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Antal NagyDepartment of Image Processing
and Computer GraphicsUniversity of Szeged
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 1
Human perception Image degradation Convolution, Furier Transform Noise Image operations
◦ Frequency filters◦ Spatial filtering
Inverse filtering Wiener filtering
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 2
Aim◦ to improve the perception
of information images for human viewers
◦ to provide ‘better’ input for other automated image
processing techniques
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 3
No general theory for determining what is good image enhancement◦ If it looks good, it is good!?
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 4
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 5
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 6
http://www.youtube.com/watch?v=_d_l5nsnIvM
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 7
Focus◦ Noise reduction techniques
Quantitative measures can determine which techniques are most appropriate◦ How does it improve e.g.
the result of the next automated image processing step? E.g. image segmentation
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 8
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 9
Non-linear mapping◦ E.g., non-linear sensitivity, image of the straight
line is not straight e.t.c. Blurring
◦ Image of a point is blob Moving during the image acquisition Probabilistic noise
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 10
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 11
),(),(),(),(
),(),(),(),(
vuNvuFvuHvuG
yxyxfyxhyxg
Frequency domain
Spatial domain
◦ Multiplication point by point
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 12
)()()*( hFfFhfF
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 13
* =
=·
Multiplication
Convolution
Fourier transf.Inverse
Fourier transf.
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 14
dvduvyuxgvufyxgf
duuxgufxgf
,),(),)((
)( ))((
Definition
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 15
ationdifferenti '*'*)'*(
vitydistributi )*()*()(*
ityassociativ *)*()*(*
multipl.scalar ity withassociativ )(**)()*(
itycommutativ **
gfgfgf
hfgfhgf
hgfhgf
gafgfagfa
fggf
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 16
otherwise,0
,2/1if,1)(
xxg
2/1
2/1
)()(x
x
dfxgf
x
x
x
)(xf
Even functions that are not periodic can be expressed as the integrals of sines and/or cosines multiplied by a weighting function.
The formulation in this case is the Fourier transform.
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 17
∑∑ ==
Taught mathematics in Paris
Eventually traveled to Egypt with Napoleon to become the secretary of the Institute of Egypt
After fall of Napoleon worked at Bureau of Statistics
Elected to National Academy of Sciences in 1817
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 18
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 19
dueuFxf
dxexfuF
ixu
ixu
2
2
)()(
)()(
),(1 Lf
(invers transform)(invers transform)
(continous)(continous)
base-functionsbase-functions
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 20
dudvevuFyxf
dydxeyxfvuF
yvxui
yvxui
)(2
)(2
),(),(
),(),(
base-functionsbase-functions
(invers transform)(invers transform)
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 21
))(2cos(
Re )(2
vyuxi
e vyuxi
u=0, v=0 u=1, v=0 u=2, v=0u=-2, v=0 u=-1, v=0
u=0, v=1 u=1, v=1 u=2, v=1u=-2, v=1 u=-1, v=1
u=0, v=2 u=1, v=2 u=2, v=2u=-2, v=2 u=-1, v=2
u=0, v=-1 u=1, v=-1 u=2, v=-1u=-2, v=-1 u=-1, v=-1
u=0, v=-2 u=1, v=-2 u=2, v=-2u=-2, v=-2 u=-1, v=-2
u
v
wavelength:
22/1 vu
F(0,0) - value is by far the largest component of the image,
Other frequency components are usually much smaller,
The magnitude of F(X,Y) decreases quickly◦ Instead of displaying the |F(u,v)| we display
log( 1 + |F(u,v)| ) real function usually
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 22
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 23
x
y
v
u
),(1log vuF),( yxf
The 2D Fourier transform can be separated
The edges on the image appears as point series in perpendicular direction in Fourier transform of the image and vice versa.
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 24
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 25
Image-spaceImage-space
Frequency spaceFrequency spaceooriginal rotation linearity riginal rotation linearity shiftshift scale scale
Noise unknown subtraction not possible Periodic noise
◦ N(u,v) can be estimated from G(u,v)
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 26
),(),(),(
and
),(),(),(
vuNvuFvuG
yxyxfyxg
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 27
Gaussian◦ In an image due to
factors Electronic circuit
noise Sensor noise due to
poor illumination High temperature
Rayleigh◦ Range imaging
Exponential and gamma◦ Laser imaging
Impulse◦ Faulty switching
Uniform density◦ Practical situations
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 28
22 2/)(
2
1)(
noiseGaussian
zzezp
azfor 0
for )(2
)(
noiseRayleigh
/)( 2
azeazbzp
baz
4
)4(
and
4/
2
b
baz
0zfor 0
0for )!1(
ap(z)
noise (gamma) Erlang1b
zeb
z azb
22
and
a
b
a
bz
0a where
0zfor 0
0for )(
noise lExponentia
zae
zpaz
22 1
and
1
a
az
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 29
otherwise 0
if 1
)(
Noise Uniform
bzaabzp
12
and2
22 ab
baz
otherwise 0
for
for
)(
noise peper)-and-(salt Impulse
bzP
azP
zp b
a
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 30
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 31
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 32
Electrical and electromechanical interference
Spatial dependent noise Can be reduced via
frequency domain filtering ◦ Pair of impulses
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 33
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 34
T: A=[a(i,j)] → B=[b(i,j)]
b(i,j)=T{a(i,j), S(i,j), i, j}
intensity enviroment position
Global: b(i,j)=T{A} (S(i,j)=A) (e.g. Fourier-transformation)
Local: T{a(i,j), S} given size of S and independent from the position (e.g. convolution with a mask)
Local, adaptive: T{a(i,j), S(i,j), i, j} the size of S(i,j) is independent from the size of image (e.g. adaptive thresholding)
Point operation: T{a(i,j)} (e.g. gamma-correction, histogram-equalization)
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 35
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 36
D0: cutoff frequency
All frequencies less than D0 will be passed,
Other frequencies will be filtered out.Bluring and ringing propertiesScope: noise filtering
otherwise,0
if,1),(
20
22 DYXYXH ILPF
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 37
F
F-1.
Input image
Frequency-mask
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 38
Original 5
15 30
80 230
Cutoff frequencies
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 39
n: order of the filter
Properties: Smooth transition in blurring No ring effect (continouos filter) Smoothed edges
nBLPF
DYX
YXH 2
0
22
1
1),(
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 40
Original 5
15 30
80 230
Cutoff frequenciesCutoff frequencies
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 41
022 2/)(),( Dvu
GLPF evuH
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 42
Original 5
15 30
80 230
Cutoff frequenciesCutoff frequencies
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 43
),(1),( vuLPFvuHPF
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 44
Bandreject and Bandpass Filters Notch Filters
◦ Rejects or passes frequencies in a predefined neighborhood about the frequency rectangle
◦ Zero-phase-shift filters Symmetric about the origin
(u0,v0) (-u0,-v0)
◦ Product of highpass filters whose centers have been translated to the centers of the notches.
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 45
Q
kkkNR vuHvuHvuH
1
),(),(),(
),(1),( vuHvuH NRNP
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 46
noisyimage
frequencymask
frequencyimage
filteredimage
11
00
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 47
Mean Filter
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 48
where g input image, S(x,y) neighborhood of (s,t) point,
mn number of pixels in neighborhood.
3x3 neighborhood
1
1
1
1
),(9
1),(ˆ
u v
vyuxgyxf
xySts
tsgmn
yxf),(
),(1
),(ˆ
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 49
111
111
111
9
1
:where
),,)((
),(),(
),(9
1),(ˆ
1
1
1
1
1
1
1
1
h
jihf
vuhvyuxg
vyuxgyxf
u v
u v
AvAveerraagingging ◦ same weight for every pixels in neighborhood,same weight for every pixels in neighborhood,
Weighted averageWeighted average ◦ weights for pixels in the neighborhood (generally decreasing with weights for pixels in the neighborhood (generally decreasing with
the distance).the distance).
The sum of the Noise Filtering/smoothing The sum of the Noise Filtering/smoothing masks elements is 1! masks elements is 1!
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 50
121
242
121
16
1
111
111
111
9
1
Smooth when the difference between the intensity value of the given pixel and the mean of the neighborhood is larger than threshold value defined in advance.
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 51
otherwise),,(
,),(),( if),,(),('
jif
Tjifjigjigjig
Arithmetic mean filter
Geometric mean filter◦ Lose less image details
Harmonic mean filter◦ Works well for
salt noise, Gaussian◦ Fails for pepper noise
Contraharmonic mean filter◦ Q>0 pepper, Q<0 salt
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 52
xySts
tsgmn
yxf),(
),(1
),(ˆ
mn
Sts xy
tsgyxf
1
),(
),(),(ˆ
xySts tsg
mnyxf
),( ),(1
),(ˆ
xy
xy
Sts
Q
Sts
Q
tsg
tsg
yxf
),(
),(
1
),(
),(
),(ˆ
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 53
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 54
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 55
Median filter
Max and min filters
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 56
),(median),(ˆ),(
tsgyxfxySts
),(min),(ˆ
),(max),(ˆ
),(
),(
tsgyxf
tsgyxf
xy
xy
Sts
Sts
50%
0%
100%
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 57
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 58
Behavior changes based on statistical characteristic in the filter region◦ Improved filtering power◦ Increase in filter complexity◦ Noise only!
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 59
Adaptive, Local noise reduction filter◦ Mean◦ Variance
Local region Sxy
◦ g(x,y) intensity value◦ the variance of the noise corrupting f(x,y) to
form g(x,y) ◦ mL local mean◦ local variance
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 60
2
2L
Adaptive, Local noise reduction filter1. If is zero, the filter should return the value of
g(x,y) Zero noise case
2. If the local variance is high relative to the filter should return a value close to g(x,y)
Edges should be preserved3. If the variances are equal the filter should
return the arithmetic mean value Local noise averaging
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 61
2
2
Simplest approach to restoration is direct inverse filtering
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 62
experience:H: quickly decreasing function,N: not (so quickly) decreasinglet us cut the high frequencies
),(
),(),(ˆ
vuH
vuGvuF
),(
),(),(
),(
),(),(
),(
),(),(ˆ
vuH
vuNvuF
vuH
vuNvuF
vuH
vuGvuF
No additive noise
If we only know the degradation function◦ Can not recover the undegraded image
H(u,v ) is not known Get around the zero or small value problem
◦ To limit the filter frequencies to values near the origin H(0,0) is usually the highest value of H(u,v) in the
frequency domain
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 63
Inverse filtering makes no explicit provision for handling noise
Approach incorporates◦ Degradation function◦ Statistical characteristics of noise
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 64
Method◦ Considering images and noise as random variable◦ Objective is to find an estimate of the
uncorrupted image f such that the mean square error between them is minimized.
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 65
f̂
argument theof value
expected theis where
ˆ 22
E
ffEe
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary
66
spectrum
Wiener filter
input
result
original
spectrum of the result
Edge preserving filters◦ E.g. anisotropic diffusion
Wavelet denoising Point operations
◦ E.g. gamma correction, histogram operations Iterative filtering E.t.c.
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 67
We always should consider some kind of noise model◦ Even when working on phantom data
Should do in automatic way◦ Have to chose carefully the method
Depends on the given task◦ Determining the parameters
What we gain?◦ Less problem afterwards◦ Better final result
Even no other technique will be applied
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 68
Digital Image Processing◦ Gonzalez and Woods◦ www.ImageProcessingPlace.com
Course on Image Processing at University of Szeged◦ Attila Kuba, Kálmán Palágyi
Image Restoration presentation◦ Attila Kuba◦ SSIP 2006
17th SSIP 2009, 2 - 11 July, Debrecen, Hungary 69