filter factor analysis of an iterative multilevel...
TRANSCRIPT
Filter factor analysis of an iterative multilevelregularizing method
Marco Donatelli
Department of Physics and MathematicsUniversity of Insubria
Outline
1 Restoration of blurred and noisy imagesThe model problemProperties of the PSFIterative regularization methods
2 Multigrid regularizationMultigrid methodsIterative Multigrid regularizationComputational CostFilter factor analysis of the TL
3 Numerical experiments
4 Conclusions
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 2 / 30
Restoration of blurred and noisy images
Outline
1 Restoration of blurred and noisy imagesThe model problemProperties of the PSFIterative regularization methods
2 Multigrid regularizationMultigrid methodsIterative Multigrid regularizationComputational CostFilter factor analysis of the TL
3 Numerical experiments
4 Conclusions
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 3 / 30
Restoration of blurred and noisy images The model problem
Image restoration with Boundary Conditions
Using Boundary Conditions (BCs), the restored image f is obtainedsolving: (in some way ...)
Af = g + ξ
• g = blurred image,
• ξ = noise (random vector),
• A = two-level matrix depending on the point spread function (PSF)and the BCs.
The PSF is the observation of a single point (e.g., a star in astronomy).
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 4 / 30
Restoration of blurred and noisy images The model problem
Coefficient matrix structure
The matrix-vector product computed in O(n2 log(n)) ops for n × n imageswhile the inversion costs O(n2 log(n)) ops only in the periodic case.
BCs A
Dirichlet Toeplitzperiodic circulant
Neumann (reflective) Toeplitz + Hankelanti-reflective Toeplitz + Hankel
If the PSF is symmetric with respect to each direction:
BCs A
Neumann (reflective) DCT IIIanti-reflective DST I + low-rank
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 5 / 30
Restoration of blurred and noisy images Properties of the PSF
Generating function of PSF
• The eigenvalues of A(z) are about a uniform sampling of z .
PSF Generating function z(x)
• The ill-conditioned subspace is mainly constituted by themiddle/high frequencies.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 6 / 30
Restoration of blurred and noisy images Iterative regularization methods
Iterative regularization methods
Semi-convergence behavior
Some iterative methods (Landweber, CGNE, . . . ) have regularizationproperties: the restoration error firstly decreases and then increases.
Example
0 50 100 150 200 250 30010
−1
100
101
ReasonMarco Donatelli (University of Insubria) An iterative multilevel regularization method 7 / 30
Multigrid regularization
Outline
1 Restoration of blurred and noisy imagesThe model problemProperties of the PSFIterative regularization methods
2 Multigrid regularizationMultigrid methodsIterative Multigrid regularizationComputational CostFilter factor analysis of the TL
3 Numerical experiments
4 Conclusions
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 8 / 30
Multigrid regularization Multigrid methods
The algorithm
The choices
1 We apply only the pre-smoother simply called smoother.
2 Let Ri and Pi be the restriction and the prolongation operators atthe level i , respectively.
3 We use the Galerkin approach• Pi = RT
i
• Ai+1 = RAiRTi
4 Coarser grid of size 8 × 8 independent of the size of the finer grid.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 9 / 30
Multigrid regularization Multigrid methods
The Algebraic Multigrid (AMG)
• The AMG uses only information on the coefficient matrix.
• Different classic smoothers have similar behavior:in the initial iterations they are not able to reduce effectively the errorin the subspace generated by the eigenvectors associated to smalleigenvalues (ill-conditioned subspace)
⇓• To obtain a fast solver, the restriction is chosen in order to project
the error equation in such subspace.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 10 / 30
Multigrid regularization Multigrid methods
Image deblurring and Multigrid
• In the image deblurring the ill-conditioned subspace is related to highfrequencies, while the well-conditioned subspace is generated by lowfrequencies.
• In order to obtain a fast convergence the algebraic multigrid projectsin the high frequencies where the noise “lives” =⇒ noise explosionalready at the first iteration (it requires Tikhonov regularization[Donatelli, NLAA, 12 (2005), pp. 715–729]).
• In this case the low-pass filter projects in the well-conditionedsubspace (low frequencies) =⇒ it is slowly convergent but it can be agood iterative regularizer.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 11 / 30
Multigrid regularization Multigrid methods
Multigrid for structured matrices
Preserve the structure
• In order to apply recursively the MGM, it is necessary to keep thesame structure at each level (Toeplitz, . . . ).
• For every structure arising from the proposed BCs, there existprojectors that preserve the same structure.
Ri = KNiANi
(p), where
• KNi∈ R
Ni4×Ni is the cutting matrix that preserves the structure at
the lower level.
• p(x , y) is the generating function of the projector, which selects thesubspace where to project the linear system.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 12 / 30
Multigrid regularization Multigrid methods
Multigrid, structured matrices, and images
The cutting matrix Kniin 1D
circulant Toeplitz&DST − I DCT − III
[
1 01 0 ... ...
1 0
] [
0 1 00 1 0... ... ...
0 1 0
] [
1 1 01 1 0... ... ...
0 1 1
]
Low-pass filter: Low frequencies projection ⇒ noise reduction
2D ↔ p(x , y) = (1 + cos(x))(1 + cos(y))
ց Full weighting ր Bilinear interpolation
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 13 / 30
Multigrid regularization Iterative Multigrid regularization
Iterative multigrid regularization
The Multigrid as an iterative regularization method
If we have an iterative regularization method we can improve itsregularizing properties and/or accelerate its convergence using it assmoother in a Multigrid algorithm.
Regularization
The regularization properties of the smoother are preserved since it iscombined with a low-pass filter.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 14 / 30
Multigrid regularization Iterative Multigrid regularization
Two-Level (TL) regularization
Idea: project into the low frequencies and then apply an iterativeregularization method.
TL as a specialization of TGM
Smoother: iterative regularization method
Projector: low-pass filter
TL Algorithm
1 No smoothing at the finer level
2 At the coarser level to apply one step of the smoother instead ofto solve directly the linear system
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 15 / 30
Multigrid regularization Iterative Multigrid regularization
Multigrid regularization (applying recursively the TL)
V-cycle
Using a larger number of recursive calls (e.g. W -cycle), the algorithm“works” more in the well-conditioned subspace, but it is more difficult todefine an early stopping criterium.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 16 / 30
Multigrid regularization Computational Cost
Computational Cost
Assumptions: n × n images and m × m PSFs with m ≪ n.
• Let S(n) be the computational cost of one smoother iteration.
• The computational cost of one iteration of our multigridregularization method with γ recursive calls is
C (γ, n) ≈
13S(n), γ = 1S(n), γ = 23S(n), γ = 3
• if m ≈ n then S(n) = O(n2 log(n)).
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 17 / 30
Multigrid regularization Filter factor analysis of the TL
Filter factor of the Landweber method
• Imposing P-BCs A = Cn(z): A is a circulant matrix of size ngenerated by the function z .
• A = FnDn(z)FHn , where Fn = [eijxk ]n−1
k,j=0/√
n is the DFT matrix and
Dn(z) = diag([f (xk)]n−1k=0) with xk = 2πk
n.
• Taking x0 = 0 the jth approximation of f is
xj = Fn
j−1∑
i=0
(I − Dn(|z |2))iDn(z)FHn b = Cn(φj)C
−1n (z)b
where φj(x) = 1 − (1 − |z(x)|2)j , x ∈ (0, 2π] is the filter factor.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 18 / 30
Multigrid regularization Filter factor analysis of the TL
Filter factor of the TL method
• For TL with Landweber as smoother xj = Bnb with
Bn = Cn(p)KTn C n
2(g)KnCn(r),
where g(x) = 1−(1−|z(x)|2)j
z(x) , x ∈ (0, 2π], Kn is the cutting matrix andr , p and z are restriction, prolongation and PSF function at thecoarser level respectively.
• Bn = FnΠTn WnΠnF
Hn , where Πn is a permutation matrix and Wn is
the diagonal block matrix of size (n/2) × (n/2) with blocks ofdimension 2 × 2. For k = 0, . . . , n/2 − 1, the k-th diagonal block isgiven by
W(k)n =
1
2g(x2k)
[
p(xk)p(x(k+n/2))
]
[
r(xk) r(x(k+n/2))]
.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 19 / 30
Multigrid regularization Filter factor analysis of the TL
Filter factor of the TL method 2
• The block W(k)n has rank 1 and the nontrivial null eigenvalue λk is
λk =1
2g(x2k)
(
(pr)(xk) + (pr)(x(k+n/2)))
.
• The eigenvector associated to the null eigenvalue is
r(xk)
r(x(k+n/2))F
(k+n/2)n − F
(k)n .
This should be an high frequency (to filtering) ⇒ it provides acondition to choose r : e.g. nonnegative and decreasing in [0, pi ].
• The eigenvector associated to λk defines an analogous condition for p.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 20 / 30
Multigrid regularization Filter factor analysis of the TL
Comparison TL vs Landweber
Focus on the high frequencies for the filter factors of TL and Landweberfor j = 1000
0 1 2 3 4 5 60
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1x 10
−4
Landweber
TL
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 21 / 30
Multigrid regularization Filter factor analysis of the TL
Noise −→ 0 ?
In the noise free case the TL method does not compute the exact solution.
How to recover the high frequencies in the noise free case is a work inprogress . . .
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 22 / 30
Numerical experiments
Outline
1 Restoration of blurred and noisy imagesThe model problemProperties of the PSFIterative regularization methods
2 Multigrid regularizationMultigrid methodsIterative Multigrid regularizationComputational CostFilter factor analysis of the TL
3 Numerical experiments
4 Conclusions
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 23 / 30
Numerical experiments
An airplane
• Periodic BCs
• Gaussian PSF (A spd)
• noise = 1%
OriginalImage
Inner part 128 × 128 Observed image Restored with MGM
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 24 / 30
Numerical experiments
Restoration error: noise = 1%
ej = ‖f − f(j)‖2/‖f‖2 restoration error at the j-th iteration.
Minimum restoration error
Method minj=1,...
(ej) arg minj=1,...
(ej)
CG 0.1215 4
Richardson 0.1218 8
TL(CG) 0.1132 8
TL(Rich) 0.1134 16
MGM(Rich, 1) 0.1127 12
MGM(Rich, 2) 0.1129 5
CGNE 0.1135 178
RichNE 0.1135 352
Relative error vs. number of iterations
0.2Marco Donatelli (University of Insubria) An iterative multilevel regularization method 25 / 30
Numerical experiments
Noise = 10%
For CG and Richardson it is better to resort to normal equations.
Minimum restoration error
Method minj=1,...
(ej) arg minj=1,...
(ej)
CGNE 0.1625 30
RichNE 0.1630 59
TL(CGNE) 0.1611 48
TL(RichNE) 0.1613 97
MGM(RichNE,1) 0.1618 69
MGM(RichNE,2) 0.1621 26
MGM(Rich,1) 0.1648 3
MGM(Rich,2) 0.1630 1
Relative error vs. number of iterations
0.21Marco Donatelli (University of Insubria) An iterative multilevel regularization method 26 / 30
Conclusions
Outline
1 Restoration of blurred and noisy imagesThe model problemProperties of the PSFIterative regularization methods
2 Multigrid regularizationMultigrid methodsIterative Multigrid regularizationComputational CostFilter factor analysis of the TL
3 Numerical experiments
4 Conclusions
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 27 / 30
Conclusions
Possible generalizations
• Include the nonnegativity constraints.
• Improve the projector:
p(x , y) = (1 + cos(x))α(1 + cos(y))α, α ∈ N+.
• The γ regularization:
varying γ, the proposed multigrid is a direct (one step)regularization method with regularization parameter γ.
The computational cost increases with γ but not so much (e.g.γ = 8 ⇒ O(N1.5) where N = n2).
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 28 / 30
Conclusions
Summarizing . . . multigrid regularization method
• It is a general framework which can be used to improve theregularization properties of an iterative regularizing method.
• It leads to a smaller relative error and a flatter error curve withrespect to the smoother applied alone.
• It is fast and usually it obtains a good restored image also withoutresorting to normal equations.
• It can be combined with other techniques and it can lead to severalgeneralizations (e.g., nonnegativity constraints).
ReferenceM. Donatelli and S. Serra Capizzano, On the regularizing power ofmultigrid-type algorithms, SIAM J. Sci. Comput., 27–6 (2006) pp.2053–2076.
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 29 / 30
Conclusions
Future work
Theoretical
• A complete theoretical analysis of the regularization properties.
Applications:
• strictly nonsymmetric PSFs.
• Combination with techniques for edge enhancing (Wavelet, TotalVariation, . . . ).
Numerics/Simulations:
• A complete experimentation with all the proposed BCs (multigridmethods already exist for the arising matrices, see [Arico, Donatelli,Serra Capizzano, SIMAX, Vol. 26–1 pp. 186–214.]).
Marco Donatelli (University of Insubria) An iterative multilevel regularization method 30 / 30