Lecture 3

C7B Optimization Hilary 2011 A. Zisserman

Cost functions with special structure:

• Levenberg-Marquardt algorithm

• Dynamic Programming• chains

• applications

First: review Gauss-Newton approximation

The Optimization Tree

Summary of minimizations methods

Update xn+1 = xn+ δx

1. Newton.

H δx = −g

2. Gauss-Newton.

2J>J δx = −g

3. Gradient descent.

λδx = −g

Levenberg-Marquardt algorithm

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• Away from the minimum, in regions of negative curvature, the

Gauss-Newton approximation is not very good.

• In such regions, a simple steepest-descent step is probably the bestplan.

• The Levenberg-Marquardt method is a mechanism for varying be-

tween steepest-descent and Gauss-Newton steps depending on how

good the J>J approximation is locally.

gradient descent

Newton

• The method uses the modified Hessian

H (x,λ) = 2J>J+ λI

• When λ is small, H approximates the Gauss-Newton Hessian.

• When λ is large, H is close to the identity, causing steepest-descent

steps to be taken.

LM Algorithm

H (x,λ) = 2J>J+ λI

1. Set λ = 0.001 (say)

2. Solve δx = −H(x,λ)−1 g

3. If f(xn+ δx) > f(xn), increase λ (×10 say) and go to 2.

4. Otherwise, decrease λ (×0.1 say), let xn+1 = xn+ δx, and go to 2.

Note : This algorithm does not require explicit line searches.

Example

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3Levenberg-Marquardt method

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gradient < 1e-3 after 31 iterations

• Minimization using Levenberg-Marquardt (no line search) takes 31iterations.

Matlab: lsqnonlin

Comparison

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gradient < 1e-3 after 31 iterations

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gradient < 1e-3 after 14 iterations

Gauss-Newton

• more iterations than Gauss-Newton, but

• no line search required,

• and more frequently converges

Levenberg-Marquardt

Case study – Bundle Adjustment (non-examinable)

Notation:

• A 3D point Xj is imaged in the “i” th view as

xij = Pi Xj

P : 3× 4 matrixX : 4-vector

x : 3-vector

Problem statement

• Given: n matching image points xij over m views

• Find: the cameras Pi and the 3D points Xj such that xij = Pi Xj

Number of parameters

• for each camera there are 6 parameters

• for each 3D point there are 3 parameters

a total of 6 m + 3 n parameters must be estimated

• e.g. 50 frames, 1000 points: 3300 unknowns

Example

image sequence cameras and points

Sparse form of the Jacobian matrix

• Image point xij does not depend on the parameters of any cameraother than Pi.

• Thus,∂xij/∂P

k = 0

unless i = k.

• Similarly, image point xij does not depend on any 3D point except

Xj.

∂xij/∂Xk = 0

unless j = k.

Form of the Jacobian and Gauss-Newton Hessian for the bundle-adjustment problem consisting of 3 cameras and 4 points.

J>JJ

By taking advantage of this sparse form, one iterative update of the

LM algorithm

• H (x,λ) = 2J>J+ λI

• Solve δx = −H(x,λ)−1 g

can be solved in O(N) rather than O(N3), where N is the total number

of parameters

= 0

Application: Augmented reality

original sequence

Augmentation

Dynamic programming

• Discrete optimization

• Each variable x has a finite number of possible states

• Applies to problems that can be decomposed into a sequence of stages

• Each stage expressed in terms of results of fixed number of previous stages

• The cost function need not be convex

• The name “dynamic” is historical

• Also called the “Viterbi” algorithm

Consider a cost function of the form

where xi can take one of h values

e.g. h=5, n=6

x1 x2 x3 x4 x5 x6

f(x) : IRn → IR

f(x) =

find shortest

path

Complexity of minimization:

• exhaustive search O(hn)

• dynamic programming O(nh2)

f(x) =nXi=1

mi(xi) +nXi=2

φi(xi−1, xi)

m1(x1) +m2(x2) +m3(x3) +m4(x4) +m5(x5) +m6(x6)

φ(x1, x2) + φ(x2, x3) + φ(x3, x4) + φ(x4, x5) + φ(x5, x6)

trellis

Example 1

closeness to measurements

smoothness

f(x) =nXi=1

(xi − di)2 +nXi=2

λ2(xi − xi−1)2

f(x) =nXi=1

mi(xi) +nXi=2

φ(xi−1, xi)

d

i

x

i

Motivation: complexity of stereo correspondence

Objective: compute horizontal displacement for matches between left and right images

xi is spatial shift of i’th pixel → h = 40

x is all pixels in row → n = 256

Complexity O(40256) vs O(256× 402)

x1 x2 x3 x4 x5 x6

Key idea: the optimization can be broken down into n sub-optimizations

f(x) =nXi=1

mi(xi) +nXi=2

φ(xi−1, xi)

Step 1: For each value of x2 determine the best value of x1

• Compute

S2(x2) = minx1{m2(x2) +m1(x1) + φ(x1, x2)}

= m2(x2) +minx1{m1(x1) + φ(x1, x2)}

• Record the value of x1 for which S2(x2) is a minimum

To compute this minimum for all x2 involves O(h2) operations

x1 x2 x3 x4 x5 x6

Step 2: For each value of x3 determine the best value of x2 and x1

• Compute

S3(x3) = m3(x3) +minx2{S2(x2) + φ(x2, x3)}

• Record the value of x2 for which S3(x3) is a minimum

Again, to compute this minimum for all x3 involves O(h2) operations

Note Sk(xk) encodes the lowest cost partial sum for all nodes up to k

which have the value xk at node k, i.e.

Sk(xk) = minx1,x2,...,xk−1

kXi=1

mi(xi) +kXi=2

φ(xi−1, xi)

Viterbi Algorithm

Complexity O(nh2)

• Initialize S1(x1) = m1(x1)

• For k = 2 : n

Sk(xk) = mk(xk) + minxk−1

{Sk−1(xk−1) + φ(xk−1, xk)}bk(xk) = argmin

xk−1{Sk−1(xk−1) + φ(xk−1, xk)}

• Terminatex∗n = argmin

xnSn(xn)

• Backtrackxi−1 = bi(xi)

Example 2

Note, f(x) is not convex

f(x) =nXi=1

(xi − di)2 +nXi=2

gα,λ(xi − xi−1)

where

gα,λ(∆) = min(λ2∆2,α) =

(λ2∆2 if |∆| < √α/λα otherwise.

i

d

i

x

Note

This type of cost function often arises in MAP estimation

x∗ = argmaxxp(x|y)

measurements

= argmaxxp(y|x)p(x) Bayes’ rule

e.g. for Gaussian measurement errors, and first order smoothness

Use negative log to obtain a cost function of the form

from likelihood from prior

f(x) =nXi=1

(xi − yi)2 +nXi=2

λ2(xi − xi−1)2

∼nYi

e−(xi−yi)

2

2σ2 e−β2(xi−xi−1)2

Where can DP be applied?

Example Applications:

1. Text processing: String edit distance

2. Speech recognition: Dynamic time warping

3. Computer vision: Stereo correspondence

4. Image manipulation: Image re-targeting

5. Bioinformatics: Gene alignment

Dynamic programming can be applied when there is a linear ordering on the cost function (so that partial minimizations can be computed).

Application I: string edit distance

The edit distance of two strings, s1 and s2, is the minimum number of single character mutations required to change s1 into s2, where a mutation is one of:

1. substitute a letter ( kat cat ) cost = 1

2. insert a letter ( ct cat ) cost = 1

3. delete a letter ( caat cat ) cost = 1

Example: d( opimizateon, optimization )

op imizateon|| |||||||||optimization||||||||||||cciccccccscc

d(s1,s2) = 2

‘c’ = copy, cost = 0

Complexity

• for two strings of length m and n, exhaustive search has complexity O( 3m+n )

• dynamic programming reduces this to O( mn )

Using string edit distance for spelling correction

1. Check if word w is in the dictionary D

2. If it is not, then find the word x in D that minimizes d(w, x)

3. Suggest x as the corrected spelling for w

Note: step 2 appears to require computing the edit distance to all words in D, but this is not required at run time because edit distance is a metric, and this allows efficient search.

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audio

log(STFT)

time

short term Fourier transform

sample template

Application II: Dynamic Time Warp (DTW)

Objective: temporal alignment of a sample and template speech pattern

freq

uenc

y (H

z)

warp to match `columns’ of log(STFT) matrix

Application II: Dynamic Time Warp (DTW)

is time shift of i th column

quality of match cost of allowed moves

f(x) =nXi=1

mi(xi) +nXi=2

φ(xi−1, xi)

template

sample

(1, 0)

(0, 1)

(1, 1)

xi

Application III: stereo correspondence

Objective: compute horizontal displacement for matches between left and right images

quality of match uniqueness, smoothness

f(x) =nXi=1

mi(xi) +nXi=2

φ(xi−1, xi)

is spatial shift of i th pixelxi

left image band

right image band

normalized cross correlation(NCC)

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m(x) = α(1−NCC)2

NCC of square image regions at offset (disparity) x

• Arrange the raster intensities on two sides of a grid

• Crossed dashed lines represent potential correspondences

• Curve shows DP solution for shortest path (with cost computed from f(x))

range map

Pentagon example

left image right image

Real-time application – Background substitution

Left view Right view

Input

input left view

Results

Background substitution 1 Background substitution 2

• Remove image “seams” for imperceptible aspect ratio change

Application IV: image re-targeting

seam

Seam Carving for Content-Aware Image Retargeting. Avidan and Shamir, SIGGRAPH, San-Diego, 2007

scale

seam removal

Finding the optimal seam – s

E(I) = |∂I/∂x|+ |∂I/∂y|→ s∗ = argminsE(s)

E(I)s