Chapter 9 morphological image processing

Preview
 “Morphology “ – a branch in biology that deals with the form and
structure of animals and plants.
 “Mathematical Morphology” – as a tool for extracting
image components, that are useful in the representation and
description of region shape
 What are the applications of Morphological Image Filtering?
boundaries extraction
skeletons
convex hull
morphological filtering
thinning
Pruning
 The language of mathematical morphology is – Set theory.
 Unified and powerful approach to numerous image processing
problems

Sets in mathematical morphology represent objects in an
image:
 binary image (0 = black, 1 = white) :
the element of the set is the coordinates (x,y) of pixel
belong to the object Z2
 gray-scaled image :
the element of the set is the coordinates (x,y) of pixel
belong to the object and the gray levels Z3
Image Attributes Extracted from ImageMorphological
Image
Processing

9.1 Basic Concepts in Set Theory
 Subset
 Union
 Intersection
disjoint / mutually exclusive
 Complement
 Difference
 Reflection
 Translation

Logic Operations Involving Binary
Pixels and Images
 The principal logic operations used in image processing
are: AND, OR, NOT (COMPLEMENT).
 These operations are functionally complete.
 Logic operations are preformed on a pixel by pixel basis
between corresponding pixels (bitwise).
 Other important logic operations :
XOR (exclusive OR), NAND (NOT-AND)
 Logic operations are just a private case for a binary set
operations, such : AND – Intersection , OR – Union,
NOT-Complement.

Structuring Elements (SE)
 Structuring Elements(SE): small sets or subimages used to probe
an image under study for properties of interest.
 The origin of the SE is indicated by a black dot.
 When SE is symmetric and no dot is show the origin is at the center
of symmetry.
 When SE is symmetric B = (reflection of B)
 When working with images
 SE must be in a Rectangular Arrays (padding with the smallest
possible number of background elements).
 Images must be in a Rectangular Arrays (padding with the
smallest possible number of background elements).
 For Images a background border is provided to accommodate
the entire SE when its origin is on the border.

9.2.1 Erosion
 Erosion is used for shrinking of element A by using element B
 Erosion for Sets A and B in Z2, is defined by the
following equation:
 This equation indicates that the erosion of A by B is the set of
all points z such that B, translated by z, is contained in A.
 Erosion can be used to
 Shrinks or thins objects in binary images
 Remove image components(how?)
 Erosion is a morphological filtering operation in which
image details smaller than the structuring elements are
filtered(removed)

Suppose that the structuring element is a 3×3 square
Note that in subsequent diagrams, foreground pixels are
represented by 1's and background pixels by 0's.
The structuring element is now superimposed over each
foreground pixel ( input pixel ) in the image. If all the
pixels below the structuring element are foreground
pixels then the input pixel retains it’s value. But if any
of the pixels is a background pixel then the input pixel
gets the background pixel value.
1 1 1
1 1 1
1 1 1
Structuring element

9.2.2 Dilation
 Dilation is used for expanding an element A by using structuring
element B
 Dilation of A by B and is defined by the following equation:
 This equation is based 0n obtaining the reflection 0f B
about its origin and shifting this reflection by z.
 The dilation of A by B is the set of all displacements z,
such that and A overlap by at least one element. Based
On this interpretation the equation of (9.2-1) can be
rewritten as:
 Relation to Convolution mask:
- Flipping
- Overlapping

Suppose that the structuring element is a 3×3 square .
Note that in subsequent diagrams, foreground pixels are
represented by 1's and background pixels by 0's.
To compute the dilation of a binary input image by this structuring
element, we superimpose the structuring element on top of the
input image so that the origin of the structuring element
coincides with the input pixel position.
If the center pixel in the structuring element coincides with a
foreground pixel in the image underneath, then the input pixel
is set to the foreground value.
1 1 1
1 1 1
1 1 1
Structuring element

9.2.2 Dilation – A More interesting
Example (bridging gaps)

Duality between dilation and erosion
 Dilation and erosion are duals of each other with respect to set
complementation and reflection. That,
Prove of (9.2-5)
 One of the simplest uses of erosion is for eliminating irrelevant details
(in terms of size) from a binary image

Erosion vs. Dilation
 Erosion:
 Shrinks or thins objects in binary images
 Remove image components(how?)
 Erode away the boundaries of regions of foreground
pixels
 Areas of foreground pixels shrink in size, and holes
within those areas become larger
 Dilation:
 Grows or thickens object in a binary image
 Bridging gaps
 Fill small holes of sufficiently small size

9.3 Opening And Closing
 Opening – smoothes contours , eliminates protrusions
 Closing – smoothes sections of contours, fuses narrow
breaks and long thin gulfs, eliminates small holes and
fills gaps in contours
 These operations are dual to each other
 These operations are can be applied few times, but has
effect only once

 Opening –
 First – erode A by B, and then dilate the result by B
 In other words, opening is the unification of all B objects
Entirely Contained in A

 Closing –
 First – dilate A by B, and then erode the result by B
 In other words, closing is the group of points, which the
intersection of object B around them with object A – is
not empty

Use of opening and closing for morphological filtering

9.4 The Hit-or-Miss Transformation
 A basic morphological tool for shape detection.
 Let the origin of each shape be located at its center of
gravity.
 If we want to find the location of a shape , say – X ,
at (larger) image, say – A :
 Let X be enclosed by a small window, say – W.
 The local background of X with respect to W is defined as
the set difference (W - X).
 Apply erosion operator of A by X, will get us the set of
locations of the origin of X, such that X is completely
contained in A.
 It may be also view geometrically as the set of all locations of
the origin of X at which X found a match (hit) in A.

Cont.
 Apply erosion operator on the complement of A by the local
background set (W – X).
 Notice, that the set of locations for which X exactly fits inside
A is the intersection of these two last operators above.
This intersection is precisely the location sought.
Formally:
If B denotes the set composed of X and it’s background –
B = (B1,B2) ; B1 = X , B2 = (W-X).
The match (or set of matches) of B in A, denoted is:

 The reason for using these kind of structuring element –
B = (B1,B2) is based on an assumed definition that,
two or more objects are distinct only if they are disjoint
(disconnected) sets.
 In some applications , we may interested in detecting
certain patterns (combinations) of 1’s and 0’s.
and not for detecting individual objects.
 In this case a background is not required.
and the hit-or-miss transform reduces to simple erosion.
 This simplified pattern detection scheme is used in some of
the algorithms for – identifying characters within a text.

9.5 Basic Morphological Algorithms
(Applications)
 1 – Boundary Extraction
 2 – Region Filling
 3 – Extraction of Connected Components
 4 – Convex Hull
 5 – Thinning
 6 – Thickening
 7 – Skeletons
 8 – Pruning

9.5.1 Boundary Extraction
 First, erode A by B, then make set difference between
A and the erosion
 The thickness of the contour depends on the size of
constructing object – B

9.5.2 Region Filling
 This algorithm is based on a set of dilations,
complementation and intersections
 p is the point inside the boundary(given) , with the value of 1

 The process stops when
 The result that given by union of A and X(k), is a set contains
the filled set and the boundary

9.5.3 Extraction of Connected
Components
 This algorithm extracts a component by selecting a
point on a binary object A
 Works similar to region filling, but this time we use in
the conjunction the object A, instead of it’s
complement

This shows automated
inspection of chicken-
breast, that contains
bone fragment
The original image is
thresholded
We can get by using
this algorithm the
number of pixels in
each of the connected
components
Now we could know if
this food contains big
enough bones and
prevent hazards

9.5.4 Convex Hull
 A is said to be convex if a straight line segment joining any
two points in A lies entirely within A
 The convex hull H of set S is the smallest convex set
containing S
 Convex deficiency is the set difference H-S
 Useful for object description
 This algorithm iteratively:
 Apply the hit-or-miss transform to A with the first B element
Unions it with A
Repeat with second B element
Let The convex hull is :

9.5.5 Thinning
 The thinning of a set A by a structuring element B, can
be defined by terms of the hit-and-miss transform:
 9.5-6
 A more useful expression for thinning A symmetrically
is based on a sequence of structuring elements:
{B}={B1, B2, B3, …, Bn}
 Where Bi is a rotated version of Bi-1. Using this concept
we define thinning by a sequence of structuring
elements:

9.5.5 Thinning cont
 The process is to thin by one pass with B1 , then thin
the result with one pass with B2, and so on until A is
thinned with one pass with Bn.
 The entire process is repeated until no further changes
occur.
 Each pass is preformed using the equation:

9.5.6 Thickening
 Thickening is a morphological dual of thinning.
 Definition of thickening .
 As in thinning, thickening can be defined as a
sequential operation:
 the structuring elements used for thickening have the
same form as in thinning, but with all 1’s and 0’s
interchanged.

9.5.6 Thickening - cont
 A separate algorithm for thickening is often used in
practice, Instead the usual procedure is to thin the
background of the set in question and then complement
the result.
 In other words, to thicken a set A, we form C=Ac , thin C
and than form Cc.
 depending on the nature of A, this procedure may result in
some disconnected points. Therefore thickening by this
procedure usually require a simple post-processing step to
remove disconnected points.

9.5.6 Thickening example preview
 We will notice in the next example 9.22(c) that the
thinned background forms a boundary for the
thickening process, this feature does not occur in the
direct implementation of thickening
 This is one of the reasons for using background
thinning to accomplish thickening.

9.5.7 Skeleton
 The notion of a skeleton of a set A :S(A) is intuitively
defined, we deduce from this figure that:
a) If z is a point of S(A) and (D)z is the largest disk
centered at z and contained in A (one cannot find a
larger disk that fulfils this terms) – this disk is called
“maximum disk”.
b) The disk (D)z touches the boundary of A at two or
more different places.

9.5.7 Skeleton
 The skeleton of A is defined by terms of erosions and
openings:
 with
 Where B is the structuring element and indicates k
successive erosions of A:
 k times, and K is the last iterative step before A erodes to an empty
set, in other words:
 in conclusion S(A) can be obtained as the union of skeleton
subsets Sk(A).

9.5.7 Skeleton
 A can be also reconstructed from subsets Sk(A) by
using the equation:
 Where denotes k successive dilations of
Sk(A) that is:

9.5.8 Pruning
 Removing parasitic component
 Complement to thinning and skeletonizing by successive
elimination its end point.

Morphological Reconstruction
 Involve two images and SE
 Marker(F): starting point for transformation.
 Mask(G): constrains the transformation.
 SE: define connectivity.
 F and G are both binary images and

Geodesic Dilation & Erosion
 The geodesic dilation of size 1 of the marker image F with
respect to the mask G
 Where ∩ is logical AND
 The geodesic dilation of size n of the F with respect to G
 Where
 The geodesic erosion of size 1 of the marker image F with
respect to the mask G
 Where ∪ is logical OR
 The geodesic erosion of size n of the F with respect to G
 Where

Morphological reconstruction by
dilation & erosion
(1) Morphological reconstruction by dilation of a mask image
G from a marker image F
(1) Morphological reconstruction by erosion of a mask image
G from a marker image F

Sample Applications
 (1) Opening by reconstruction:
 Restore exactly the shapes of the objects that remain after
erosion.
 The opening by reconstruction of size n of an image F is the
reconstruction by dilation of F from the erosion of size n of F:
 (2) Filling holes
 If I(x,y) is a binary image A marker image F is defined:
 is a binary image equal to I with all holes filled.

Sample Apps. Cont.
 (3) Border clearing:
1. Removing objects that touch (are connected to)the
border
2. Subtract from original image

9.6 Gray-Scale Images
 f(x,y): the input image
 b(x,y): a structuring element (a subimage function)
 (x,y) are integers.
 f(x,y) and b(x,y) are functions that assign a gray-level
value (real number or real integer) to each distinct pair
of coordinate (x,y)
 For example the domain of gray values can be 0-255,
whereas 0 – is black, 255- is white.

9.6.1 Erosion – Gray-Scale
 The erosion of f by a flat structuring element b at any
location (x,y) is defined as the minimum value of the
image in the region coincides with b when the origin
of b is at (x,y).
 The erosion of f by a non flat structuring element :

9.6.1 Erosion– Gray-Scale example 1

9.6.1 Erosion– Gray-Scale (cont)
 General effect of performing an erosion in grayscale
images:
1. If all elements of the structuring element are positive, the
output image tends to be darker than the input image.
2. The effect of bright details in the input image that are
smaller in area than the structuring element is reduced,
with the degree of reduction being determined by the
grayscale values surrounding by the bright detail and by
shape and amplitude values of the structuring element
itself.

9.6.1 Dilation – Gray-Scale
 The Dilation of f by a flat structuring element b at any
location (x,y) is defined as the maximum value of the
image in the region outlined by when the origin of is
at (x,y).
 The Dilation of f by a non flat structuring element :

9.6.1 Dilation – Gray-Scale (cont)
 The general effects of performing dilation on a gray
scale image is twofold:
1. If all the values of the structuring elements are positive
than the output image tends to be brighter than the
input.
2. Dark details either are reduced or eliminated,
depending on how their values and shape relate to the
structuring element used for dilation

9.6.1 Dilation – Gray-Scale example

9.6.1 Dilation & Erosion– Gray-Scale
 Similar to binary image grayscale erosion and dilation
are duals with respect to function complementation and
reflection.

9.6.2 Opening And Closing
 Similar to the binary algorithms
 Opening –
 Closing –
 In the opening of a gray-scale image, we remove
small light details, while relatively undisturbed
overall gray levels and larger bright features
 In the closing of a gray-scale image, we remove small
dark details, while relatively undisturbed overall gray
levels and larger dark features

 Opening a G-S picture is describable as pushing object
B under the scan-line graph, while traversing the
graph according the curvature of B

 Closing a G-S picture is describable as pushing object
B on top of the scan-line graph, while traversing the
graph according the curvature of B
 The peaks are usually
remains in their
original form

Opening: Decreased size
of small bright details. No
changes to dark region
Closing: Decreased size
of small dark details. No
changes to bright region

9.6.3 Some Applications of Gray-Scale
Morphology
1) Morphological smoothing
2) Morphological gradient
3) Top-hat & bottom-hat transformation
4) Textural segmentation
5) Granulometry

9.6.3 Some Applications of Gray-
Scale Morphology
 Morphological smoothing
 Perform opening followed by a closing
 The net result of these two operations is to remove or
attenuate both bright and dark artifacts or noise.
 Morphological gradient
 Dilation and erosion are use to compute the morphological
gradient of an image, denoted g:
 It uses to highlights sharp gray-level transitions in the
input image.
 Obtained using symmetrical structuring elements tend to
depend less on edge directionality.

Scale Morphology
 Morphological smoothing
 Morphological gradient

Scale Morphology
 Top-hat transformation
 Called (white top-hat) and Is defined as:
 Detect structures of a certain size.
 Light objects on a dark background.
 Bottom-hat transformation
 Called (black top-hat )and Is defined as:
 Detect structures of a certain size.
 Dark objects on a bright background

Scale Morphology
 Top-hat transformation
 Bottom-hat transformation

Scale Morphology
 Textural segmentation
 The objective is to find the boundary between
different image regions based on their textural
content.
1. Performing closing with successive larger structuring
element till removing the small blobs
2. Performing Simple opening
3. Performing Simple thresholding

Scale Morphology
 Granulometry
 Granulometry is a field that deals principally with
determining the size distribution of particles in an
image.
 Because the particles are lighter than the background, we can
use a morphological approach to determine size distribution.
To construct at the end a histogram of it.
 Based on the idea that opening operations of particular size
have the most effect on regions of the input image that
contain particles of similar size.
 This type of processing is useful for describing regions
with a predominant particle-like character.

Scale Morphology
 Granulometry

9.6.4 Gray-Scale Morphological
Reconstruction
 Let f and g denote the marker and mask images
respectively and
 The geodesic erosion of size 1 of f with respect to g:
 where V denotes the point-wise maximum operator
 The geodesic erosion of size n of f with respect to g:
 With
 Reconstruction by erosion of g by f:


Reconstruction
 Let f and g denote the marker and mask images
respectively and
 The geodesic dilation of size 1 of f with respect to g:
 where ^ denotes the point-wise minimum operator
 The geodesic dilation of size n of f with respect to g:
 With
 Reconstruction by dilation of g by f:

Reconstruction
 Opening by reconstruction of size n of an image f:
 Closing by reconstruction of size n of an image f:
 Top-hat by reconstruction:
 Subtracting from an image opening by reconstruction

Chapter 9 morphological image processing

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