GIEE
ICS
R01943121
陳柏淳
2013 06
Content
1
Abstruct
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3
2
Introduction
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4
2.1
Multi

media Content Analysis
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4
2.2
Machine Learning
and Design Challenges
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5
2.3
An Introduct
or
y Example
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7
3
Learning
Pattern Recognition From Example
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11
4
Hyperplane Classifiers
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13
5
Optimal Margin Support Vector Classifiers
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6
Kernels
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18
6
.1
Product Features
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18
6
.2
Polynomial Feature Spaces Induced by Kernels
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defined.
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6.3
Examples of Kernels
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7
Multi

class SV Classifiers
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7
.1
One

against

all Method
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7
.2
One

against

one
Method
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7.3
Considering All Data At Once Method
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8
Applications
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8
.1
LIBSVM
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8
8
.2
Experiment
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2
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9
Conclusion
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10
Reference
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1
Abstract
Learning general functional dependencies is one of the main goals in machine
learning. Recent progress in kernel

based methods has focused on designing flexible
and powerful input representations. This tutorial addresses the complementary issue
of problems
involving complex outputs such as multiple dependent output variables
and structured output spaces.
Some
propose to generalize multiclass Support Vector
Machine learning in a formulation that involves features extracted jointly from inputs
and outputs. Th
e resulting optimization problem is solved efficiently by a cutting
plane algorithm that exploits the sparseness and structural decomposition of the
problem.
Some
demonstrate the versatility and effectiveness on problems ranging
from supervised grammar lea
rning and named

entity recognition, to taxonomic text
classification and sequence alignment.
The tutorial starts with an overview of the concepts of structural risk
minimization.
T
hen describe linear Support Vector Machines (SVMs) for separable
and non

sep
arable
data, working through a non

trivial example in detail. We describe
a mechanical analogy, and discuss
when SVM solutions are unique and when they are
global. We describe how support vector training can
be practically implemented, and
discuss in detai
l the kernel mapping technique which is used to construct
SVM
solutions which are nonlinear in the data. We show how Support Vector machines can
have very large
(even inﬁnite) VC dimension by computing the VC dimension for
homogeneous polynomial and Gaussi
an
radial basis function kernels. While very high
VC dimension would normally bode ill for generalization
performance, and while at present there exists no theory which shows that good
generalization performance
is guaranteed for SVMs, there are several ar
guments
which support the observed high accuracy of SVMs,
which we review. Results of
some experiments which were inspired by these arguments are also presented.
We give numerous examples and proofs of most of the key theorems.
2
Introduction
2.1
Multi

media Content Analysis
With the growth of the semiconductor technology, hand

held devices nowadays
are equipped with more and more powerful VLSI architectures, like high

quality
CMOS senso
rs, large storage devices,.. ,
and so on. People can easily u
se this
kind of
product to take and store pictures, and to search for the related images in
the database.
Since the storage on the hand

held devices with cameras becomes
larger and larger, it
is quite often that the user took hundreds of thousands of
pictu
res and kept them in the
storage device without classifying them. It will be a
big burden to manually classify
and tag this huge amount of photos.
To effi
ciently manage such huge amount of
c
ollections, it is necessary to have
access to high

level informati
on about the contents
obtained in the image
.
Since it might be troublesome for people to manually
manipulate, index, sort,
fi
lter, summarize, or search
through the photos they take
, it is
necessary to
establish a system which can automatically analyze the
contents of
p
hotos. Organizing
this huge amount of photos into categories and providing effective
indexing
is imperative for
”real

time”
browsing and retrieval
.
The technique to
accomplish such tasks is called ”
Image Classifi
cation” or
”Image Categorizati
on”.
Early works only consider general purpose semantic
classes, such as outdoor
scenes
versus indoor scenes
, or city scenes versus
landscape scenes .
However, all these
previous works rely only on the low

level features and
the
y can handle very simple
classifi
cation problems as they only consider a few
classes. To classify such huge
amount of informati
ve image data, it is diffi
cult
to use only low

level features since
those features do not strongly correlate with
human perception. Therefore, a semantic
modeling step has to be employed to
bridge this semantic gap. Many software
solutions have been proposed to
provide an intelligent indexing and im
age content
analysis platform
.
One way to bridge this semantic gap is to map the low

level
features to the sem
antic
concepts. The generation of concept features usually involves
two stages
of processing: (1) concept learning and (2) concept detection and score
mapping
to produce the required feature. The concept detector may take any forms
includin
g Support Vector
Machines (SVM). Among this
machine learning
algorithms,
the supervised learning methods, SVM are the most commonly
used algorithms.
2.2
Machine Learning and Design Challenges
Machine learning algorithms, SVM, are gaining attention in many
fie
lds. To bri
dge the semantic gap encountered in the multimedia content analysis,
the supervised learning methods are adopted to project the low

level feature space
to higher

level of semantic concept space. In order to do the mapping, there are
a
lot of researchers who take the strategy to combine local region concept information
to get the rich information that are hidden in one image. In this scheme,
a two

stage
image featu
re extraction is required. The fi
rst stage contains image
sampling, such
as
image partitioning or key point extraction, and classifying those
samples into different
concept classes. Then in the next stage, a new image representation
can be obtained
by manipulating the local region concept labels. This
method is becoming more an
d
more important and achieves good performance in
many applications.
In contrast to
the generative model, like GMM, that models the distribution
of the data of a given
class, discriminative models, on the other hand, let the data
speak for themselves, and
SVM is the
one of the most popular one
. Support
Vector Machine (SVM) is one of
the most powerfu
l techniques for real

world pat
tern recogniti
on and data mining
problems
. In recent years, SVM proposed
by Cortes and Vapnik has become the
state

of

the

art cl
assi
fie
r for some supervised
classi
fication problems , especially in
the fi
eld of multimedia content
analysis. SVMs are famous for their strong
g
eneralization guarantees derived
from the max

margin property, and for their ability
to use very high dimension
al
feature spaces usi
ng the kernel functions
.
The SVM
classi
fier fi
nds a hyperplane which separates two

class data with
maximal margin.
Since the data sets are not always linearly separable, the SVM
takes the kernel
method to deal with this problem . Ther
e have been a variety
of SVM software
packages that provide ef
fi
cient SVM implementation, and
LIBSVM
is the most
commonly adopted one.
Ever since the SVM classi
fier was proposed
, there have been
many
researchers devoted themselves into this area. Most of
the researchers focus on
the algorithm part to make l
arge learning practical
, extend the original
binary
classi
fi
er into multi

class , or even generalize the SVM classi
fi
er to
h
andle arbitrary
output type
.
Since the computations involved in the SV
M algorithm are similar to that
involved
in the Arti
fi
cial Neural Network (ANN), some comparisons have also be
made between the two algorithms. The development of ANN followed a heuristic
path, with applications and extensive experimentation preceding the
ory. In contrast,
the developmen
t of SVM involved sound theory fi
rst, then implementation
and
experiments. A signi
fi
cant advantage of SVM is that while ANN can suffer
from
multiple local minima, the solution to an SVM is global and unique. The
reason that
SVM often outperforms ANN in practice is that SVM can deal with
bigger problems
than ANN,
and SVM is less prone to over fitting
.
When the problem to deal with is
getting more complex,
say, the dimension is quite large, then the SVM algorithm
often achieve
s
the state

of

the

art performance.
However, there is a signifi
cant lack of
hardware architecture implementations
for SVM classifi
er for solving the real
world
problems.
2.3
An Introductory Example
Suppose we are given empirical data
Here, the domain X
is some non

empty set that the patterns xi
are taken from .T
he
are called
labels or targets.
Unless stated otherwise, indices
i and j will always be
understood to run over the training set,
i.e.
i
;
j
,1;...;m.
Note that we have not made
any assumptio
ns on the domain X other than it being a set. In
order to study the
problem of learning, we need additional structure. In learning, we want to be
able to
generalize to unseen data points. In the case of pattern recognition, this means that
given
some new p
attern x
∈
X; we want to predict the corresponding y
∈
{
±
1
}
: By this
we mean,
loosely speaki
ng, that we choose y such that (
x ,
y
)
is in some sense similar
to the training
examples. To this end, we need similarity measures in X and in
{
±
1
}
The
latter is easy, as two
target values can only be identical or di
ﬀ
erent. For the
former, we require a similarity measure
i.e. a function that, given two examples x and
x
’
; returns a real number characterizing their
similarity. For reasons that will be
come
clear later, the function k is called a ke
rnel
.A type of similarity measure that is of
particular mathematical appeal are dot products. For
instance, given two vectors
x
;
x
’
∈
𝑅
𝑁
the canonical dot product is deﬁned as
Here,
(
)
denotes the ith entry of x.
The geometrical interpretation of this dot
p
roduct is that it computes the cosine of the angle
between the vectors x and x’,
provided they are normalized to length 1. Moreover, it allows
computation of the
length of a vector x
as
√
∙
,
and of the distance between two vectors as
the length
of the di
ﬀ
erence vector. Therefore, being able to compute dot products mounts to
being able to carry out all geometrical constructions that can be formulated in terms of
angles,
lengths and d
istances.
Note, however, that we have not made the assumption
that the patterns live in a dot product
space. In order to be able to use a dot product as
a similarity measure, we therefore ﬁrst need to
transform them into some dot product
space
H; which nee
d not be identical to
𝑅
𝑁
.
To this end
,
we use a map
The space
is
called a feature space. To summarize, there are three beneﬁts to
transform the
data into
1.
It lets us deﬁne a similarity measure from the dot product in
,
2.
It allows us to deal
with the patterns geometrically, and thus lets us study learning
algorithm using linear algebra and analytic geometry.
3.
The freedom to choose the mapping
will enable us to design a large variety of
learning algorithms. For instance, consider a situation where the inputs already
live in a dot product space. In that case, we could directly deﬁne a similarity
measure as the dot product. However, we might stil
l choose to ﬁrst apply a
non

linear map
to change the
representation into one that is more suitable for a
given problem and learning algorithm.
We are now in the position to describe a pattern recognition learning algorithm that is
arguable one of the si
mplest possible. The basic idea is to compute the means of the
two classes
in feature space,
where
𝑚
+
and
𝑚
−
are the number of examples with positive and negative labels,
respectively (see
Figure 1). We then assign a new point x to the class whose
mean is
closer to it. This geometrical
construction can be formulated in terms of dot products.
Half

way in between
+
and
−
lies the
point
We compute the class
of x by checking whether the vector connecting c
and x encloses an angle smaller
than
p=2
with the vector
connecting the class
means, in other words
Here, we have deﬁned the o
ﬀ
set
It will be proved instructive to rewrite this expression in terms of the patterns
in the input
domain
To this end, note that we do not have a dot product in
,
all
we have is the similarity
measure k (cf. (5)). Therefore, we need to rewrite everything
in terms of the kernel k evaluated
on input patterns. To this end, substitute (6) and (7)
into (8) to get
the decision function
Similarly, the o
ﬀ
set becomes
Let us consider one well

known special case of this type of classiﬁer. Assume that the
class
means have the same distance to the origin (hence b
=
0), and that k can be
viewed as a density,
i
.e. it is positive and has integral 1,
In order to state this assumption, we have to require that we can deﬁne an integral on
If the above holds true, then (10) corresponds to the so

called Bayes decision
boundary
separating the two classes, subject
to the assumption that the two classes
were generated from
two probability distributions that are correctly estimated by the
Parzen windows estimators of
the two classes
Given some point x; the label is then simply computed by checking which of the
two,
𝑝
1
(
)
or
𝑝
2
(
)
,
is larger, which directly leads to (10). Note that this decision is the
best we can do if we
have no prior information about the probabilities of the two
classes. For further details, see [1].
Classiﬁer (10) is quite close to the
types of
learning machines that we will be interested in. It is
linear in the feature space, and
while in the input domain, it is represented by a kernel expansion
in terms of the
training points. It is example

based in the sense that the kernels are cent
e
red on
the
training examples, i.e. one of the two arguments of the kernels is always a training
example.
The main points that the more sophisticated techniques to be discussed later
will deviate from
(10) are in the selection of the examples that the kerne
ls are
cent
e
red on, and in the weights that
are put on the individual data in the decision
function. Namely, it will no longer be the case that
all training examples appear in the
kernel expansion, and the weights of the kernels in the
expansion will no lo
nger be
uniform. In the feature space representation, this statement
corresponds to saying that
we will study all normal vectors
w
of
decision hyperplanes that can be
represented as
linear combinations of the training examples. For instance, we might want
to
remove
the inﬂuence of patterns that are very far away from the decision boundary, either
since
we expect that they will not improve the generalization error of the decision
function, or since
we would like to reduce the computational cost of evaluating
the
decision function (cf. (10)).
The hyperplane will then only depend on a subset of
training examples, called support vectors.
3
Learning Pattern Recognition From Example
With the above example in mind, let us now consider the problem of pattern
r
ecognition in a
more formal setting [5, 6], following the introduction of Schoolkopf .
et al.
[7]. In two

class pattern
recognition, we seek to estimate a function
based on input
–
output training data (1). We assume that the data were generated
independen
tly
from some unknown (but ﬁxed) probability distribution P
( x , y ).
Our
goal is to learn a function
that will correctly classify unseen examples
(x ,y ),
i.e. we
want
f(x) =
y for examples
(x , y)
that
were also generated from P
(x , y).
If we put no
restriction on the class of functions that we choose our estimate f from, however,
even
a function which does well on the training data, e.g. by satisfying f
(
) =
for all i
=
1
,...,m,
need not generalize well to unseen examples. To see this, note th
at for each
function f
and any test set
(
1
̅
̅
̅
,
1
̅
̅
̅
)
,
.
.
.
,
(
𝑚
̅
̅
̅
̅
,
𝑚
̅
̅
̅
̅
)
𝜖
𝑅
𝑁
×
{
±
1
}
,satisfying
{
̅
1
̅
,
.
.
.
,
̅
𝑚
̅
}
∩
{
1
,
…
,
𝑚
}
=
{
}
,
there exists another function
𝑓
∗
such that
𝑓
∗
(
)
=
for all i = 1 , … ,m , yet
𝑓
∗
(
̅
)
≠
̅
for all
i
= 1 , …, m .
As we are
only giv
en the training data, we have no means of selecting which
of the two
f
unctions (and hence which of the completely di
ﬀ
erent sets of test label predictions) is
preferable. Hence, only minimizing the training error (or empirical risk),
does
not imply a small test error (called risk), averaged over test examples drawn
from the
underlying distribution P
(x ,
y
),
Statistical learning theory [5, 6, 8, 9], or VC (Vapnik
–
Chervonenkis) theory, shows
that it is
imperative to restrict the class of
functions that
f is chosen from to one which
has a capacity that
is suitable for the amount of available training data. VC theory
provides bounds on the test
error. The minimization of these bounds, which depend on
both the empirical risk and the
capacity
of the function class, leads to the principle of
structural risk minimization [5]. The best

known capacity concept of VC theory is the
VC dimension, deﬁned as the largest number h of
points that can be separated in all
possible ways using functions of the
given class. An example
of a VC bound is the
following: if h
<
m is the VC dimension of the class of functions that the
learning
machine can implement, then for all functions of that class, with a probability of at
least 1

η
; the bound
holds, w
here the c
onﬁdence term f is deﬁned as
Tighter bounds can be formulated in terms of other concepts, such as the annealed VC
entropy
or the Growth function. These are usually considered to be harder to evaluate,
but they play a
fundamental role in the conceptual
part of VC theory [6]. Alternative
capacity concepts that can
be used to formulate bounds include the fat shattering
d
imension [10].
The bound (18) deserves some further explanatory remarks. Suppose we wanted
to learn a
‘dependency’ where P(x ,
y
)
=
P
(x)
∙
P(
y
)
,
i.e. where the pattern x contains no
information about
the label y; with uniform P
(y).
Given a training sample of ﬁxed size,
we can then surely come up
with a learning machine which achieves zero training
error (provided we have no examples
contradic
ting each other).
H
owever, in order to
reproduce the random labelling, this machine
will necessarily require a large VC
dimension h: Thus, the conﬁdence term (19), increasing
monotonically with h; will be
large, and bound (18) will not support possible hop
es that due to
the small training
error, we should expect a small test error. This makes it understandable how
(18) can
hold independent of assumptions about the
underlying distribution P(x , y)
:
it always
holds (provided that h<
m), but it does not alway
s
make a non

trivial prediction

a
bound on
an error rate becomes void if it is larger than the maximum error rate. In
order to get non

trivial
predictions from (18), the function space must be restricted
such that the capacity (e.g. VC
dimension) is small en
ough (in relation to the available
amount of data).
4
Hyperplane Classifiers
In the present section, we shall describe a hyperplane learning algorithm that can
be performed
in a dot product space (such as the feature space that we introduced
previously).
As described in
the previous section, to design learning algorithms, one
needs to come up with a class of
functions whose capacity can be computed.
Vapnik and Leaner [11] considered the class of hyperplanes
correspondin
g to decision
functions
and
proposed a learning algorithm for separable problems, termed the generalized
portrait, for
constructing f from empirical data. It is based on two facts. First, among
all hyperplanes
separating the data, there exists a unique one yielding the maximum
margi
n of separation
between the classes,
Second, the capacity decreases with increasing margin.
To construct this optimal hyperplane (cf. Figure 2), one solves the following
o
ptimization
problem:
A way to solve (23) is through its Lagrangian dual:
w
here
The Lagrangian L has to be minimized with respect to the primal variables w and b
and
maximized with
respect to the dual variables
𝛼
.
For a non

linear problem like
(23), called the
primal problem, there are several closely related problems of which
the Lagrangian dual is an
important one. Under certain conditions, the primal and dual
problems have the same optimal
objective values. Therefore, we can instead solve the
dual which may b
e an easier problem than
the primal.
In part
icular, we will see in Section 5
that when working in feature spaces, solving
the dual may be the only way to train SVM.
Let us try to get some intuition for this primal
–
dual relation. Assume
(
̅
,
̅
)
is an
op
timal
solution of the primal with the optimal objective value
Thus, no
(w , b)
satisﬁes
With (26), there is
α
>0 such that for all w ,
b
We do not provide a rigorous proof here but details can be found in, for example,
Reference
[13]. Note that for
general convex programming this result requires some
additional conditions
on constraints which are now satisﬁed by our simple linear
inequalities.
Therefore, (27) implies
On the other hand, for any
α
,
so
Therefore, with (28), the inequality in (29) becomes an equality. This property is
the strong
duality where the primal and dual have the same optimal objective value. In
addition, putting
(
̅
,
̅
)
into (27), with
𝛼
𝑖
̅
≥
0 and
which is u
sually called the
complementarity condition.
To simplify the dual, as
L(w
,
b,
α
)
is convex when
α
is ﬁxed, for any given
α
,
l
eads
to
and
As
α
is now given, we may wonder what (32) means. From the deﬁnition of the
L
agrangian, if
we can decrease
in
L(w,b,
α
)
as much as we want. Therefore, by
substituting (33) into (24), the dual problem can be written as
As
−
∞
is deﬁnitely not the maximal objective value of the dual, the dual optimal
solution does
not happen when
Therefore, the dual problem is
simpliﬁed to
ﬁ
nding multipliers
𝛼
which
This is the dual SVM problem that we usually refer to. Note that (30), (32), and
𝛼
>
=0 for all i,
are called the Karush
–
Kuhn
–
Tucker (KKT) optimality conditions of the
primal problem.
Except an abnormal situation where all o
ptimal
𝛼
are zero, b can be
computed using (30).
The discussion from (31) to (33) implies that we can consider a
di
ﬀ
erent form of dual
problem:
Thi
s is the so

called Wolfe dual for convex optimization, which is a very early work in
duality
[14]. For
convex and di
ﬀ
erentiable problems, it is equivalent to the
L
agrangian dual though the
derivation of the Lagrangian dual more easily shows the
strong duality results. Some notes
about the two duals are
in, for example, [15,
Section 6
.4].
Following the
above discussion, the hyperplane decision function can be written
as
The solution vector
w has an expansion in terms of a subset of the training patterns,
namely
those patterns whose
𝛼
is non

zero, called support vectors. By (30), the
Support Vector
s lie on
the margin (cf. Figure 2). All remaining examples of the
training set are irrelevant: their
constraint (23) does not play a role in the
o
ptimization,
and they do not appear in expansion
(33). This nicely captures our intuition of the
problem: as t
he hyperplane (cf. Figure 2) is
completely determined by the patterns
closest to it, the solution should not depend on the other
examples.
The structure of the optimization problem closely resembles those that typically
arise in
Lagrange’s formulation of m
echanics. Also there, often only a subset of the
constraints become
active. For instance, if we keep a ball in a box, then it will
typically roll into one of the corners.
The constraints corresponding to the walls which
are not touched by the ball are irre
levant, the
walls could just as well be removed.
Seen in this light, it is not too surprising that it is possible to give a mechanical
interpretation
of optimal margin hyperplanes [16]: If we assume that each support
vector
exerts a
perpendicular forc
e of size
𝛼
and sign
on a solid plane sheet
lying along the hyperplane, then
the solution satisﬁes the requirements of mechanical
stability. Constraint (32) states that the
forces on the sheet sum to zero; and (33)
implies that the torques also sum
to zero, via
∑
×
(
𝑦
𝑖
𝛼
𝑖
𝒘
‖
𝒘
‖
)
=
𝒘
×
(
𝒘
‖
𝒘
‖
)
=
0
.
There are theoretical arguments supporting the good generalization performance
of the
optimal hyperplane [5, 8, 17
–
19]. In addition, it is computationally attractive,
since it can be
constructed by solving a
quadratic programming problem.
5
Optimal Margin Support Vector Classifiers
We now have all the tools to describe support vector machines [1, 6]. Everything
in the last
section was formulated in a dot product space. We think of this space as the
feature space
described in Section 1. To express the formulas in terms of
the input
patterns living in X ,
we
thus need to employ (5), which expresses the dot product
of
bold face feature vectors x ,
x
’
in
terms of the kernel
k evaluated on input patterns x ,
x
’ ,
This can be done since all feature vectors only occurred in dot products. The weight
vector (cf.(33)) then becomes
an expansion in feature space,
and will
thus typically no
longer correspond
to the image of a single vector from input space. We thus obtain
decision functions of the more
general form (cf. (38))
and the following quadratic program (cf. (35)):
Working in the feature space somewhat forces
us to solve the dual problem
instead of the
primal. The dual problem has the same number of variables as the
number of training data.
However, the primal problem may have a lot more (even
inﬁnite) variables depending on the
dimensionality of the feature s
pace (i.e. the length
of
Φ
(
x
)
). Though our derivation of the dual
problem in Section 3 considers
problems in ﬁnite

dimensional spaces, it can be directly extended
to problems in
Hilbert spaces [21].
6
Kernels
We now take a closer look at the issue of the
similarity measure, or kernel, k: In
this section,
we think of X as a subset of the vector space
𝑅
𝑁
,
(
𝑁𝜖
Ν
)
,
endowed with
the canonical dot
product (3).
6.1
Product features
Suppose we are given patterns x
ϵ
𝑅
𝑁
where most information is contained in
the
d
th order
products (monomials) of entries
[
]
of x,
where
𝑗
1
,
…
,
𝑗
𝑑
𝜖
{
1
,
…
,
𝑁
}
.
In that case, we might prefer to
extract
these product
features, and
work in the feature space
of all products of
d
entries. In visual
recognition problems, where
images are often represented as vectors, this would
amount to extracting features which are
products of individual pixels.
For instance, in
𝑅
2
,
we can collect all monomial feature extractors of degree 2 in
the nonlinear map
This approach works ﬁne
for small toy examples, but it fails for realistically sized
problems: for
N

dimensional input patterns, there exist
di
ﬀ
erent monomials (43), comprising a feature space
of dimensionality
For instance,
already 16
x
16 pixel input images and a monomial
degree d
=
5 yield a
dimensionality of
10
10
.
In certain cases described below, there exists, however, a way of computing dot
products in
these high

dimensional feature spaces without explicitly mapping into
them: by means of kernels
non

linear in the input
space
𝑅
𝑁
.
Thus, if the subsequent
processing can be carried out using dot
products exclusively, we are able to deal with
the high dimensionality.
6.2
Polynomial f
eature spaces induced by kernels
In order to compute dot products of the form
(
Φ
(
x
)
∙
Φ
(
x′
)
)
we employ kernel
representations of
the form
which allow us to compute the value of the dot product in
without having to
carry out the
map
Φ
.
This method was used by Boser et al. to extend the generalized
portrait hyperplane
classiﬁer [8] to non

linea
r support vector machines [4]. Aizerman
et al. called
the linearization
space, and used in the context of the potential
function classiﬁcation method to express the dot
product between elements of
in
terms of elements of the input space [3].
What does
k look like for the case of polynomial features? We start by giving an
example [6]
for N
=
d
=
2: For the map
dot
p
roducts in
take the form
i.e. the desired kernel k is simply the square of the dot product in input space. Note
that it is
possible to modify
(
x
,
x′
)
𝑑
such that it maps into the space of all monomials
up to degree d[6],
deﬁning
6.3
Examples of
kernels
When considering feature maps, it is also possible to look at things the other way
around, and
start with the kernel. Given a ker
nel function satisfying a mathematical
condition termed
positive deﬁniteness, it is possible to construct a feature space such
that the kernel computes the
dot product in that feature space. This has been brought
to the attention of the machine learning
co
mmunity by [3, 4, 6]. In functional analysis,
the issue has been studied under the heading of
reproducing kernel Hilbert space
(RKHS).
Besides (50), a popular choice of kernel is the Gaussian radial basis function
[3]
An illustration is in Figure 3. For
an overview of other kernels, see [1].
7
Multi

class SV Classifiers
Support vector machines were origin
ally designed for binary classifi
cation.
How
to effectively extend it for multi

class classi
fi
cation is still an ongoing research
issue.
C
urrently there
are mainly two types of approaches for multi

class
SVM. One is
one

against

all while the other is one

against

one as shown in Fig.4.1 and Fig. 4.2.
Based on the training process or solving the optimization problem
differently, it can
also be divided into
two kinds of approaches. One is “all

together”
methods that
consider all the training data at once, the other is by constructing
and
combining
several binary classifi
ers
.
7.1
One

against

all Method
In the case of
combining several binary classifi
ers, the
one

against

all method
constructs
K
SVM models where
K
is the number of classes
. The
i
th SVM is
trained with all of the examples in the
i
th class with positive labels, and all other
examples with negative labels. The problem fo
rmulation is as follows.
Given
l
training data (
1
,
1
), ..., (
,
), where
∈
𝑅
𝑑
, j = 1, ...,
l
and
∈
1, ...,K is
label of
. The mth SVM classifi
er solves the following problem:
(52)
where the training data
can be mapped to the higher
dimensional space the
using
the function
as shown in Fig. 4.3, and
C
is the penalty parameter to adjust
the
n
umber of training errors. Minimizing
1
2
(
𝑚
)
𝑚
is equivalent to maximize
the
m
argin between two groups of data as shown in Fig. 4.4 (a) an
d (b). From the
equations above, it can be observed that the main goal of SVM is to
fi
nd a balance
between the regularization term
1
2
(
𝑚
)
𝑚
and the training errors as illustrated
in
Fig. 4.4 (c).
After solving 4.1, the parameters can be obtained, and
thus there will be
K
decision functions:
The classifi
cation result, or the label of
x
can be obtained by
fi
nding the decision
function with the largest value:
(
53
)
(
54
)
(54)
can
be solved by using the Lagrange multiplier theory in its dual form
:
subject to
where
Only a few
𝛼
will be greater than zero. The corresponding xis are exactly the
support vectors, which lie on the margin.
Introducing the parameters obtained above into 4.3, the decision function for
the mth classifi
er can be expressed as the function of support vectors shown as
follows:
where
K
(
,
x
)
is the kernel function obtained from
:
(
55
)
(
56
)
(
57
)
(
58
)
(
59
)
And the label of
x
can be found
by:
The kernel function may take many forms, and the most commonly used are the
linear kernel, polynomial kernel, and the “Radial Basis Function” (RBF) kernel
shown as follows respectively:
(
60
)
7.2
One

against

one
Method
Likewise, the one

against

one method constructs
2
=
K
(
K
−
1
)
/
2 binary
classi
fi
ers where each one is trained on data from two classes. The most common
strategy to make the final classifi
cation decision is to vote for the
corresponding
class
in one of the
K
(
K
−
1
)
/
2 binary classifi
ers and classify the data to the class
with the
largest votes. This strategy is also called the “Max Wins”
strategy
.
That is, for the
classifi
er constructed from class
i
and class
j
, the following
process
is conducted to
vote for the candidate class: if
sign
(
(
)
Φ
(
)
)
says
x
is in the
i
th class, then
the vote for the
i
th class is added by one. Otherwise, the
j
th is increased by one.
7.3
Considering All Data At Once
Method
Different
from cons
tructing multi

class SVM classifi
er by combining binary
c
lassi
fi
ers, there also exist some works that handle the multi

class problems by
solving
one single optimization problem. That is, instead of iteratively sampling the
d
ata
with two differen
t labels, positive and negative, these kinds of methods consider
a
ll the training data at once by solving one problem. The concept is very similar to
the
one

against

all in that it also constructs
K
decision functions where each decision
function separates
vectors of the corresponding class from the other vectors.
Various
methods can be found in. Generally, the problem formulation
is as follows:
Then the decision function is
:
(
61
)
(
62
)
(
63
)
which is the same as
(54)
of the one

against

all method.
Hsu et al. implemented the multi

class SVM
classifi
er algorithms in a tool
called
“BSVM” using the one

against

one method, and two considering all
data at once
m
ethods with the one solving a single optimization problem,
and the other using,
respectiv
ely. The latter two methods are called “KBB”
and “SPOC” in the BSVM
tool.
Assume there are
L
support vectors, and the
decision function of “KBB” is:
where
From the above constraints, the term
𝑚
𝐴
−
𝛼
𝑚
can be expressed as a new
parameter
𝛼
𝑚
′
as follows:
The decision function of “SPOC” is as follows:
where
(
64
)
(
65
)
(
66
)
(
67
)
(
68
)
From the above constraints, it is clear that the lower and upper bound of
𝛼
𝑚
is:
By comparing the two decision functions,
(64)
and
(67)
, and their constraints,
it
is clear that the “SPOC” method is more hardware

friendly and the reasons are
in
t
hree

folds. One is the range of the weighting
𝛼
𝑚
is smaller and more consistent
compared with the weighting
𝛼
𝑚
′
, another is the decision function is of “SPOC” is
simpler that requires no bias term, and the other is the number of support vectors
obtained from the “SPOC” method is smaller in our experimental results.
I
t is
reported
that the considering all data at once
methods in general need fewer support
vectors although it might sacrifi
ce some
classifi
cation accuracy. Because the number
of support vectors affects the memory
size required to keep them and the computation
iteration
s required to compute
the decision function, the considering all data at once
method will be the candidate
in the proposed hardware architecture. Since the
considering all data at once
m
ethods also constructs K classifi
ers and differ from the
one

against

a
ll method
only in the functions that the objective function is subjected to.
The name of “one

against

all” method will stand for the considering all data at once
method
.
(
69
)
8
Applications
8.1
LIBSVM
Researchers have applied
SVM on di
ﬀ
erent applications. Some of
them feel that
it is easier
and more intuitive to deal with
v
ϵ
[
0 , 1
]
than C
ϵ
[0 ,
∞
).
Here, we brieﬂy
summarize some work
which use LIBSVM to solve SVM.
In Reference [39],
r
esearchers from HP Labs discuss the topics of personal email agent. Data
classi
ﬁ
cation is an important component for which the authors use n

SVM because
they think
‘the n parameter is more intuitive than the C parameter’.
Martin et al. [40]
applies machine learning methods to detect and localize boundaries of
natural images.
8.2
Experi
m
ent
(a)
System overview
: (
Cardiac dysrhythmia detector
)
SVM
Output:
Classified data
1:
abnormal
0: normal
Input:
ECG signal
(
heart beat
)
with 5 features
(b)
Experiment Result
Use LIBSVM to construct model file , and send to my .cpp
file .The following is the
related files
:
(for simplification , here only shows parts of the files)
Model file
Test file
The red

squared data in test file is the ground truth label of the heart beat , and
the red

squared data in
the result file is the result label classified by SVM . If the two
labels are the same , then match , or the error would increase by one.
The follow
ing is
the simulation result:
Result file
Performance measurement:
Precision = tp / tp+fp
Recall = tp / tp+fn
(
implemented by C/C++
)
9
Conclusion
One of the most appealing features of kernel algorithms is the solid foundation
provided by
both statistical learning theory and functional analysis. Kernel methods
let us interpret (and
design) learning algorithms geometrically in feature spaces
non

linearly related to the input
space, and combine statistics and geometry in a
promising way. Kernels provide an elegant
framework for studying three fundamental
issues of machine learning:
*
Similarity measures

the kernel can be viewed as a (non

linear) similarity
measure, and
should ideally incorporate prior knowledge about the problem at hand
* Data representation

as described above, kernels induce representations of the
data in a
linear
space
* Function class

due to the representer theorem, the kernel implicitly also
determines the
function class which is used for learning.
Support vector machines have been one of the major kernel methods for data
classiﬁcation.
Its original form
requires a parameter C
∈
[
0
,
∞
)
which controls the
trade

o
ﬀ
between the
classiﬁer capacity and
the training errors. Using the
v

parameterization, the parameter C is
replaced by a parameter
v
∈
[
0
,
1
]
.
10
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