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Image Super-resolution Using Statistical Learning - PowerPoint PPT Presentation

Image Super-resolution Using Statistical Learning. Preliminary Exam Presentation Karl Ni Professor Truong Nguyen Professor Nuno Vasconceles Professor William Hodgkiss. Outline. Problem Description: Image super-resolution Background Information Non-statistical Techniques

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Image Super-resolution Using Statistical Learning

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Image Super-resolutionUsing Statistical Learning

Preliminary Exam Presentation

Karl Ni

Professor Truong Nguyen

Professor Nuno Vasconceles

Professor William Hodgkiss

Outline

• Problem Description: Image super-resolution

• Background Information

• Non-statistical Techniques

• Statistical Techniques

• Classification-based Approaches

• Contributions

• Regression-based Approach and Rationale

• Spatial Domain SVM Superresolution

• Frequency Domain SVM Superresolution

• Results

• Conclusion

Problem Description

• Low-resolution image transferred to high resolution

• Single-frame image superresolution is a way to fill in the missing information for a larger image, specifically what values these pixels take on.

Low Freq

Coefficients

Low Freq

Coefficients

All Zeros

Frequency Domain

Non-statistics-based Interpolation Techniques

• B-Splines Methods

• Bilinear

• Bicubic

• Cosine Domain Upscaling: Zero Padding.

Current Statistical Learning Techniques

• Instead of blindly guessing or filling in information, we can use prior knowledge included in a training set.

• Nearest Neighbor: Freeman, Jones, Pasztor

• Expectation Maximization: Atkins and Bouman

• Image estimation lowers MSE

On the correlation between known and unknown information

• There’s some relationship between every even and every odd components, just as there is some relationship between high frequencies and low frequencies.

Knowledge Base

Decision

Operations

Informed Decision

Observations

Using past observations for future decisions

• Prior knowledge of the values and locations of the missing information.

• Exploit this knowledge as a relationship between known and unknown

Application to Machine Perception

• Would like pattern recognition for application of knowledge base to observation data

• Call input random variables X, the data.

• Call associated labels for input random variables Y.

• Have pairs:

{ (x1, y1), (x2, y2), …, (xN, yN), … }

Statistical Learning: (Classification)

• Two types of variables:

• X : vector of observations

(features) in the world

• Y : state (class) of the

world

• X, Y are related by a

f: function (unknown)

Knowledge Base:

{ (x1, y1), (x2, y2), …, (xN, yN)}

Decision

Operations based on

Cost Function

h(xobs)

Informed Decision = ydec

Observations = xobs

Statistical Learning Goal

• Goal: Make h(x) = f(x) given training data as knowledge base

Application to Super-resolution

Pixel Locations

• What is the feature set x?

• Can be a single pixel

• Can be a vector of all the pixels

• Can be linear transformation of pixels

• Can be kernelized transformation of pixel values

• Can be anything! (reasonable)

• What is the h(x) that we are trying to learn?

• Can be filter coefficients with input x = original pixels

• Can be actual pixel values

• Can be anything! (reasonable)

Expectation Maximization For Filter Design

C.B. Atkins, et. al., 1998, “Classification Based Methods in

Optimal Image Interpolation”, PhD Dissertation, Purdue

University, West Lafayette, IN, USA

Minimizing the Risk Function (1/2)

• We wish to learn the relationship of f(x) = y, and model it the best we can with h(x)

• “0-1” loss function:

0, if y = h(x, α)

1, if y ≠ h(x, α)

• Minimize the risk, defined as expected loss:

R(α) = EX,Y{ L[ y, h(x, α)] }

= ∫ PX,Y (x, y) L[y, h(x, α)] dx dy

= 0 • PX,Y[y= h(x, α)] + 1 • PX,Y[y ≠ h(x, α)]

= PX,Y[y ≠ h(x, α)]

L[ y, h(x, α)] =

Minimizing the Risk Function (2/2)

• What function h(x, α) minimizes the risk?

h* = argminhR (α, x)

= argminhEX,Y{ L[ y, h(x, α)] }

= argminh PX,Y[y ≠ h(x)]

h*(x) = argminh PY|X[y ≠ h(x) | x]

= argminh 1 – PY|X[y = h(x) | x ]

= argmaxhPY|X[ h(x) | x ]

= argmaxiPY|X[ i | x ]

• In other words, the optimal value of h(x) = i, given an observation x, is the value which maximizes the posterior PY|X(i | x)

Learning Algorithms

• Determine the value i that maximizes PY|X(i | x)

• All methods must assume a model

• Two Different Philosophies:

• Generative Method

• Model p(x,y) and use Bayesian rules to calculate p(y|x).

• Possibly biased due to the unknown assumptions

• Discriminant Method

• Model p(y|x) directly and map accordingly.

• Possibly highly variable with few data points

• Relatively new field in the past decade, universally applied

Linear Discriminants

• Underlying concept is to use discriminant functions to estimate a boundary or regression in feature space.

• Use a hyperplane

to create boundary

of classes:

wTx + b = 0.

• A correct decision

is given by

y•g(x) = y•(wTx + b)

> 0 ?

wTx + b = 0 divides hyperspace into two subspaces.

Distance to origin is

b/||w||, where ||w|| is the norm of w.

Distance to closest point is:

wTxi + b

γ = mini

||w||

wTxi + b

γ = mini

||w||

Support Vector Machines:Classification

• Recall the minimum distance to nearest point is:

• This is called the margin.

• It is natural that we would wish to maximize the margin. (Maximize this minimum distance.)

• The SVM classifier that maximizes the margin under some normalization is

The Soft Margin

• Perhaps data is not well behaved. Not all data can be separated.

• Introduce an extra “slack variable”.

Support Vector Machine Regression

• Classification has a tendency of “discretizing” our output

• Can think of regression as kind of like a continuous version of classification, which would need infinite number of classes

• Will approximate the function that given the known image information

• Function is the relationship between known and unknown elements

Support Vector Regression

• Soft Margin SVMs for Classification:

Wish to minimize { ||w||2 + C Σξi } subject to

yi (wTxi + b) ≥ 1 – ξi

ξi ≥ 0, for all i

• Soft Margin SVMs for Regression:

Wish to minimize { ||w||2 + C Σ( ξi+ + ξi- ) } subject to

-ξi- ≤ | yi - (wTxi + b) | – ε ≤ ξi+

ξi-, ξi+ ≥ 0

• Rearranging, the Lagrangian can be written:

L(w,b,ξ+,ξ-) = wTw + Σαi-((wTxi+b)-yi-ε+ξi-)

+ Σαi+(yi-(wTxi+b)i-ε+ξi+) + Σri-ξi- + Σri+ξi+

SVR Cost Function and Dual 2/2

• The optimization is, for an ε and C chosen a priori:

max W(α+, α-) = -εΣ (α+ + α-) + Σi (αi+ - αi-)yi - ½ Σi Σk (αi+ - αi-) (αk+ - αk-) k ( xi ,xk)

subject to

0 ≤ α+, α-≤ C, Σi (αi+ - αi-) = 0

• The regression estimate will then have the form of:

f(x) = Σi (α+ - α-) k (x, xi) + b

Contributions

• Unsupervised learning using SVM

• Application of Support Vector Regression to Superresolution Problem

• Direct Application (i.e. pixel prediction)

• Indirect Application (i.e. filter coefficients)

FILTER COEFFICIENTS

c11

c12

c13

c21

c22

c23

c31

c32

c33

SVR Spatial Filter Selection

• Regression to find spatial domain filters

• Direct regression actually works better

f

Regression

Support Vector Regression:Frequency Domain

• DCT Domain for statistical purposes (regression)

• A downsampled version of an image is all the even samples.

• From the DCT-II of a downsampled image, can we reconstruct the DCT-II of the original image?

Decimation in Time

K. R. Rao, P. Yip 1988, “Discrete Cosine Transform: Algorithms, Advantages, Applications”, San Diego, CA, USA: Academic Press

Applying Learning to DIT & DIS

• We can rewrite decimation in time for DCT as a linear combination of the time/spatial domain terms corresponding to even and odd samples

• Decimation in Time

DCT-II(x) = k(m) (DCT-I(xeven) + DCT-I(xodd) + DCT-II(xeven) + DCT-II(xodd))

• Decimation in Time

DCT-II(x) = k(m) { X1 + X2 + X3 + X4 }

• Overall Idea

DCT-II(x) = Input Signal + f (Input Signal) + g (Remaining Terms),

f is exactly known and g is to be estimated

• DCT-II(X2N) = DCT-II(XN) + Known + Estimated.

Support Vector Regression

• DCT-II(X2N) = DCT-II(XN)+Known+Estimated.

• Given DCT-II(XN), can we determine the estimated terms that will give us DCT-II(X2N)?

• Our regression is thus,

Estimated= Σi (αi+ – αi-) K ( xi, DCT-II[XN] ) + b

• This is done for all lower coefficients.

• LibSVM & LSSVM, Regression package

Bilinear Filtering

SVR Spatial Filtering

SVR Filtering

Bilinear

Spatial Domain (5x5 Filter) vs Bilinear Filtering

Small training set (10 frames)

SVR Frequency Reconstruction vs Bilinear Interpolation

Bilinear Interpolation

SVR Frequency Regression

Small training set (10 frames)

PSNR Values

Method

PSNR

Bilinear

23.301

Bicubic

22.209

Spatial

25.995

Frequency

26.843

Comparison of SVR Algorithms

Small training set (10 frames)

SVM Frequency Reconstruction vs Bilinear Interpolation

Bilinear Interpolation

SVR Frequency Regression

4 x 4 features

8 x 8 features

Effect of Dimensionality

Structured Frequency Regression

Structured versus Direct SVR Regression

• Direct Frequency Regression

Future Work

• Apply superresolution to the error residual in video

• Denoising algorithms using Support Vector Regression

• Markov Random Fields, or some interrelationship between predicted values

• Motion Prediction Values