The max-tree is a hierarchical representation of an image that is the basis for a large family of morphological filters.

If we apply a threshold operation to an image, we obtain a binary image containing one or several connected components. If we apply a lower threshold, all the connected components from the higher threshold are contained in the connected components from the lower threshold. This naturally defines a hierarchy of nested components that can be represented by a tree. whenever a connected component A obtained by thresholding with threshold t1 is contained in a component B obtained by thresholding with threshold t1 < t2, we say that B is the parent of A. The resulting tree structure is called a component tree. The max-tree is a compact representation of such a component tree. 1, 2, 3, 4

In this example we give an intuition of what a max-tree is.



Salembier, P., Oliveras, A., & Garrido, L. (1998). Antiextensive Connected Operators for Image and Sequence Processing. IEEE Transactions on Image Processing, 7(4), 555-570. DOI:10.1109/83.663500


Berger, C., Geraud, T., Levillain, R., Widynski, N., Baillard, A., Bertin, E. (2007). Effective Component Tree Computation with Application to Pattern Recognition in Astronomical Imaging. In International Conference on Image Processing (ICIP) (pp. 41-44). DOI:10.1109/ICIP.2007.4379949


Najman, L., & Couprie, M. (2006). Building the component tree in quasi-linear time. IEEE Transactions on Image Processing, 15(11), 3531-3539. DOI:10.1109/TIP.2006.877518


Carlinet, E., & Geraud, T. (2014). A Comparative Review of Component Tree Computation Algorithms. IEEE Transactions on Image Processing, 23(9), 3885-3895. DOI:10.1109/TIP.2014.2336551

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.lines import Line2D
from skimage.morphology import max_tree
import networkx as nx

Before we start : a few helper functions

def plot_img(ax, image, title, plot_text, image_values):
    """Plot an image, overlaying image values or indices."""
    ax.imshow(image, cmap='gray', aspect='equal', vmin=0, vmax=np.max(image))

    for x in np.arange(-0.5, image.shape[0], 1.0):
        ax.add_artist(Line2D((x, x), (-0.5, image.shape[0] - 0.5),
                             color='blue', linewidth=2))

    for y in np.arange(-0.5, image.shape[1], 1.0):
        ax.add_artist(Line2D((-0.5, image.shape[1]), (y, y),
                             color='blue', linewidth=2))

    if plot_text:
        for i, j in np.ndindex(*image_values.shape):
            ax.text(j, i, image_values[i, j], fontsize=8,

def prune(G, node, res):
    """Transform a canonical max tree to a max tree."""
    value = G.nodes[node]['value']
    res[node] = str(node)
    preds = [p for p in G.predecessors(node)]
    for p in preds:
        if (G.nodes[p]['value'] == value):
            res[node] += ', %i' % p
            prune(G, p, res)
    G.nodes[node]['label'] = res[node]

def accumulate(G, node, res):
    """Transform a max tree to a component tree."""
    total = G.nodes[node]['label']
    parents = G.predecessors(node)
    for p in parents:
        total += ', ' + accumulate(G, p, res)
    res[node] = total
    return total

def position_nodes_for_max_tree(G, image_rav, root_x=4, delta_x=1.2):
    """Set the position of nodes of a max-tree.

    This function helps to visually distinguish between nodes at the same
    level of the hierarchy and nodes at different levels.
    pos = {}
    for node in reversed(list(nx.topological_sort(canonical_max_tree))):
        value = G.nodes[node]['value']
        if canonical_max_tree.out_degree(node) == 0:
            # root
            pos[node] = (root_x, value)

        in_nodes = [y for y in canonical_max_tree.predecessors(node)]

        # place the nodes at the same level
        level_nodes = [y for y in
                       filter(lambda x: image_rav[x] == value, in_nodes)]
        nb_level_nodes = len(level_nodes) + 1

        c = nb_level_nodes // 2
        i = - c
        if (len(level_nodes) < 3):
            hy = 0
            m = 0
            hy = 0.25
            m = hy / (c - 1)

        for level_node in level_nodes:
            if(i == 0):
                i += 1
            if (len(level_nodes) < 3):
                pos[level_node] = (pos[node][0] + i * 0.6 * delta_x, value)
                pos[level_node] = (pos[node][0] + i * 0.6 * delta_x,
                                   value + m * (2 * np.abs(i) - c - 1))
            i += 1

        # place the nodes at different levels
        other_level_nodes = [y for y in
                             filter(lambda x: image_rav[x] > value, in_nodes)]
        if (len(other_level_nodes) == 1):
            i = 0
            i = - len(other_level_nodes) // 2
        for other_level_node in other_level_nodes:
            if((len(other_level_nodes) % 2 == 0) and (i == 0)):
                i += 1
            pos[other_level_node] = (pos[node][0] + i * delta_x,
            i += 1

    return pos

def plot_tree(graph, positions, ax, *, title='', labels=None,
              font_size=8, text_size=8):
    """Plot max and component trees."""
    nx.draw_networkx(graph, pos=positions, ax=ax,
                     node_size=40, node_shape='s', node_color='white',
                     font_size=font_size, labels=labels)
    xlimit = ax.get_xlim()
    for v in range(image_rav.min(), image_rav.max() + 1):
        ax.hlines(v - 0.5, -3, 10, linestyles='dotted')
        ax.text(-3, v - 0.15, "val: %i" % v, fontsize=text_size)
    ax.hlines(v + 0.5, -3, 10, linestyles='dotted')
    ax.set_xlim(-3, 10)

Image Definition

We define a small test image. For clarity, we choose an example image, where image values cannot be confounded with indices (different range).

image = np.array([[40, 40, 39, 39, 38],
                  [40, 41, 39, 39, 39],
                  [30, 30, 30, 32, 32],
                  [33, 33, 30, 32, 35],
                  [30, 30, 30, 33, 36]], dtype=np.uint8)


Next, we calculate the max-tree of this image. max-tree of the image

P, S = max_tree(image)

P_rav = P.ravel()

Image plots

Then, we visualize the image and its raveled indices. Concretely, we plot the image with the following overlays: - the image values - the raveled indices (serve as pixel identifiers) - the output of the max_tree function

# raveled image
image_rav = image.ravel()

# raveled indices of the example image (for display purpose)
raveled_indices = np.arange(image.size).reshape(image.shape)

fig, (ax1, ax2, ax3) = plt.subplots(1, 3, sharey=True, figsize=(9, 3))

plot_img(ax1, image - image.min(), 'Image Values',
         plot_text=True, image_values=image)
plot_img(ax2, image - image.min(), 'Raveled Indices',
         plot_text=True, image_values=raveled_indices)
plot_img(ax3, image - image.min(), 'Max-tree indices',
         plot_text=True, image_values=P)

Visualizing threshold operations

Now, we investigate the results of a series of threshold operations. The component tree (and max-tree) provide representations of the inclusion relationships between connected components at different levels.

fig, axes = plt.subplots(3, 3, sharey=True, sharex=True, figsize=(6, 6))
thresholds = np.unique(image)
for k, threshold in enumerate(thresholds):
    bin_img = image >= threshold
    plot_img(axes[(k // 3), (k % 3)], bin_img, 'Threshold : %i' % threshold,
             plot_text=True, image_values=raveled_indices)

Max-tree plots

Now, we plot the component and max-trees. A component tree relates the different pixel sets resulting from all possible threshold operations to each other. There is an arrow in the graph, if a component at one level is included in the component of a lower level. The max-tree is just a different encoding of the pixel sets.

  1. the component tree: pixel sets are explicitly written out. We see for instance that {6} (result of applying a threshold at 41) is the parent of {0, 1, 5, 6} (threshold at 40).

  2. the max-tree: only pixels that come into the set at this level are explicitly written out. We therefore will write {6} -> {0,1,5} instead of {6} -> {0, 1, 5, 6}

  3. the canonical max-treeL this is the representation which is given by our implementation. Here, every pixel is a node. Connected components of several pixels are represented by one of the pixels. We thus replace {6} -> {0,1,5} by {6} -> {5}, {1} -> {5}, {0} -> {5} This allows us to represent the graph by an image (top row, third column).

# the canonical max-tree graph
canonical_max_tree = nx.DiGraph()
for node in canonical_max_tree.nodes():
    canonical_max_tree.nodes[node]['value'] = image_rav[node]
canonical_max_tree.add_edges_from([(n, P_rav[n]) for n in S[1:]])

# max-tree from the canonical max-tree
nx_max_tree = nx.DiGraph(canonical_max_tree)
labels = {}
prune(nx_max_tree, S[0], labels)

# component tree from the max-tree
labels_ct = {}
total = accumulate(nx_max_tree, S[0], labels_ct)

# positions of nodes : canonical max-tree (CMT)
pos_cmt = position_nodes_for_max_tree(canonical_max_tree, image_rav)

# positions of nodes : max-tree (MT)
pos_mt = dict(zip(nx_max_tree.nodes, [pos_cmt[node]
                                      for node in nx_max_tree.nodes]))

# plot the trees with networkx and matplotlib
fig, (ax1, ax2, ax3) = plt.subplots(1, 3, sharey=True, figsize=(20, 8))

plot_tree(nx_max_tree, pos_mt, ax1, title='Component tree',
          labels=labels_ct, font_size=6, text_size=8)

plot_tree(nx_max_tree, pos_mt, ax2, title='Max tree', labels=labels)

plot_tree(canonical_max_tree, pos_cmt, ax3, title='Canonical max tree')


Total running time of the script: ( 0 minutes 1.731 seconds)

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