RANDOM WALKS FOR
IMAGE SEGMENTATION
IEEE Transaction on pattern analysis and machine
intelligence, November 2006
Leo Grady, Member, IEEE
Outline
Introduction
Algorithm
Dirichlet
Problem
Behavioral Properties
Result

Demo
2
Introduction
K

way image segmentation
User

defined
seeds
user

defined
labels
How to labels and unseeded pixel ?
K objects
3
Introduction
Algorithm is resolving the question:
Given a random walker starting at this location, what is
the probability that it first reaches each of the K seed
points ?
Probability = ? A random walker first reach each of
the K seed points.
K

tuple
vector
A pixel
Seed point
Probability?
4
Introduction
5
The probability a random walker first
reaches a seed point.
The solution to the
Dirichelet
problem
with boundary conditions at the locations
of the seed points and the seed point in
question fixed to unity while the others
are set to zero.
Introduction
Goal
1. location of weak (or missing) boundaries.
2. noise robustness.
3. ability to identify multiple objects simultaneously.
4. fast computation (and editing).
5. avoidance of small/trivial solutions.
6
Algorithm
1. generating the graph weights
2. establishing the system of equations to solve the
problem.
3. the practical details of implementation.
7
Defining a graph
Graph
G = ( V , E )
And edge, e, spanning two vertices, v
i
and
v
j
, is
denoted by
e
ij
.
The weight of an edge,
e
ij
, is denoted by
w
(
e
ij
) or
w
ij
.
Degree
of a vertex is
Assume this graph is
connected
and
undirected
.
8
Edge Weights
9
Gaussian weighting function
The only free parameter
g
i
indicates the image
intensity at pixel
i
Combinatorial
Dirichlet
Problem
10
The
Dirichlet
integral
A
harmonic function
is a function that satisfies the
Laplace equation
Dirichlet
problem

finding a harmonic function
subject to its boundary values.
Combinatorial
Dirichlet
Problem(cont.)
11
Combinatorial
Laplacian
matrix
Where
L
ij
is indexed by vertices v
i
and
v
j
.
The
m
x
n
edge

node
incidence matrix
as
Incidence matrix is indexed by edge
e
ij
and node
v
k
.
Combinatorial
Dirichlet
Problem(cont.)
12
A combinatorial formulation of the
Dirichlet
integral
C is the
m
x
m
constitutive matrix
(the diagonal matrix
with the weights of each edge along the diagonal).
Combinatorial
Dirichlet
Problem(cont.)
13
Partition the vertices into two sets,
V
M
(marked/seed nodes)
V
U
(unseeded nodes)
Finding the critical point yields
Combinatorial
Dirichlet
Problem(cont.)
14
The probability (potential) assumed at node, v
i
, for
each label, s, by .
Define the set of labels for the seed points as a
function
Define the vector for each label, s, at node
as
Solving the combinatorial
Dirichlet
problem
15
For one label
For all labels
X has K columns taken by each and M has
colums
given by each
Equivalences between random walks
and electrical circuits
16
Three fundamental equations of circuit theory.
These three equations may be combined into the
linear system
It is equivalent to with f = 0.
Algorithm Summary
17
1.
Using , map the image
intensities to edge weights in the lattice.
2.
Obtain a set,
V
M
, of marked (labeled) pixels with
K labels, either interactively or automatically.
3.
Solve outright for the potentials or
solve for each label except the
final one, f. Set
4.
Obtain a final segmentation by assigning to each
node, v
i
, the label corresponding to
Overview of segmentation computation
18
Analogies
19
Assigns an unseeded pixel to a label, given a weighted
graph:
If a random walker leaving the pixel is most likely to first
reach a bearing label
s
, assign the pixel to label
s
.
If the seeds are alternately replaced by grounds/unit
voltage sources, assign the pixel to the label for which its
seeds being “on” produces the greatest electrical potential.
Assign the pixel to the label for which its seeds have the
largest
effective conductance
.
If a
2

tree
is drawn randomly from the graph, assign the
pixel to the label for which the pixel is most likely to remain
connected to.
Effective Conductance
20
Effective conductance
Dirichlet
integral
equals the effective conductance
between nodes labeled “1” (“on”) and those
labeled “0” (“off”).
x is intended to include both
x
M
and
x
U
i
j
i
j
equals
Unit voltage
Current flow
Effective Conductance (cont.)
21
Effective conductance between two nodes, v
i
,
v
j
is
given by
where T is a set of edges defining a connected tree
the sum is over all possible trees in the graph
Where TT(
i
,
j
) is used to represent the set of edges
defining a
2

tree
, such that node v
i
is in one component
and
v
j
is in another.
2

tree
22
A
2

tree
is defined to be a tree with one edge
removed.
v
i
and
v
j
are indifferent components and
v
t
is in the
same component as
v
j
.
Note that
2

tree (cont.)
23
Then, the following expressions are equivalent
The segmentation is computed from the potentials
by assigning the pixel to the label for which it has
greatest potential (probability).
Behavioral Properties
24
1.
Weak Boundary detection
2.
Noise robustness
3.
Assignment of ambiguous regions
Weak Boundaries
25
Weak Boundaries

Comparison
26
Noise Robustness
27
Ambiguous Unseeded Regions
28
Demo Videos
29
http://www.cns.bu.edu/~lgrady/Random_Walker_I
mage_Segmentation.html
Brain
Lung tumor
Aorta

3D
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Definition of a harmonic function
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