[j] + i
if image[n] != 0 and image[n] != image[i]:
row[k] = image[i]
col[k] = image[n]
data[k] = np.sqrt(
distances[j]**2 + (height[i] - height[n])**2
)
k += 1
return k
@numba.jit(nopython=True, cache=False) # change this to True with Numba 1.0
def _build_paths(jgraph, indptr, indices, path_data, visited, degrees):
indptr_i = 0
indices_j = 0
# first, process all nodes in a path to an endpoint or junction
for node in range(jgraph.shape[0]):
if degrees[node] > 2 or degrees[node] == 1:
for neighbor in jgraph.neighbors(node):
if not visited.edge(node, neighbor):
n_steps = _walk_path(
jgraph,
node,
neighbor,
visited,
degrees,
indices,
path_data,
indices_j,
)
indptr[indptr_i + 1] = indptr[indptr_i] + n_steps
indptr_i += 1
indices_j += n_steps
# everything else is by definition in isolated cycles
for node in range(jgraph.shape[0]):
if degrees[node] > 0:
neighbor = jgraph.neighbors(node)[0]
if not visited.edge(node, neighbor):
n_steps = _walk_path(
jgraph,
node,
neighbor,
visited,
degrees,
indices,
path_data,
indices_j,
)
indptr[indptr_i + 1] = indptr[indptr_i] + n_steps
indptr_i += 1
indices_j += n_steps
return indptr_i + 1, indices_j
@numba.jit(nopython=True, cache=False) # change this to True with Numba 1.0
def _walk_path(
jgraph, node, neighbor, visited, degrees, indices, path_data, startj
):
indices[startj] = node
start_node = node
path_data[startj] = jgraph.node_properties[node]
j = startj + 1
while not visited.edge(node, neighbor):
visited.set_edge(node, neighbor, True)
visited.set_edge(neighbor, node, True)
indices[j] = neighbor
path_data[j] = jgraph.node_properties[neighbor]
if degrees[neighbor] != 2 or neighbor == start_node:
break
n1, n2 = jgraph.neighbors(neighbor)
nextneighbor = n1 if n1 != node else n2
node, neighbor = neighbor, nextneighbor
j += 1
return j - startj + 1
def _build_skeleton_path_graph(graph):
max_num_cycles = graph.indices.size // 4
buffer_size_offset = max_num_cycles
degrees = np.diff(graph.indptr)
visited_data = np.zeros(graph.data.shape, dtype=bool)
visited = NBGraphBool(
graph.indptr, graph.indices, visited_data, graph.shape,
np.broadcast_to(1.0, graph.shape[0])
)
endpoints = (degrees != 2)
endpoint_degrees = degrees[endpoints]
num_paths = np.sum(endpoint_degrees)
path_indptr = np.zeros(num_paths + buffer_size_offset, dtype=int)
# the number of points that we need to save to store all skeleton
# paths is equal to the number of pixels plus the sum of endpoint
# degrees minus one (since the endpoints will have been counted once
# already in the number of pixels) *plus* the number of isolated
# cycles (since each cycle has one index repeated). We don't know
# the number of cycles ahead of time, but it is bounded by one quarter
# of the number of points.
n_points = (
graph.indices.size + np.sum(np.maximum(0, endpoint_degrees - 1))
+ buffer_size_offset
)
path_indices = np.zeros(n_points, dtype=int)
path_data = np.zeros(path_indices.shape, dtype=float)
m, n = _build_paths(
graph, path_indptr, path_indices, path_data, visited, degrees
)
paths = sparse.csr_matrix(
(path_data[:n], path_indices[:n], path_indptr[:m]),
shape=(m - 1, n)
)
return paths
[docs]
class Skeleton:
"""Object to group together all the properties of a skeleton.
In the text below, we use the following notation:
- N: the number of points in the pixel skeleton,
- ndim: the dimensionality of the skeleton
- P: the number of paths in the skeleton (also the number of links in the
junction graph).
- J: the number of junction nodes
- Sd: the sum of the degrees of all the junction nodes
- [Nt], [Np], Nr, Nc: the dimensions of the source image
Parameters
----------
skeleton_image : array
The input skeleton (1-pixel/voxel thick skeleton, all other values 0).
Other Parameters
----------------
spacing : float or array of float, shape ``(ndim,)``
The scale of the pixel spacing along each axis.
source_image : array of float, same shape as `skeleton_image`
The image that `skeleton_image` represents / summarizes / was generated
from. This is used to produce visualizations as well as statistical
properties of paths.
keep_images : bool
Whether or not to keep the original input images. These can be useful
for visualization, but they may take up a lot of memory.
value_is_height : bool
Whether to consider the value of a float skeleton to be the "height"
of the image. This can be useful e.g. when measuring lengths along
ridges in AFM images.
Attributes
----------
graph : scipy.sparse.csr_matrix, shape (N + 1, N + 1)
The skeleton pixel graph, where each node is a non-zero pixel in the
input image, and each edge connects adjacent pixels. The graph is
represented as an adjacency matrix in SciPy sparse matrix format. For
more information see the ``scipy.sparse`` documentation as well as
``scipy.sparse.csgraph``. Note: pixel numbering starts at 1, so the
shape of this matrix is ``(N + 1, N + 1)`` instead of ``(N, N)``.
nbgraph : NBGraph
A thin Numba wrapper around the ``csr_matrix`` format, this provides
faster graph methods. For example, it is much faster to get a list of
neighbors, or test for the presence of a specific edge.
coordinates : array, shape (N, ndim)
skeleton_pixel_id i -> coordinates[i]
The image coordinates of each pixel in the skeleton.
Some values in this matrix are non-sensical — you should only access
them from node ids.
paths : scipy.sparse.csr_matrix, shape (P, N + 1)
A csr_matrix where element [i, j] is on if node j is in path i. This
includes path endpoints. The number of nonzero elements is N - J + Sd.
n_paths : int
The number of paths, P. This is redundant information given `n_paths`,
but it is used often enough that it is worth keeping around.
distances : array of float, shape (P,)
The distance of each path. Note: not initialized until `path_lengths()`
is called on the skeleton; use path_lengths() instead
skeleton_image : array or None
The input skeleton image. Only present if `keep_images` is True. Set to
False to preserve memory.
source_image : array or None
The image from which the skeleton was derived. Only present if
`keep_images` is True. This is useful for visualization.
"""
def __init__(
self,
skeleton_image,
*,
spacing=1,
source_image=None,
keep_images=True,
value_is_height=False,
):
graph, coords = skeleton_to_csgraph(
skeleton_image,
spacing=spacing,
value_is_height=value_is_height,
)
if np.issubdtype(skeleton_image.dtype, np.floating):
self.pixel_values = skeleton_image[coords]
elif np.issubdtype(skeleton_image.dtype, np.integer):
self.pixel_values = skeleton_image.astype(float)[coords]
else:
self.pixel_values = None
self.graph = graph
self.nbgraph = csr_to_nbgraph(graph, self.pixel_values)
self.coordinates = np.transpose(coords)
self.paths = _build_skeleton_path_graph(self.nbgraph)
self.n_paths = self.paths.shape[0]
self.distances = np.empty(self.n_paths, dtype=float)
self._distances_initialized = False
self.skeleton_image = None
self.skeleton_shape = skeleton_image.shape
self.skeleton_dtype = skeleton_image.dtype
self.source_image = None
self.degrees = np.diff(self.graph.indptr)
self.spacing = (
np.asarray(spacing) if not np.isscalar(spacing) else
np.full(skeleton_image.ndim, spacing)
)
if keep_images:
self.keep_images = keep_images
self.skeleton_image = skeleton_image
self.source_image = source_image
[docs]
def path(self, index):
"""Return the pixel indices of path number `index`.
Parameters
----------
index : int
The desired path.
Returns
-------
path : array of int
The indices of the pixels belonging to the path, including
endpoints.
"""
# The below is equivalent to `self.paths[index].indices`, which is much
# more elegant. However the below version is about 25x faster!
# In [14]: %timeit mat[1].indices
# 128 µs ± 421 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)
# In [16]: %%timeit
# ...: start, stop = mat.indptr[1:3]
# ...: mat.indices[start:stop]
# ...:
# 5.05 µs ± 77.2 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
start, stop = self.paths.indptr[index:index + 2]
return self.paths.indices[start:stop]
[docs]
def path_coordinates(self, index: int):
"""Return the image coordinates of the pixels in the path.
Parameters
----------
index : int
The desired path.
Returns
-------
path_coords : array of float
The (image) coordinates of points on the path, including endpoints.
"""
path_indices = self.path(index)
return self.coordinates[path_indices]
[docs]
def path_with_data(self, index: int):
"""Return pixel indices and corresponding pixel values on a path.
Parameters
----------
index : int
The desired path.
Returns
-------
path : array of int
The indices of pixels on the path, including endpoints.
data : array of float
The values of pixels on the path.
"""
start, stop = self.paths.indptr[index:index + 2]
return self.paths.indices[start:stop], self.paths.data[start:stop]
[docs]
def path_lengths(self):
"""Return the length of each path on the skeleton.
Returns
-------
lengths : array of float
The length of all the paths in the skeleton.
"""
if not self._distances_initialized:
_compute_distances(
self.nbgraph, self.paths.indptr, self.paths.indices,
self.distances
)
self._distances_initialized = True
return self.distances
[docs]
def paths_list(self):
"""List all the paths in the skeleton, including endpoints.
Returns
-------
paths : list of array of int
The list containing all the paths in the skeleton.
"""
return [list(self.path(i)) for i in range(self.n_paths)]
[docs]
def path_label_image(self):
"""Image like self.skeleton_image with path_ids as values.
Returns
-------
label_image : array of ints
Image of the same shape as self.skeleton_image where each pixel
has the value of its branch id + 1.
"""
image_out = np.zeros(self.skeleton_shape, dtype=int)
for i in range(self.n_paths):
coords_to_wipe = self.path_coordinates(i)
coords_idxs = tuple(np.round(coords_to_wipe).astype(int).T)
image_out[coords_idxs] = i + 1
return image_out
[docs]
def path_means(self):
"""Compute the mean pixel value along each path.
Returns
-------
means : array of float
The average pixel value along each path in the skeleton.
"""
sums = np.add.reduceat(self.paths.data, self.paths.indptr[:-1])
lengths = np.diff(self.paths.indptr)
return sums / lengths
[docs]
def path_stdev(self):
"""Compute the standard deviation of values along each path.
Returns
-------
stdevs : array of float
The standard deviation of pixel values along each path.
"""
data = self.paths.data
sumsq = np.add.reduceat(data * data, self.paths.indptr[:-1])
lengths = np.diff(self.paths.indptr)
means = self.path_means()
return np.sqrt(np.clip(sumsq/lengths - means*means, 0, None))
[docs]
def prune_paths(self, indices: npt.ArrayLike) -> 'Skeleton':
"""Prune nodes from the skeleton.
Parameters
----------
indices: List[int]
List of indices to be removed.
Retruns
-------
Skeleton
A new Skeleton object pruned.
"""
# warning: slow
image_cp = np.copy(self.skeleton_image)
if not np.all(np.array(indices) < self.n_paths):
raise ValueError(
f'The path index {np.max(indices)} does not exist in this '
f'skeleton. (The highest path index is {self.n_paths}.)\n'
'If you obtained the index from a summary table, you '
'probably need to resummarize the skeleton.'
)
for i in indices:
pixel_ids_to_wipe = self.path(i)
junctions = self.degrees[pixel_ids_to_wipe] > 2
pixel_ids_to_wipe = pixel_ids_to_wipe[~junctions]
coords_to_wipe = self.coordinates[pixel_ids_to_wipe]
coords_idxs = tuple(np.round(coords_to_wipe).astype(int).T)
image_cp[coords_idxs] = 0
# optional cleanup:
new_skeleton = morphology.skeletonize(image_cp.astype(bool)) * image_cp
return Skeleton(
new_skeleton,
spacing=self.spacing,
source_image=self.source_image,
keep_images=self.keep_images
)
def __array__(self, dtype=None):
"""Array representation of the skeleton path labels."""
return self.path_label_image()
[docs]
def summarize(
skel: Skeleton,
*,
value_is_height: bool = False,
find_main_branch: bool = False,
separator: str | None = None,
) -> pd.DataFrame:
"""Compute statistics for every skeleton and branch in ``skel``.
Parameters
----------
skel : skan.csr.Skeleton
A Skeleton object.
value_is_height : bool
Whether to consider the value of a float skeleton to be the "height"
of the image. This can be useful e.g. when measuring lengths along
ridges in AFM images.
find_main_branch : bool, optional
Whether to compute main branches. A main branch is defined as the
longest shortest path within a skeleton. This step is very expensive
as it involves computing the shortest paths between all pairs of branch
endpoints, so it is off by default.
separator : str, optional
Some column names are composite, e.g. ``'coord_src_0'``. The separator
argument allows users to configure which character is used to separate
the components. The default up to version 0.12 is '-', but will change
to '_' in version 0.13.
Returns
-------
summary : pandas.DataFrame
A summary of the branches including branch length, mean branch value,
branch euclidean distance, etc.
"""
if separator is None:
warnings.warn(
"separator in column name will change to _ in version 0.13; "
"to silence this warning, use `separator='-'` to maintain "
"current behavior and use `separator='_'` to switch to the "
"new default behavior.",
np.exceptions.VisibleDeprecationWarning,
stacklevel=2, # make sure warning points to calling line
)
separator = '-'
summary = {}
ndim = skel.coordinates.shape[1]
_, skeleton_ids = csgraph.connected_components(skel.graph, directed=False)
endpoints_src = skel.paths.indices[skel.paths.indptr[:-1]]
endpoints_dst = skel.paths.indices[skel.paths.indptr[1:] - 1]
summary['skeleton_id'] = skeleton_ids[endpoints_src]
summary['node_id_src'] = endpoints_src
summary['node_id_dst'] = endpoints_dst
summary['branch_distance'] = skel.path_lengths()
deg_src = skel.degrees[endpoints_src]
deg_dst = skel.degrees[endpoints_dst]
kind = np.full(deg_src.shape, 2) # default: junction-to-junction
kind[(deg_src == 1) | (deg_dst == 1)] = 1 # tip-junction
kind[(deg_src == 1) & (deg_dst == 1)] = 0 # tip-tip
kind[endpoints_src == endpoints_dst] = 3 # cycle
summary['branch_type'] = kind
summary['mean_pixel_value'] = skel.path_means()
summary['stdev_pixel_value'] = skel.path_stdev()
for i in range(ndim): # keep loops separate for best insertion order
summary[f'image_coord_src_{i}'] = skel.coordinates[endpoints_src, i]
for i in range(ndim):
summary[f'image_coord_dst_{i}'] = skel.coordinates[endpoints_dst, i]
coords_real_src = skel.coordinates[endpoints_src] * skel.spacing
for i in range(ndim):
summary[f'coord_src_{i}'] = coords_real_src[:, i]
if value_is_height:
values_src = skel.pixel_values[endpoints_src]
summary[f'coord_src_{ndim}'] = values_src
coords_real_src = np.concatenate(
[coords_real_src, values_src[:, np.newaxis]],
axis=1,
)
coords_real_dst = skel.coordinates[endpoints_dst] * skel.spacing
for i in range(ndim):
summary[f'coord_dst_{i}'] = coords_real_dst[:, i]
if value_is_height:
values_dst = skel.pixel_values[endpoints_dst]
summary[f'coord_dst_{ndim}'] = values_dst
coords_real_dst = np.concatenate(
[coords_real_dst, values_dst[:, np.newaxis]],
axis=1,
)
summary['euclidean_distance'] = (
np.sqrt((coords_real_dst - coords_real_src)**2
@ np.ones(ndim + int(value_is_height)))
)
df = pd.DataFrame(summary)
if find_main_branch:
# define main branch as longest shortest path within a single skeleton
df['main'] = find_main_branches(df)
df.rename(columns=lambda s: s.replace('_', separator), inplace=True)
return df
@numba.jit(nopython=True, nogil=True, cache=False) # cache with Numba 1.0
def _compute_distances(graph, path_indptr, path_indices, distances):
for i in range(len(distances)):
start, stop = path_indptr[i:i + 2]
path = path_indices[start:stop]
distances[i] = _path_distance(graph, path)
@numba.jit(nopython=True, nogil=True, cache=False) # cache with Numba 1.0
def _path_distance(graph, path):
d = 0.
n = len(path)
for i in range(n - 1):
u, v = path[i], path[i + 1]
d += graph.edge(u, v)
return d
def _mst_junctions(csmat):
"""Replace clustered pixels with degree > 2 by their minimum spanning tree.
This function performs the operation in place.
Parameters
----------
csmat : NBGraph
The input graph.
pixel_indices : array of int
The raveled index in the image of every pixel represented in csmat.
spacing : float, or array-like of float, shape `len(shape)`, optional
The spacing between pixels in the source image along each dimension.
Returns
-------
final_graph : NBGraph
The output csmat.
"""
# make copy
# mask out all degree < 3 entries
# find MST
# replace edges not in MST with zeros
# use .eliminate_zeros() to get a new matrix
csc_graph = csmat.tocsc()
degrees = np.asarray(csmat.astype(bool).astype(int).sum(axis=0))
non_junction = np.flatnonzero(degrees < 3)
non_junction_column_start = csc_graph.indptr[non_junction]
non_junction_column_end = csc_graph.indptr[non_junction + 1]
for start, end in zip(non_junction_column_start, non_junction_column_end):
csc_graph.data[start:end] = 0
csr_graph = csc_graph.tocsr()
non_junction_row_start = csr_graph.indptr[non_junction]
non_junction_row_end = csr_graph.indptr[non_junction + 1]
for start, end in zip(non_junction_row_start, non_junction_row_end):
csr_graph.data[start:end] = 0
csr_graph.eliminate_zeros()
mst = csgraph.minimum_spanning_tree(csr_graph)
non_tree_edges = csr_graph - (mst + mst.T)
final_graph = csmat - non_tree_edges
return final_graph
def distance_with_height(source_values, neighbor_values, distances):
height_diff = source_values - neighbor_values
return np.hypot(height_diff, distances)
[docs]
def skeleton_to_csgraph(
skel,
*,
spacing=1,
value_is_height=False,
):
"""Convert a skeleton image of thin lines to a graph of neighbor pixels.
Parameters
----------
skel : array
An input image in which every nonzero pixel is considered part of
the skeleton, and links between pixels are determined by a full
n-dimensional neighborhood.
spacing : float, or array-like of float, shape `(skel.ndim,)`
A value indicating the distance between adjacent pixels. This can
either be a single value if the data has the same resolution along
all axes, or it can be an array of the same shape as `skel` to
indicate spacing along each axis.
Other Parameters
----------------
value_is_height : bool, optional
If `True`, the pixel value at each point of the skeleton will be
considered to be a height measurement, and this height will be
incorporated into skeleton branch lengths. Used for analysis of
atomic force microscopy (AFM) images.
Returns
-------
graph : sparse.csr_matrix
A graph of shape (Nnz, Nnz), where Nnz is the number of
nonzero pixels in `skel`. The value graph[i, j] is the distance
between adjacent pixels i and j. In a 2D image, that would be
1 for immediately adjacent pixels and sqrt(2) for diagonally
adjacent ones.
pixel_coordinates : array of float
An array of shape (Nnz, skel.ndim), mapping indices in `graph`
to pixel coordinates in `skel`.
"""
# ensure we have a bool image, since we later use it for bool indexing
skel_im = skel
skel_bool = skel.astype(bool)
ndim = skel.ndim
spacing = np.ones(ndim, dtype=float) * spacing
if value_is_height:
edge_func = distance_with_height
else:
edge_func = None
graph, pixel_indices = pixel_graph(
skel_im,
mask=skel_bool,
edge_function=edge_func,
connectivity=ndim,
spacing=spacing
)
graph = _mst_junctions(graph)
pixel_coordinates = np.unravel_index(pixel_indices, skel.shape)
return graph, pixel_coordinates
@numba.jit(nopython=True, cache=True)
def _csrget(indices, indptr, data, row, col):
"""Fast lookup of value in a scipy.sparse.csr_matrix format table.
Parameters
----------
indices, indptr, data : numpy arrays of int, int, float
The CSR format data.
row, col : int
The matrix coordinates of the desired value.
Returns
-------
dat: float
The data value in the matrix.
"""
start, end = indptr[row], indptr[row + 1]
for i in range(start, end):
if indices[i] == col:
return data[i]
return 0.
@numba.jit(nopython=True, cache=True)
def _csrset(indices, indptr, data, row, col, value):
"""Fast lookup and set of value in a scipy.sparse.csr_matrix format table.
Parameters
----------
indices, indptr, data : numpy arrays of int, int, float
The CSR format data.
row, col : int
The matrix coordinates of the desired value.
value : dtype
The value to set in the matrix.
Notes
-----
This function only sets values that already existed in the matrix.
Returns
-------
success: bool
Whether the data value was successfully written to the matrix.
"""
start, end = indptr[row], indptr[row + 1]
for i in range(start, end):
if indices[i] == col:
data[i] = value
return True
return False
[docs]
def submatrix(M, idxs):
"""Return the outer-index product submatrix, `M[idxs, :][:, idxs]`.
Parameters
----------
M : scipy.sparse.spmatrix
Input (square) matrix
idxs : array of int
The indices to subset. No index in `idxs` should exceed the
number of rows of `M`.
Returns
-------
Msub : scipy.sparse.spmatrix
The subsetted matrix.
Examples
--------
>>> Md = np.arange(16).reshape((4, 4))
>>> M = sparse.csr_matrix(Md)
>>> print(submatrix(M, [0, 2]).toarray())
[[ 0 2]
[ 8 10]]
"""
Msub = M[idxs, :][:, idxs]
return Msub
[docs]
def make_degree_image(skeleton_image):
"""Create a array showing the degree of connectivity of each pixel.
Parameters
----------
skeleton_image : array
An input image in which every nonzero pixel is considered part of
the skeleton, and links between pixels are determined by a full
n-dimensional neighborhood.
Returns
-------
degree_image : array of int, same shape as skeleton_image
An image containing the degree of connectivity of each pixel in the
skeleton to neighboring pixels.
"""
bool_skeleton = skeleton_image.astype(bool)
degree_kernel = np.ones((3,) * bool_skeleton.ndim)
degree_kernel[(1,) * bool_skeleton.ndim] = 0 # remove centre pixel
if isinstance(bool_skeleton, np.ndarray):
degree_image = ndi.convolve(
bool_skeleton.astype(int),
degree_kernel,
mode='constant',
) * bool_skeleton
# use dask image for any array other than a numpy array (which isn't
# supported yet anyway)
else:
import dask.array as da
from dask_image.ndfilters import convolve as dask_convolve
if isinstance(bool_skeleton, da.Array):
degree_image = bool_skeleton * dask_convolve(
bool_skeleton.astype(int), degree_kernel, mode='constant'
)
return degree_image
def _simplify_graph(skel):
"""Iterative removal of all nodes of degree 2 while reconnecting their
edges.
Parameters
----------
skel : skan.csr.Skeleton
A Skeleton object containing graph to be simplified.
Returns
-------
simp_csgraph : scipy.sparse.csr_matrix
A sparse adjacency matrix of the simplified graph.
reduced_nodes : tuple of int
The index nodes of original graph in simplified graph.
"""
if np.sum(skel.degrees > 2) == 0: # no junctions
# don't reduce
return skel.graph, np.arange(skel.graph.shape[0])
summary = summarize(skel, separator='_')
src = np.asarray(summary['node_id_src'])
dst = np.asarray(summary['node_id_dst'])
distance = np.asarray(summary['branch_distance'])
# to reduce the size of simplified graph
nodes = np.unique(np.append(src, dst))
n_nodes = len(nodes)
nodes_sequential = np.arange(n_nodes)
fw_map = ArrayMap(nodes, nodes_sequential)
inv_map = ArrayMap(nodes_sequential, nodes)
src_relab, dst_relab = fw_map[src], fw_map[dst]
edges = sparse.coo_matrix((distance, (src_relab, dst_relab)),
shape=(n_nodes, n_nodes))
dir_csgraph = edges.tocsr()
simp_csgraph = dir_csgraph + dir_csgraph.T # make undirected
reduced_nodes = inv_map[np.arange(simp_csgraph.shape[0])]
return simp_csgraph, reduced_nodes
def _fast_graph_center_idx(skel):
"""Accelerated graph center finding using simplified graph.
Parameters
----------
skel : skan.csr.Skeleton
A Skeleton object containing graph whose center is to be found.
Returns
-------
original_center_idx : int
The index of central node of graph.
"""
simp_csgraph, reduced_nodes = _simplify_graph(skel)
simp_center_idx, _ = central_pixel(simp_csgraph)
original_center_idx = reduced_nodes[simp_center_idx]
return original_center_idx
def _normalize_shells(shells, *, center, skeleton_coordinates, spacing):
"""Normalize shells from any format allowed by `sholl_analysis` to radii.
Parameters
----------
shells : int or sequence of floats, or None
If an int, it is used as number of evenly spaced concentric shells. If
an array of floats, it is used directly as the different shell radii in
real world units. If None, the number of evenly spaced concentric
shells is automatically calculated.
center : (D,) array of float
The scaled coordinates of the center point for Sholl analysis.
skeleton_coordinates : (N, D) array of float
The scaled coordinates of skeleton pixels. Used when shells is None or
int.
spacing : (D,) array of float
The pixel/voxel spacing of the skeleton data.
Returns
-------
radii : array of float
The computed and normalized shell radii.
"""
if isinstance(shells, (list, tuple, np.ndarray)):
shell_radii = np.asarray(shells)
else: # shells is int, number of shells, or None
# Find max euclidean distance from center to all nodes
distances = np.linalg.norm(skeleton_coordinates - center, axis=1)
start_radius = 0
end_radius = np.max(distances) # largest possible radius
if shells is None:
stepsize = np.linalg.norm(spacing)
else: # scalar
stepsize = (end_radius-start_radius) / shells
epsilon = np.finfo(np.float32).eps
shell_radii = np.arange(start_radius, end_radius + epsilon, stepsize)
if (sp := np.linalg.norm(spacing)) > (sh := np.min(np.diff(shell_radii))):
warnings.warn(
'This implementation of Sholl analysis may not be accurate if '
'the spacing between shells is smaller than the (diagonal) '
f'voxel spacing. The given voxel spacing is {sp}, and the '
f'smallest shell spacing is {sh}.',
stacklevel=2
)
return shell_radii
[docs]
def sholl_analysis(skeleton, center=None, shells=None):
"""Sholl Analysis for Skeleton object.
Parameters
----------
skeleton : skan.csr.Skeleton
A Skeleton object.
center : array-like of float or None, optional
Scaled coordinates of a point on the skeleton to use as the center
from which the concentric shells are computed. If None, the
geodesic center of skeleton is chosen.
shells : int or array of floats or None, optional
If an int, it is used as number of evenly spaced concentric shells. If
an array of floats, it is used directly as the different shell radii in
real world units. If None, the number of evenly spaced concentric
shells is automatically calculated.
Returns
-------
center : array of float
The scaled coordinates in real world units of the center of the shells.
(This might be provided as input, but it might also have been computed
within this function, and that computation is expensive, so we return
it just in case.)
shell_radii : array of float
Radii in real world units for concentric shells used for analysis.
intersection_counts : array of int
Number of intersections for corresponding shell radii.
"""
if center is None:
# By default, find the geodesic center of the graph
center_idx = _fast_graph_center_idx(skeleton)
center = skeleton.coordinates[center_idx] * skeleton.spacing
else:
center = np.asarray(center)
scaled_coords = skeleton.coordinates * skeleton.spacing
shell_radii = _normalize_shells(
shells,
center=center,
skeleton_coordinates=scaled_coords,
spacing=skeleton.spacing,
)
edges = skeleton.graph.tocoo()
coords0 = scaled_coords[edges.row]
coords1 = scaled_coords[edges.col]
d0 = distance_matrix(coords0, [center]).ravel()
d1 = distance_matrix(coords1, [center]).ravel()
bins0 = np.digitize(d0, shell_radii)
bins1 = np.digitize(d1, shell_radii)
crossings = bins0 != bins1
shells = np.minimum(bins0[crossings], bins1[crossings])
# we divide by 2 because the graph is undirected, so each edge appears
# twice in the matrix
intersection_counts = np.bincount(shells, minlength=len(shell_radii)) // 2
return center, shell_radii, intersection_counts
[docs]
def skeleton_to_nx(
skeleton: Skeleton,
summary: pd.DataFrame | None = None,
) -> nx.MultiGraph:
"""Convert a Skeleton object to a networkx Graph.
Parameters
----------
skeleton : Skeleton
The skeleton to convert.
summary : pd.DataFrame | None
The summary statistics of the skeleton. Each row in the summary table
is an edge in the networkx graph. It is not necessary to pass this in
because it can be computed from the input skeleton, but if it is
already computed, it will speed up this function.
Returns
-------
g : nx.MultiGraph
A graph where each node is a junction or endpoint in the skeleton, and
each edge is a path.
"""
if summary is None:
summary = summarize(skeleton, separator='_')
# Ensure underscores in column names
csum = summary.rename(columns=lambda s: s.replace('-', '_'))
g = nx.MultiGraph(
shape=skeleton.skeleton_shape, dtype=skeleton.skeleton_dtype
)
for row in csum.itertuples(name='Edge'):
index = row.Index
i = row.node_id_src
j = row.node_id_dst
indices, values = skeleton.path_with_data(index)
# Nodes are added if they don't exist so only need to add edges
g.add_edge(
i, j, **{
'path': skeleton.path_coordinates(index),
'indices': indices,
'values': values,
}
)
return g
[docs]
def nx_to_skeleton(g: nx.Graph | nx.MultiGraph) -> Skeleton:
"""Convert a Networkx Graph to a Skeleton object.
The NetworkX Graph should have the following graph properties:
- 'shape': the shape of the image from which the graph was created.
- 'dtype': the dtype of the image from which the graph was created.
and the following edge properties:
- 'path': an (N, Ndim) array of coordinates in the image of the pixels
traced by this edge.
- 'values': an (N,) array of values for the pixels traced by this edge.
The `skeleton_to_nx` function can produce such a graph from a `Skeleton`
object.
Parameters
----------
g: nx.Graph
Networkx graph to convert to Skeleton.
Returns
-------
Skeleton
Skeleton object corresponding to the input graph.
Notes
-----
Currently, this method uses the naive, brute-force approach of regenerating
the entire image from the graph, and then generating the Skeleton from the
image. It is probably low-hanging fruit to instead generate the Skeleton
compressed-sparse-row (CSR) data directly.
"""
image = np.zeros(g.graph['shape'], dtype=g.graph['dtype'])
all_coords = np.concatenate([path for _, _, path in g.edges.data('path')],
axis=0)
all_values = np.concatenate([
values for _, _, values in g.edges.data('values')
],
axis=0)
image[tuple(all_coords.T)] = all_values
return Skeleton(image)
def _merge_paths(p1: npt.NDArray, p2: npt.NDArray):
"""Join two paths together that have a common endpoint."""
return np.concatenate([p1[:-1], p2], axis=0)
def _merge_edges(g: nx.Graph, e1: tuple[int], e2: tuple[int]):
middle_node = set(e1) & set(e2)
new_edge = sorted(
(set(e1) | set(e2)) - {middle_node},
key=lambda i: i in e2,
)
d1 = g.edges[e1]
d2 = g.edges[e2]
p1 = d1['path'] if e1[1] == middle_node else d1['path'][::-1]
p2 = d2['path'] if e2[0] == middle_node else d2['path'][::-1]
n1 = len(d1['path'])
n2 = len(d2['path'])
new_edge_values = {
'skeleton_id':
g.edges[e1]['skeleton_id'],
'node_id_src':
new_edge[0],
'node_id_dst':
new_edge[1],
'branch_distance':
d1['branch_distance'] + d2['branch_distance'],
'branch_type':
min(d1['branch_type'], d2['branch_type']),
'mean_pixel_value': (
n1 * d1['mean_pixel_value'] + n2 * d2['mean_pixel_value']
) / (n1+n2),
'stdev_pixel_value':
np.sqrt((
d1['stdev_pixel_value']**2 *
(n1-1) + d2['stdev_pixel_value']**2 * (n2-1)
) / (n1+n2-1)),
'path':
_merge_paths(p1, p2),
}
g.add_edge(new_edge[0], new_edge[1], **new_edge_values)
g.remove_node(middle_node)
def _remove_simple_path_nodes(g):
"""Remove any nodes of degree 2 by merging their incident edges."""
to_remove = [n for n in g.nodes if g.degree(n) == 2]
for u in to_remove:
v, w = g[u].keys()
_merge_edges(g, (u, v), (u, w))
```