Digital Image Processing: Image Segmentation

CSC447: Digital Image
Processing
Chapter 10:
Prof. Dr. Mostafa Gadal-Haqq M. Mostafa
Computer Science Department
Faculty of Computer & Information Sciences
AIN SHAMS UNIVERSITY

 Segmentation attempts to partition the pixels of
an image into groups that strongly correlate
with the objects in an image
 Typically the first step in any automated
computer vision application
Image Segmentation
2CSC447: Digital Image Processing Prof. Dr. Mostafa GadalHaqq.

Image Segmentation
• Segmentation algorithms generally are based
on one of two basis properties of intensity
values
• Discontinuity: to partition an image based
on abrupt changes in intensity (such as
edges)
• Similarity: to partition an image into regions
that are similar according to a set of
predefined criteria.

Image Segmentation
 Image Segmentation

Image Segmentation
 Detection of discontinuities:
 There are three basic types of gray-level
discontinuities:
 points , lines , edges
 the common way is to run a mask through
the image

Point Detection:
• Note that the mark is the same as the mask of
Laplacian Operation (in chapter 3)
• The only differences that are considered of
interest are those large enough (as determined
by T) to be considered isolated points.
|R| >T

Point Detection:

Line Detection
• Horizontal mask will result with max response when a line
passed through the middle row of the mask with a constant
background.
• the similar idea is used with other masks.
• note: the preferred direction of each mask is weighted with
a larger coefficient (i.e.,2) than other possible directions.
R1 R2 R3 R4

Line Detection
• Apply every masks on the image
• let R1, R2, R3, R4 denotes the response of
the horizontal, +45 degree, vertical and -45
degree masks, respectively.
• if, at a certain point in the image
|Ri| > |Rj|, for all j≠i,
• that point is said to be more likely
associated with a line in the direction of
mask i.

Line Detection
• Alternatively, if we are interested in detecting all
lines in an image in the direction defined by a
given mask, we simply run the mask through the
image and threshold the absolute value of the
result.
• The points that are left are the strongest
responses, which, for lines one pixel thick,
correspond closest to the direction defined by
the mask.

Line Detection

Edge Detection Approach
 Segmentation by finding pixels on a region
boundary.
 Edges found by looking at neighboring pixels.
 Region boundary formed by measuring gray
value differences between neighboring pixels

Edge Detection
• an edge is a set of connected pixels that
lie on the boundary between two regions.
• an edge is a “local” concept whereas a
region boundary, owing to the way it is
defined, is a more global idea.

Edge Detection

Edge Detection
 Detection of discontinuities: Image Derivatives

Edge Detection
• First column: images and gray-
level profiles of a ramp edge
corrupted by random Gaussian
noise of mean 0 and = 0.0,
0.1, 1.0 and 10.0, respectively.
• Second column: first-derivative
images and gray-level profiles.
• Third column : second-
derivative images and gray-
level profiles.

Edge Detection
 Gradient Operator
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Edge Detection
 Prewitt and Sobel Operators

Edge Detection

Edge Detection
 The Laplacian

Edge Detection

Edge Detection
 The Laplacian of Gaussian (LoG)

The Hough Transform

The Hough Transform
 Global processing: The Hough Transform

The Hough Transform

Region-Based Segmentation

What is a Region?
 Basic definition :- A group of connected
pixels with similar properties.
 Important in interpreting an image because
they may correspond to objects in a scene.
 For that an image must be partitioned into
regions that correspond to objects or parts of
an object.

Region-Based vs. Edge-Based
Region-Based
 Closed boundaries
 Multi-spectral
images improve
segmentation
 Computation based
on similarity
Edge-Based
 Boundaries formed
not necessarily
closed
 No significant
improvement for
multi-spectral images
 Computation based
on difference
 36CSC447: Digital Image Processing Prof. Dr. Mostafa GadalHaqq.

Image Thresholding
•What is thresholding?
•Simple thresholding
•Adaptive thresholding

Thresholding – A Key Aspect
 Most algorithms involve establishing a
threshold level of certain parameter.
 Correct thresholding leads to better
segmentation.
 Using samples of image intensity available,
appropriate threshold should be set
automatically in a robust algorithm i.e. no
hard-wiring of gray values

Automatic Thresholding
 Use of one or more of the following:-
1. Intensity characteristics of objects
2. Sizes of objects
3. Fractions of image occupied by objects
4. Number of different types of objects
 Size and probability of occurrence – most
popular
 Intensity distributions estimate by histogram
computation.

Automatic Thresholding Methods
 Some automatic thresholding schemes:
1. P-tile method
2. Iterative threshold selection
3. Adaptive thresholding

Thresholding Methods
 P-tile Method:- If object
occupies P% of image pixels
then set a threshold T such
that P% of pixels have
intensity below T.
 Iterative Thresholding:-
Successively refines an
approx. threshold to get a
new value which partitions
the image better.
 21
2
1
 T
 41 CSC447: Digital Image Processing Prof. Dr. Mostafa GadalHaqq.

P-Tile Thresholding
• Thresholding is usually the first
step in any segmentation approach
• Single value thresholding can be
given mathematically as follows:
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Tyxf
Tyxf
yxg
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P-Tile Thresholding
• Basic global thresholding:
• Based on the histogram of an image
Partition the image histogram using
a single global threshold

P-Tile Thresholding
• Basic global thresholding:
• The success of this technique very
strongly depends on how well the
histogram can be partitioned

Iterative P-Tile Thresholding
• The Basic global thresholding:
1. Select an initial estimate for T (typically the
average grey level in the image)
2. Segment the image using T to produce two
groups of pixels: G1 consisting of pixels with
grey levels >T and G2 consisting pixels with
grey levels ≤ T
3. Compute the average grey levels of pixels
in G1 to give μ1 and G2 to give μ2

Iterative P-Tile Thresholding
• The Basic global thresholding:
4. Compute a new threshold value:
5. Repeat steps 2 – 4 until the difference in T
in successive iterations is less than a
predefined limit T∞
This algorithm works very well for finding thresholds
when the histogram is suitable.
2
21  
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P-Tile Thresholding

P-Tile Thresholding
• Limitation of P-Tile thresholding:

P-Tile Thresholding

Adaptive Thresholding
 Adaptive Thresholding is used in scenes with
uneven illumination where same threshold
value not usable throughout complete image.
 In such case, look at small regions in the
image and obtain thresholds for individual
sub-images. Final segmentation is the union
of the regions of sub-images.

 Thresholding – Basic Adaptive Thresholding

Summary
 Segmentation is the most essential step
in most scene analysis and automatic
pictorial pattern recognition problems.
 Choice of the technique depends on the
peculiar characteristics of individual
problem in hand.

Digital Image Processing: Image Segmentation

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