Aniso Diff Filtering
-
Upload
maruni-devi -
Category
Documents
-
view
51 -
download
0
Transcript of Aniso Diff Filtering
1Segmentierung medizinischer BilddatenStefan Burkhardt
8 Anisotropic diffusion filtering
8 Anisotropic diffusion filtering
� Images contain of the image itself and noise
�Noise: random, little disturbances of the image
�To improve the segmentation: the noise should be reduced
�Condition:
•Remove noise
•But: keep the image information unchanged
2Segmentierung medizinischer BilddatenStefan Burkhardt
8 Anisotropic diffusion filtering
Diffusion
�Physical process for balancing concentration changes
�Now:
• the image intensity can be seen as a “concentration”
•The noise can be modelled as little concentration inhomogeneities
�These inhomogeneities could be smoothed by diffusion
�Diffusion should only be perpendicular e.g. to edges
3Segmentierung medizinischer BilddatenStefan Burkhardt
8.1 Physical background
Physical background of diffusion
�Given: a concentration distribution u
�Fick’s law:
�Concentration gradient causes a flux j
� j aims to compensate the gradient
�D: diffusion tensor, in general a positive definite, symmetric matrix
u∇⋅−= Dj
4Segmentierung medizinischer BilddatenStefan Burkhardt
8.1 Physical background
Physical background of diffusion (2)
�Diffusion is mass transport without destroying mass or creating new mass
�Continuity equation
� t denotes the time, �tu the deviation of u with respect to t
���
����
�
∂∂+
∂∂−=−=∂
yxut
jjjdiv
5Segmentierung medizinischer BilddatenStefan Burkhardt
8.1 Physical background
Physical background of diffusion (3)
�diffusion equation
�Application in many physical transport process, e.g. for heat transfer it is called heat-transfer-equation
� Image processing:
� Identify the concentration with the grey value at a certain location
( )uut ∇⋅=∂ Ddiv
6Segmentierung medizinischer BilddatenStefan Burkhardt
8.1 Physical background
Physical background of diffusion (4)
�Diffusion tensor:
•Constant over the whole image: homogeneous (or linear) diffusion
•Often it is a function of the structure of the image itself: nonlinear diffusion
•Isotropic: j and the concentration gradient are parallel
•Anisotropic: otherwise
7Segmentierung medizinischer BilddatenStefan Burkhardt
8.2 Linear diffusion
Linear isotropic diffusion
�Mostly used for smoothing images
�The image I itself is the initial starting for the diffusion process
�We use D = 1 since D only influences the speed of the diffusion
),()0,,(
div
yxIyxu
uut==∂
8Segmentierung medizinischer BilddatenStefan Burkhardt
8.2 Linear diffusion
Linear isotropic diffusion (2)
t=4 t=8 t=12
t=20t=16 t=24 t=40
t=0
9Segmentierung medizinischer BilddatenStefan Burkhardt
8.2 Linear diffusion
Linear isotropic diffusion (3)
�Advantages:
•Continuously simplifying of the image
•Reducing the noise in the image
�Disadvantages:
•Linear isotropic diffusion does not only reduce noise
•It also blues important features like edges
•No a-priori knowledge is taken into account
•Result: it makes edges harder to identify
10Segmentierung medizinischer BilddatenStefan Burkhardt
8.3 Nonlinear diffusion
Nonlinear diffusion
� Improvement:
•Preservation of the edges, only smooting between edges
•Need to assign a position specific diffusivity
�Adapting the diffusivity g to the gradient in the actual image u(x,y,t)
�We obtain the equation
( )( )uuu ∇∇=∂ 2div gt
11Segmentierung medizinischer BilddatenStefan Burkhardt
8.3 Nonlinear diffusion
0ufor1
ufor0
→∇→∞→∇→
g
g
Nonlinear diffusion (2)
�Conditions for g
�Perona& Malik:
( )2
2
2
1
1
λu
ug∇
+=∇
12Segmentierung medizinischer BilddatenStefan Burkhardt
8.3 Nonlinear diffusion
Nonlinear diffusion (3)
t=4 t=8 t=12
t=20t=16 t=24 t=40
t=0
13Segmentierung medizinischer BilddatenStefan Burkhardt
8.3 Nonlinear diffusion
Nonlinear diffusion (4)
t=4 t=20 t=40
t=40t=20t=4
Linear diffusion
Nonlineardiffusion
14Segmentierung medizinischer BilddatenStefan Burkhardt
8.3 Nonlinear diffusion
Nonlinear diffusion (5)
�Advantages
•Steering of the diffusion at each point of the image is possible
•So diffusion can be reduced on edges
•The result: edge-preserving image smoothing
•But: the smoothing of the edges cannot be completely circumvented
�Next: anisotropic approaches
15Segmentierung medizinischer BilddatenStefan Burkhardt
8.4 Nonlinear anisotropic diffusion
Nonlinear anisotropic diffusion
�Until now: determination of the amount of diffusion depending on local features (here: the gradient).
�Other features could be possible
� Idea: we could define the diffusion tensor so that the diffusion goes around some structures
�Here: diffusion should preserve edges
•On edges: no diffusion over edges, but diffusion parallel to edges should be enabled
16Segmentierung medizinischer BilddatenStefan Burkhardt
8.4 Nonlinear anisotropic diffusion
Nonlinear anisotropic diffusion (2)
�Combination of two features
•Non-linearity
-The diffusion at border is much less than the diffusion elsewhere
•Anisotropy
-Diffusion should be perpendicular to edge
-No diffusion over edges
17Segmentierung medizinischer BilddatenStefan Burkhardt
8.4 Nonlinear anisotropic diffusion
Nonlinear anisotropic diffusion (3)
�How to define the diffusion tensor D?
�Remember: D is a positive definite symmetric matrix
•It has two different eigenvalues and two eigenvectors
•Therewith the eigenvectors are perpendicular
•The eigenvectors gives the main diffusion direction
•The corresponding eigenvalues the strength of diffusion in the direction of the eigenvectors.
18Segmentierung medizinischer BilddatenStefan Burkhardt
�Definition of the eigenvectors
�We want to stop the diffusion over the edge. Therewith we need as one eigenvector the direction of the gradient.
�The second is perpendicular and can be seen as the tangential vector to the edge
8.4 Nonlinear anisotropic diffusion
[ ][ ] ���
����
�
−=
∇∇=
x
y
v
vv
u
uv
1
12
1
Nonlinear anisotropic diffusion (4)
uv
uv
∇⊥∇
2
1 ||
19Segmentierung medizinischer BilddatenStefan Burkhardt
�Definition of the eigenvalues
�First: We want to stop the diffusion over the edge (direction v1). We use the non-linearity and define
�Perpendicular to the edge the diffusion should not be stopped
8.4 Nonlinear anisotropic diffusion
( )2
1 ug ∇=λ
Nonlinear anisotropic diffusion (5)
12 =λ
20Segmentierung medizinischer BilddatenStefan Burkhardt
�Now, we obtain D as
�Result is an edge-enhancing anisotropic diffusion
8.4 Nonlinear anisotropic diffusion
T
���
�
�
���
�
�
⋅���
����
�⋅���
�
�
���
�
�
=||
||
0
0
||
||
2
12121 vvvvD
λλ
Nonlinear anisotropic diffusion (6)
21Segmentierung medizinischer BilddatenStefan Burkhardt
8.4 Nonlinear anisotropic diffusion
Nonlinear anisotropic diffusion (7)
t=4 t=8 t=12
t=20t=16 t=24 t=40
t=0
22Segmentierung medizinischer BilddatenStefan Burkhardt
8.4 Nonlinear anisotropic diffusion
Nonlinear anisotropic diffusion (8)
t=4 t=20 t=40
t=40t=20t=4
Nonlineardiffusion
Nonlinearanisotropicdiffusion
23Segmentierung medizinischer BilddatenStefan Burkhardt
8.5 Numerical implementations
Numerical implementation
�We have
�Looking for a solution u(x,y,t)
�Either for a specific t = t0
�Or for t��
�Problem: the equation cannot be analytically solved
�Numerical solution is necessary
),()0,,( yxIyxu =
24Segmentierung medizinischer BilddatenStefan Burkhardt
�1. step: discretizing the time axis
•Determination of a time step ��
-�� dependson thenumerical stability
•only the times
are taken into account
�We obtain u(x,y,ti)
8.5 Numerical implementations
Numerical implementation (2)
τ∆⋅== itt i
25Segmentierung medizinischer BilddatenStefan Burkhardt
�2. step: finding an approximation for the derivative with respect to t
�The Taylor serie for u(x,y,t) leads to
�Neglecting the remain and transform the equation leads to
8.5 Numerical implementations
( )ζττ Rtyxut
tyxutyxu +∂∂⋅∆+=∆+ ),,(),,(),,(
Numerical implementation (3)
),,(),,(),,( 1 iii tyxut
tyxutyxu∂∂⋅∆+=+ τ
26Segmentierung medizinischer BilddatenStefan Burkhardt
�Conditions to the step size:
�Dimensions: m, grid width: h
�Costs per iteration: very low
�Efficiency: low (many iterations needed to get a result)
8.5 Numerical implementations
m
h
2
2
<∆τ
Numerical implementation (4)
27Segmentierung medizinischer BilddatenStefan Burkhardt
�Until now we used the so-called explicit scheme
�Other notation
with
•I: identity matrix
•ui values of u(x,y,ti) stacked in vector form
•Al(uk) matrix version of the diffusion tensors
8.5 Numerical implementations
Notation
( ) im
l
il
i uuAIu ⋅��
���
� ⋅∆+= �=
+
1
1 τ
28Segmentierung medizinischer BilddatenStefan Burkhardt
�Semi-implicit scheme
�Additive operator splitting
8.5 Numerical implementations
( ) im
l
il
i uuAIu ⋅��
���
� ⋅∆−=−
=
+ �1
1
1 τ
Improvements
( )( ) im
l
il
i mm
uuAIu ⋅⋅∆⋅−=−
=
+ �1
1
1 1 τ
29Segmentierung medizinischer BilddatenStefan Burkhardt
�Semi-implicit scheme: We can write the derivative of uwith respect to t as
� Implicit: because ui+1 is used
�Semi-implicit: operator Al(ui) is combined with ui+1
�Transformation of the equation leads to
8.5 Numerical implementations
( ) 1
1
1+
=
+
⋅=∆
−� i
m
l
il
ii
A uuuu
τ
Semi-implicit scheme
( ) im
l
il
i uuAIu ⋅�
��
⋅∆−=−
=
+ �1
1
1 τ
30Segmentierung medizinischer BilddatenStefan Burkhardt
�Features:
•Stable for ��< �, but: convergenceusually becomesslower for too large ��
•Cost per iteratorion: high, because thematrix must bebuilt and inverted in each iteration, O(n2)
•Efficiency: medium (better than theexplicit schemebecauseof thepossible larger �, but further improvementis possible)
�Question: How could the invertion of thematrix bemoreeffective
•Hint: each Al is a simple tri-diagonal matrix
8.5 Numerical implementations
Semi-implicit scheme (2)
31Segmentierung medizinischer BilddatenStefan Burkhardt
�Objective: split the derivation operator for each spatial direction
�Then we get for each direction a simple tri-diagonal matrix
�Such matrices can be very efficiently inverted
�Advantage:
•Keep the unrestrictedness of ��
•Make thecalculation moreeffective
8.5 Numerical implementations
Additive operator splitting (AOS)
32Segmentierung medizinischer BilddatenStefan Burkhardt
�One can write
as
8.5 Numerical implementations
( )�
��
⋅∆− �=
m
l
il
1
uAI τ
Additive operator splitting (2)
( )[ ]�=
⋅⋅∆−m
l
imm 1
1
1uAI τ
33Segmentierung medizinischer BilddatenStefan Burkhardt
�For thecomputation of the inversematrix weuse theapproximation
�Thenow occuring error is
•only of second and higher order
•This means: the first derivatives haveno errors
8.5 Numerical implementations
( ) 111 −−− +≈+ BABA
Additive operator splitting (3)
34Segmentierung medizinischer BilddatenStefan Burkhardt
�And weapproximate the inversematrix by
�Therewith weobtain theequation
8.5 Numerical implementations
( ) ( )[ ]1
11
1
1
1−
=
−
=�� ⋅⋅∆−≈�
��
⋅∆−m
l
im
l
il m
muAIuAI ττ
Additive operator splitting (4)
( )[ ] im
l
ii mm
uuAIu1
11
1 1−
=
+ � ⋅⋅∆−= τ
35Segmentierung medizinischer BilddatenStefan Burkhardt
�Differencesand improvements
•A split in theoperator for each spatial direction has beenperformed
•Thepixels can bearranged for each operator separately
•As result each operator is tri-diagonal matrix and isinverted separately.
•Such 3-diagonal matrices can be inverted in linear time O(n), not in O(n2)
8.5 Numerical implementations
Additive operator splitting (5)
36Segmentierung medizinischer BilddatenStefan Burkhardt
�Features
•Stable for ��< �
•Cost per iteratorion: low, but a littlehigher than for theexplicit scheme
�Efficiency: high
8.5 Numerical implementations
Additive operator splitting (6)
37Segmentierung medizinischer BilddatenStefan Burkhardt
8.5 Numerical implementations
highlow��<�AOS
mediumhigh��<�Semi-implicit
lowvery lowexplicit
efficiencyCost per iteration
StabilityScheme
Conclusion
m
h
2
2
<∆τ