Transcript By:Gaurav Mittal 11/7/2015 LNMIIT DIP Workshop - Noise Removal Technique by Gaurav Mittal.

By:Gaurav Mittal
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Review of Noise
 Definition - Any unwanted signal that degrades the
quality of input image is noise.
 Sources of noise
 Imaging sensors can be affected by ambient conditions
 Interference can be added to an image during
transmission
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Aim of Image Restoration

Attempts to restore images that have been degraded

Identify the degradation process and attempt to
reverse it
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
We can consider a noisy image to be modelled
as follows:
g ( x, y)  f ( x, y)   ( x, y)
where f(x, y) is the original image pixel, η(x, y)
is the noise term and g(x, y) is the resulting
noisy pixel
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Filtering to Remove Noise
 We can use spatial filters of different kinds to remove
different kinds of noise. The kind of filters that we will
study are
 Mean filters
 Order statistic filters
 Adaptive filters
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Mean Filters
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We will study the following mean filters Arithmetic mean filter
Geometric mean filter
Harmonic mean filter
Contraharmonic mean filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Arithmetic mean filter
The arithmetic mean filter is a very simple one and is
calculated as follows:
ˆf ( x, y)  1
g ( s, t )

m n ( s ,t )S xy
1/
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This is implemented as the
simple smoothing filter
Blurs the image to remove
noise
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Geometric mean filter
Geometric Mean:


fˆ ( x, y )    g ( s, t )
( s ,t )S xy

1
mn
Achieves similar smoothing to the arithmetic mean, but
tends to lose less image detail
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Noise Removal Examples
Original
Image
Image
Corrupted
By Gaussian
Noise of zero
mean and 400
variance
After A 3*3
Arithmetic
Mean Filter
11/7/2015
After A 3*3
Geometric
Mean Filter
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Harmonic mean filter
Harmonic Mean:
fˆ ( x, y ) 
mn

( s ,t )S xy
1
g ( s, t )
Works well for salt noise, but fails for pepper noise
Also does well for other kinds of noise such as Gaussian
noise
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Harmonic mean filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Contraharmonic mean filter
 g ( s, t )
Q 1
fˆ ( x, y ) 
( s ,t )S xy
 g ( s, t )
Q
( s ,t )S xy
Q is the order of the filter and adjusting its value changes
the filter’s behaviour
Positive values of Q eliminate pepper noise
Negative values of Q eliminate salt noise
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Q negative
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Q positive
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Noise Removal Examples (cont…)
Image Corrupted By Pepper
Noise of 0.1 probability
11/7/2015
Result of Filtering Above
With 3*3 Contraharmonic
LNMIIT DIP Workshop - Noise Removal
Q = 1.5
Technique by Gaurav Mittal
Noise Removal Examples (cont…)
Result of Filtering Above
With 3*3 Contraharmonic
LNMIIT DIP Workshop - Noise Removal
Q = -1.5
Image Corrupted By Salt
Noise of 0.1 probability
11/7/2015
Technique by Gaurav Mittal
Contraharmonic Filter: Here Be Dragons
Choosing the wrong value for Q when using the
contraharmonic filter can have drastic results
Negative Q in pepper
Positive Q in salt
Results of selecting wrong sign of Q
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Order Statistics Filters
Spatial filters that are based on ordering the pixel values
that make up the neighbourhood operated on by the
filter
Useful spatial filters include
 Median filter
 Max and min filter
 Midpoint filter
 Alpha trimmed mean filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Median Filter
Median Filter:
fˆ ( x, y)  median{g (s, t )}
( s ,t )S xy
Excellent at noise removal, without the smoothing
effects that can occur with other smoothing filters
Particularly good when salt and pepper noise is present
Note : In Lab do not create a program or function with
the name of median.m since it is already a function
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Max and Min Filter
Max Filter:
fˆ ( x, y)  max {g (s, t )}
( s ,t )S xy
Min Filter:
fˆ ( x, y)  min {g (s, t )}
( s ,t )S xy
Max filter is good for pepper noise and min is good for
salt noise
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Min filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Max filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Midpoint Filter
Midpoint Filter:
ˆf ( x, y)  1  max {g ( s, t )}  min {g ( s, t )}

( s ,t )S xy
2 ( s ,t )S xy
Good for random Gaussian and uniform noise
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Midpoint Filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Alpha-Trimmed Mean Filter
Alpha-Trimmed Mean Filter:
fˆ ( x, y) 
1
g r (s, t )

mn d ( s ,t )S xy
We can delete the d/2 lowest and d/2 highest grey levels
So gr(s, t) represents the remaining mn – d pixels
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Alpha-Trimmed Mean Filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Noise Removal Examples
Image
Corrupted
By Salt And
Pepper Noise
Result of 1
Pass With A
3*3 Median
Filter
Result of 2
Passes With
A 3*3 Median
Filter
Result of 3
Passes With
A 3*3 Median
Filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Noise Removal Examples (cont…)
Image
Corrupted
By Pepper
Noise
Image
Corrupted
By Salt
Noise
Result Of
Filtering
Above
With A 3*3
Max Filter
Result Of
Filtering
Above
With A 3*3
Min Filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Noise Removal Examples (cont…)
Image
Corrupted
By Uniform
Noise
Image Further
Corrupted
By Salt and
Pepper Noise
Filtered By
5*5 Arithmetic
Mean Filter
Filtered By
5*5 Geometric
Mean Filter
Filtered By
5*5 Median
Filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Filtered By
5*5 Alpha-Trimmed
Mean Filter
Adaptive Filters
The filters discussed so far are applied to an entire image
without any regard for how image characteristics vary
from one point to another
The behaviour of adaptive filters changes depending
on the characteristics of the image inside the filter
region
We will take a look at the adaptive median filter
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Adaptive Median Filtering
The median filter performs relatively well on impulse
noise as long as the spatial density of the impulse noise
is not large
The adaptive median filter can handle much more
spatially dense impulse noise, and also performs some
smoothing for non-impulse noise
The key insight in the adaptive median filter is that the
filter size changes depending on the characteristics of
the image
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Adaptive Median Filtering (cont…)
Remember that filtering looks at each original pixel
image in turn and generates a new filtered pixel
First examine the following notation:
 zmin
= minimum grey level in Sxy
 zmax
= maximum grey level in Sxy
 zmed
= median of grey levels in Sxy
 zxy
= grey level at coordinates (x,
 Smax
=maximum allowed size of Sxy
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
y)
Adaptive Median Filtering (cont…)
Level A:
Level B:
11/7/2015
A1 = zmed – zmin
A2 = zmed – zmax
If A1 > 0 and A2 < 0, Go to level B
Else increase the window size
If window size ≤ repeat Smax level A
Else output zmed
B1 = zxy – zmin
B2 = zxy – zmax
If B1 > 0 and B2 < 0, output zxy
Else output zmed
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Adaptive Filtering Example
Image corrupted by salt
and pepper noise with
probabilities Pa = Pb=0.25
11/7/2015
Result of filtering with a 7
* 7 median filter
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal
Result of adaptive median
filtering with i = 7
Thank-You
Reference:- Digital Image Processing, Gonzalz and woods
11/7/2015
LNMIIT DIP Workshop - Noise Removal
Technique by Gaurav Mittal