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Title: Conic section classifier : a novel concept class with a tractable learning algorithm
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Banerjee, Arunava
Vemuri, Baba C.
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U F College of Engineering
Department of Computer & Information
Science & Engineering

Technical Report

UF CISE-TR 472, 2009

Conic Section Classifier: A Novel Conecpt Class with a

Tractable Learning Algorithm

Santhosh Kodipaka, Arunava Banerjee, Baba C. Vemuri

Submitted to IEEE Trans. on Pattern Analysis and Machine Intelligence March 16, 2009

Center for Vision, Graphics and Medical Imaging
E331, CSE Building, PO BOX 116120, Gainesville, FL 32611, USA; +1 (352) 392 5770

L 1^^ ^ ^^ ^ ^^ ^ ^^ ^ ^ ^ ^ ^


Conic Section Classifier: A New Concept Class

with a Tractable Learning Algorithm

Santhosh Kodipaka, Arunava Banerjee, and Baba C. Vemuri
April 10, 2009

Abstract-In several computer vision and medical diagnosis applications, features used for supervised learning are often high-
dimensional and the available samples are sparse. This leads to a severely under-constrained learning problem. One can approach
this either by reducing the feature dimensionality or by limiting the classifier to a simpler concept class. We propose a new concept
class suited for such data sets, that is based on conic sections. Each class is represented by a conic section in the input space,
described by its focus (point), directrix (hyperplane) and eccentricity (value). Class labels are assigned to data-points based on the
eccentricities attributed to them by the class descriptors. The concept class can represent non-linear discriminant boundaries with
merely four times the number of parameters as a linear discriminant. Learning involves updating the class descriptors. We also present
a tractable learning algorithm for binary classification. For each descriptor, we track its feasible space that results in identical labeling
for classified points. We show favorable learning performance compared to many state-of-the-art classifiers on several data sets.

Index Terms-Machine learning, Concept learning, Classifier design and evaluation, Geometric algorithms
----------- ----------

Many notable problems in medical diagnosis, object
recognition, text-categorization, etc., can be posed in the
general framework of supervised learning theory. The
learning problem in such instances can be formulated
as follows: One is given a dataset of N labeled tuples
{(Xi, yl),..., (XN, yN)}, where each X, is a data point
represented in some input space X, and yj is it's associ-
ated class label from an output space Y. The input space
X in which the data points, X's, lie can be any appropri-
ately defined space with some degree of structure. In the
majority of cases X is set to be the Euclidean space RM.
y is in general a two-element output space. The ultimate
goal of learning is to find the best function f : X y
(called the concept) that minimizes the expected risk on X.
A lower bound on the expected risk, given a set of training
data, is furnished by the empirical risk or training error
defined as f 1 I(yi / f(Xi)), where I is the indicator
Formulated as above, the learning problem is still ill-
posed since there exist uncountably many functions, f's,
that yield zero empirical risk. To make this problem
well-defined, one further restricts the f's to a particu-
lar class of functions, known as the concept class, and
subsequently identifies the best member of that class
which minimizes the empirical risk. Such a classifier
might however over-fit the training data and perform
poorly on as yet unseen data points. A notion of gen-
eralizability for a given classifier is therefore introduced
as a regularizer to control the expected risk on unseen
This research was in part supported by NIH R01 NS046812 to BCV.
Authors are with the Department of Computer & Information Science
& Engineering, ITU- .. :I of Florida, Gainesville, FL 32611. Email:

data. Since the distribution of data in X is not known
a priori, a conservative bound on the generalization
capacity of a classifier can be quantified as a function of
the classifier's empirical error and a formalization of the
complexity of the classifier's concept class. Although Sta-
tistical Learning Theory [1] does provide formal bounds
for generalization error, such bounds are often weak. The
common practice therefore is to estimate the general-
ization error via such protocols as the holdout method,
cross-validation and bootstrapping, as reviewed in [2].
The classifier from the concept class that yields the least
generalization error, empirically measured using one of
the techniques above, is chosen for the purpose of future

1.1 Motivation
Without detailed prior knowledge regarding the nature
of a dataset, it is not possible in principle to predict
which of a given set of concept classes will yield the
smallest generalization error. Practitioners therefore re-
sort to applying as many classifiers with different con-
cept classes as possible, before choosing the one that
yields the least generalization error. Every new concept
class with a corresponding tractable learning algorithm is
consequently a potential asset to a practitioner since it expands
the set of classifiers that can be applied to a dataset.
The learning task becomes remarkably difficult when
the number of training samples available is far fewer
than the number of features used to represent each sam-
ple. We encounter such high dimensional sparse datasets
in several applications like the diagnosis of Epilepsy
based on brain MRI scans [3], the diagnosis of vari-
ous types of Cancer from micro-array gene expression


data [4], spoken letter recognition [5] and object recogni-
tion from images [6], to name only a few. The supervised
learning problem is severely under constrained when
one is given N labeled data points that lie in ]RM where
N < M. This situation arises whenever the "natural"
description of a data point in the problem domain is very
large and the cost of collecting large number of labeled
data points is prohibitive.
In such scenarios, learning even a simple classifier
such as a linear discriminant is under-constrained be-
cause one has to solve for M + 1 parameters given
only N constraints. Additional objectives, such as max-
imizing the functional margin of the discriminant, are
usually introduced to fully constrain the problem. The
learning problem becomes progressively difficult as the
concept class gets richer, since such concepts require
larger number of parameters to be solved for, given the
same number of constraints. This often leads to over-
fitting and the generalization capacity of the classifier
There are two kinds of traditional solutions to this
quandary. In the first approach, the classifier is restricted
to the simplest of concept classes like the Fisher Discrim-
inants [7], the Linear Support Vector Machine (SVM), etc.
In the second approach, the dimensionality of the dataset
is reduced either by a prior feature selection [8], [9] or by
projecting the data onto discriminative subspaces [10].
The criterion for projection may or may not incorporate
discriminability of the data, such as in PCA versus Large
Margin Component Analysis [11], respectively. The as-
sumption underlying the second approach is that there
is a smaller set of compound features that is sufficient for
the purpose of classification. Our principal contribution
in this paper, expands the power of the first approach
noted above, by presenting a novel concept class along
with a tractable learning algorithm, well suited for high-
dimensional sparse data.

1.2 Synopsis
We introduce a novel concept class based on conic
sections in Section 2. Each member class in the dataset
is assigned a conic section in the input space, parame-
terized by its focus point, directrix plane and a scalar
valued eccentricity. The eccentricity of a point is defined
as the ratio between its distance to a fixed focus and to a
fixed directrix plane. The focus and directrix descriptors
of each class attribute eccentricities to all points in the
input space ]RM. A data point is assigned to the class
to which it is closest in eccentricity value. The concept
class is illustrated in Figure-1. The resultant discriminant
boundary for two-class classification turns out to be a
pair of polynomial surfaces of at most degree 8 in ]RMand
thus has finite VC dimension [12]. Yet, it can represent
these highly non-linear discriminant boundaries with
merely four times the number of parameters as a lin-
ear discriminant. In comparison, a general polynomial
boundary of degree d requires O(\I ) parameters, where
M is the dimensionality of the input space.

Fig. 1. Overview of the concept class. Circles and
squares are data points from classes 1, & 2. The parabola
and the ellipse are the class conic sections with eccentric-
ities 1, & -0.7. Both the classes share a common directrix
line here. The faint dotted lines represent distances of
points to the foci and directrices. For one point in each
class, the eccentricities attributed by the class descriptors
are shown. Points are assigned to the class to which they
are closest to, in eccentricity value. The thick curve is the
resultant discriminant.

Given a labeled dataset, learning involves arriving at
appropriate pair of conic sections (i.e., their directrices,
foci, and eccentricities) for the classes, that reduces em-
pirical risk and results in a discriminant that is simpler
and hence more generalizable.In Section 3, we present
a tractable geometric algorithm for binary classifica-
tion, that updates the class descriptors in an alternating
manner. This paper expands upon preliminary results
presented in [13] by tracking larger feasible spaces for
the class conic section descriptors. We demonstrate the
efficacy of our technique in Section 4, by comparing it
to well known classifiers like Linear and Kernel Fisher
Discriminants and kernel SVM on several real datasets.
Our classifier consistently performed better than LFD, as
desired. In the majority of cases, it out-performed state-
of-the-art classifiers. We discuss concluding remarks in
Section 5. A list of symbols used to represent different ge-
ometric entities in this paper are given in the Appendix.

1.3 Related Work
Conic sections have been used extensively in several
Graphics and Computer Vision problems like curve fit-
ting [14], [15], and recovering conics from images [16] to
infer structure from motion, etc. The principal reasons
for this usage is that a large variety of curves can be
represented with very few parameters and that the conic
sections are very common in occurrence. Within the
domain of supervised learning, there is one instance in


which this notion was used. One can obtain a conic
section by intersecting a cone with a plane at a certain
angle. The angle is equivalent to the eccentricity and
when varied results in different conic sections. This
notion was combined with neural networks in [17] to
learn such an angle at each node. However, the other
descriptors, namely the focus and directrix are fixed at
each node unlike our approach.
Support Vector Machines (SVM) and Kernel Fisher
Discriminant (KFD) with polynomial kernel also yield
polynomial boundaries like our method. The set of dis-
criminants due to these classifiers can have a non-empty
intersection with those due to the conic section concept
class. That is, they do not subsume the boundaries that
result from the latter and vice-versa. We emphasize here
that there is no known kernel equivalent to the conic
section concept class for SVM or KFD, and hence the
concept class is indeed novel. A detailed comparison to
these concept classes is presented in Section 2.1.

1.4 Contributions
In this paper, we introduce a concept class based on conic
sections accompanied by a geometric learning algorithm.
The concept class has finite VC dimension, can represent
highly non-linear boundaries with merely 4 (M + 1)
parameters, and subsumes linear discriminants. In the
learning phase for two-class classification, we track a
feasible space for each descriptor in ]RMthat results in
identical labeling of classified points. The feasible space
is represented as a compact geometric object, from which
desirable descriptor updates are chosen. We reduce the
computation of linear subspace in which the compact
geometric object lies, into that of a Gram-Schmidt or-
thogonalization. We also employ a stiffness criterion that
is used to pursue simpler discriminants instead of highly
non-linear ones, thereby performing model selection in
the learning phase. The performance of the concept
class on several high dimensional sparse datasets is
comparable to and sometimes better than state-of-the-art

A conic section in R2 is defined as the locus of points
whose distance from a given point (the focus) and that
from a given line (the directrix), form a constant ratio
(the eccentricity). Different kinds of conic sections such as
ellipse, parabola and hyperbola, are obtained by fixing
the value of the eccentricity to < 1, 1 and > 1,
respectively. Conic sections can be defined in higher
dimensions by making the directrix a hyperplane of co-
dimension 1. Together, a fixed focus point and directrix
hyperplane generate an eccentricity function (Eqn.1) that
attributes to each point X e ]RM a scalar valued eccen-
tricity defined as:

(X)= QX where IlQ|= 1 (1)

Hereafter, we use I|.|| to denote the Euclidean L2 norm.
F e RM is the focus point and (b + QTX) is the
orthogonal distance of X to the directrix represented as
{b, Q}, where b ]R is the offset of the directrix from
the origin and Q E ]Rm is the unit vector that is normal
to the directrix. The locus of points that correspond to
e(X) e is an axially symmetric conic section in RM.
At e = 0, the conic section collapses to the focus point.
As le oo, it becomes the directrix hyperplane itself.
We are now in a position to formally define the con-
cept class for binary classification. Each class, k E {1, 2},
is represented by a distinct conic section parameterized
by the descriptor set: focus, directrix and eccentricity, as
Ck {Fk, (bk, Qk), eC}. For any given point X, each class
attributes an eccentricity Ek(X), as defined in Eqn.1, in
terms of the descriptor set Ck. We refer to (E1(X), E2(X))
as the class attributed eccentricities of X. We label a point
as belonging to that class k, whose class eccentricity eC is
closest to the sample's class attributed eccentricity Ek(X),
as in Eqn.2. The label assignment procedure is illustrated
in Figure-1.


-/-*' (Jk(X) ekJ)

The resultant discriminant boundary (Eqn.3) is the locus
of points that are equidistant to the class representative
conic sections, in eccentricity. The discriminant boundary
is defined as g {X : g(X) 0} where g(X) is given
by Eqn.3. The discriminant boundary equation can be
expanded to:

> ((ri elhl)h2)

((r2 e2h2)h1), where
(X Fk)T(X -F
XTQk + bk

Upon re-arranging the terms in Eqn.4 and squaring
them, we obtain a pair of degree-8 polynomial surfaces,
in X, as the discriminant boundary.
((rh2)2 +(r2h2)2 -((e e)hh2)2 )2 (2rr22hih2)2 -0
Depending upon the choice of the conic section de-
scriptors, {C1, C2}, the resultant discriminant can yield
lower order polynomials as well. The boundaries due to
different class conic configurations in R2, are illustrated
in Figure-2. When the normals to the directrices, Qi, Q2,
are not parallel, the discriminant is highly non-linear
(Figure-2(a)). A simpler boundary is obtained when
directrices are coincident, as in Figure-2(b). We obtain
linear boundaries when either directrices perpendicu-
larly bisect the line joining the foci (Figure-2(c)) or the
directrices are parallel and the eccentricities are equal
(Figure-2(d)). A list of symbols used in this paper are
given in the Appendix.
The concept class, referred to as Conic Section classi-
fier (CSC) hereafter, has several notable features. Learn-
ing involves arriving at conic descriptors so that the


e = 0 584 0 722 illness= 1 35

e,= 0 584, eo= 0 722, stifness= 3 00

S= -0584 .e 0 722 stfness= 295

e1= 0 584, e2= 0 584, stillness= 3 00

0 Focus1
.*.. Conic1
o Focus2

-- Disc Bndry

3 2 1 0 1 2 3

Fig. 2. Discriminant boundaries for different class conic configurations in R2: (a) Non-linear boundary for a random

configuration. (b) Simpler boundary: directrices are coincident. Sign of e2 is flipped to display its conic section. (c)

Linear boundary: directrices perpendicularly bisect the line joining foci. (d) Linear boundary: directrices are parallel

and eccentricities are equal. (See Sec-2)


given data samples are well separated. Our learning
technique, presented in Section 3, pursues boundaries
that are simple and ensures large functional margins.
Our intent is not to fit conic sections to samples from
each class, but learn a generalizable boundary between
classes with fewer parameters. Regardless of the dimen-
sionality of the input space, the discriminant is always
linear under certain conditions. The linear discriminants
can be arrived at, when the directrices for the two
classes are identical, the foci lie symmetrically on the two
opposite sides of the directrix, the line joining the foci
is normal to the directrix and/or the class eccentricities
are equal and lie in a certain range near zero. The con-
cept class therefore subsumes linear discriminants. We
can obtain boundaries ranging from simple to complex,
by varying the class conic descriptors. The number of
parameters necessary to specify discriminant boundaries
due to the conic section concept class is 4* (M+ 1). This
is far less than the M2 parameters required for even a
general quadratic surface.

2.1 Comparisons to other Classifiers
We compare CSC to Support Vector Machine (SVM)
and Kernel Fisher Discriminants (KFD) with polynomial
kernels as they appear to be related to CSC in the type
of discriminants represented. For both the classifiers, the
discriminant boundary can be defined as:
b + w(XTX + 1)d 0 (8)
where w is a weight vector. Here the decision surface is
a linear combination of N degree-d polynomial surfaces
defined from each of the data points Xi. The methods
differ in their generalization criteria to arrive at the
weights wi. Note that the discriminants due to CSC
(Eqn.7) cannot be expressed in terms of the boundary
due to (Eqn.8). There could be a non-empty intersection
between the set of the polynomial surfaces represented
by CSC and those due to Eqn.8. We point out that
there is no kernel which matches this concept class, and
therefore, the concept class is indeed novel.
KFD seeks boundaries that maximize the Fisher cri-
terion [7], i.e. maximize inter-class separation while
minimizing within class variance. The learning criterion
used in SVM is to pursue large functional margin, result-
ing is lower VC dimension [18] and thereby improving
generalizability. CSC uses a similar criterion and in fact
goes a step further. The degree of the polynomial kernel
is a model-selection parameter in kernel based classifiers
like SVM and Kernel Fisher Discriminants (KFD). As
CSC learning will involve arriving at simpler boundaries
for the same constraints on training samples, the degree
of the polynomial is also being learnt in effect. (See
The advantage of SVM over CSC is that learning in the
former involves a convex optimization. The optimization
in KFD is reduced to that of a matrix inverse problem for

binary classification. However, the equivalent numerical
formulation for CSC turns out to be non-convex and
intractable. We therefore use novel geometric approaches
to represent the entire feasible space for constraints on
each of the conic descriptors and then pick a local
optimum. In fact, we reduced the feasible space pursuit
problem in CSC into that of a Gram-Schmidt orthogo-
nalization [19] (Section 3.3.2).
When SVM deals with high-dimensional sparse
datasets, most of the data points end up being sup-
port vectors themselves, leading to about N (M + 1)
parameters whereas CSC employs only 4 (M + 1)
parameters. The boundary due to KFD also involves
the same number of parameters as SVM. In summary,
CSC has some unique benefits over the state-of-the-art
techniques that makes it worth exploring. These include
incorporating quasi model-selection into learning, and
shorter description of the discriminant boundary. We
have also found that CSC out-performed SVM and KFD
with polynomial kernel in many classification experi-
ments as listed in Table-2.

We introduce a novel incremental algorithm for the two-
class Conic Section Classifier in this section. We assume
a set of N labeled samples P { (Xi, y),..., (XN, YN)},
where Xi e RM and the label yi {-1, +1}, to be
sparse in a very high dimensional input space such that
N < M. Learning involves finding the conic section de-
scriptors, {C1, C2}, that can minimize empirical learning
risk and simultaneously result in simpler discriminant
boundaries. The empirical risk, L..rr is defined as:

Ler = 1 I( yi g(Xi) > 0) (9)
where I is the indicator function. A brief description of
the algorithm is presented next.

3.1 Overview
In the learning phase, we perform a constrained update
to one conic descriptor at a time, holding the other
descriptors fixed; the constraint being that the resultant
boundary continues to correctly classify points that are already
correctly classified in previous iteration. The feasible space
for each descriptor within which these constraints are
satisfied will be referred to as its Null Space. We pick
a solution from each Null Space in a principled manner
such that one or more misclassified points are learnt.
The sets of descriptors that will be updated alternately
are {(ei, 62), F1, F2, {bl, Qi}, {b2, Q2}}.
To begin with, we initialize the focus and directrix de-
scriptors for each class such that the Null Spaces are large.
The initialization phase is explained in detail in Sec-
tion 3.5. The subsequent learning process is comprised
of two principal stages. In the first stage, given fixed
foci and directrices we compute attributed eccentricities


Input: Labeled Samples P
Output: Conic Section Descriptors C1, C2

Initialize the class descriptors {Fk, {bk, Qk}}, k E {1, 2}
Compute (Ei(X,),E2(Xi)) VXi, P
Find the best class-eccentricities (ec, e2)
for each descriptor {Fi, F2, {bl, Qi}, {b2, Q2}}
Determine the classifying range for (Eli, E2i)
Find its feasible space due to these constraints.
for each misclassified point Xmc
Compute a descriptor update to learn Xmc
end for
Pick updates with least empirical error
Then pick an update with largest Stift-,i..'
end for
until the descriptors C1, C converge

Fig. 3. Learning the class descriptors C1, C2

(Eqn.1) for each point Xi, denoted as (Eli, 2i). We then
compute an optimal pair of class eccentricities (ei, e2),
that minimizes the empirical risk Le,, in O(N2) time,
as described in Section 3.2.1. For a chosen descriptor
(focus or directrix) to be updated, we find feasible
intervals of desired attributed eccentricities, such that
yi -g(Xi) > 0 VXi, i.e., the samples are correctly classified
(Section 3.2.2).
In the second stage, we solve for the inverse problem.
Given class-eccentricities, we seek a descriptor solution
that causes attributed eccentricities to lie in the desired
feasible intervals. This results in a set of geometric
constraints on the descriptor, due to Eqn.4, that are dealt
with in detail in Section 3.3. We compute the entire
Null Space, defined as the equivalence class of the given
descriptor that results in the same label assignment for
the data.For each misclassified point, we pick a solution
in this Null Space, that learns it with a large margin
while ensuring a simpler decision boundary in ]RM. In
Section 3.4.1, we introduce a stiffness criterion to quantify
the extent of the non-linearity in a discriminant bound-
ary. Among the candidate updates due to each misclas-
sified point, an update is chosen that yields maximum
stiffness, i.e., minimal non-linearity. The second stage is
repeated to update the foci {FI, F2} and the directrices
{{b, Q1}, {b2, Q2}}, one at a time. The two stages are
alternately repeated until either the descriptors converge
or there can be no further improvement in classification
and ratti., t Note that all through the process, the learn-
ing accuracy is non-decreasing since a previously classified
point is never misclassified due to subsequent updates. A
summary of the algorithm is listed in Figure-3. We
discuss the first phase in detail in the following section.

3.2 Learning in Eccentricity Space
The focus and directrix descriptors of both the classes
induce a non-linear mapping, E*(X), due to Eqn.1, from


(a) (b)
Fig. 4. (a) Shaded regions in this ecc-Space belong to
the class with label -1. Learning involves updating the
eccentricity maps so as to shift the misclassified points
into desired regions. (b) The discriminant boundary is the
pair of thick lines. Within the shaded rectangle, any choice
of class eccentricity descriptors (the point of intersection
of two thick lines) results in identical classification.

the input space into R 2, as:

E*(X) (1(X), E2(X)) (10)

This space of attributed eccentricities will be referred to
as the eccentricity space (ecc-Space). We defined this space
so as to determine the best pair of class eccentricities
simultaneously. In Figure-4(a), the x and y axes represent
the maps i(X) and E2(X) respectively, as defined in
Eqn.1. For any given choice of class eccentricities, ec and
62, the discriminant boundary equivalent in ecc-Space ,
|lE(X) e61 E2(X) e21 0, becomes a pair of mutu-
ally orthogonal lines with slopes +1, -1, respectively, as
illustrated in the figure. These lines intersect at (61, 62),
which is a point in ecc-Space The lines divide ecc-Space
into four quadrants with opposite pairs belonging to
the same class. It should be noted that this discrimi-
nant corresponds to an equivalent non-linear decision
boundary in the input space R". We use ecc-Space only
as a means to explain the learning process in the input
space. The crucial part of the algorithm is to learn the
eccentricity maps 1 for each class by updating the foci
and directrices. In this section, we first present an O(N2)
time algorithm to find the optimal class eccentricities.
Next, we determine resultant constraints on the focus
and directrix descriptors due to the classified points.

3.2.1 Finding Optimal Class-Eccentricities (1, C2)
We now present an O(N2) algorithm to find the optimal
pair of class-eccentricities resulting in the least possible
empirical error (Eqn.9), given fixed foci and directrices.
The discriminant boundary (Eqn.3) in ecc-Space is com-
pletely defined by the location of class eccentricities.
Consider a pair of orthogonal lines with slopes +1 and
-1 respectively, passing through each mapped sample
in ecc-Space, as illustrated in Figure-4(b). Consequently,
these pairs of lines partition the ecc-Space into (N+ 1)2 2D
rectangular intervals. We now make the critical observa-
tion that within the confines of such a 2D interval, any choice


of a point that represents class eccentricities results in identical
label assignments (see Figure-4(b)). Therefore, the search
is limited to just these (N + 1)2 intervals. The interval
that gives the smallest classification error is chosen. The
cross-hair is set at the center of the chosen 2D interval
to obtain large functional margin. Whenever, there are
multiple 2D intervals resulting in the least empirical
error, the larger interval is chosen so as to obtain a larger

3.2.2 Geometric Constraints on Foci and Directrices
Having fixed the class eccentricities, the learning con-
straint on attributed eccentricities for each Xi, with a
functional margin 6 > 0, is given by Eqn.11. Assuming
that the descriptors of class 2 are fixed, the constraint on
E~(X,) due to Eqn.11 is derived in Eqn.12. Now, if we
are interested in updating only focus F1, the constraints
on F1 in terms of distances to points, Xi, are given by

yi (I (Xi) C1| |E2(Xi) C2|) > 6 (11)
yi (|E (X,) elC) > (6 + yAE2i) = wli
li = (yei wi1) > y l1(Xi) > (yCei + wi) 1= u (12)

li > YiF > u1i (13)
Here AE2i -= E(Xi) ec2, and hli is the distance to the
directrix hyperplane of class 1 (Eqn.4). In Eqn.13, the
only unknown variable is Fl. Similarly, we can obtain
constraints for all the other descriptors. Whenever the
intervals due to the constraints are unbounded, we apply
bounds derived from the range of attributed eccentrici-
ties in the previous iteration. The margin, 6, was set to
1% of this range.
In the second stage of the learning algorithm, we
employ a novel geometric technique to construct the
Null Space of say, F1, for distance constraints (Eqn.13)
related to the currently classified points. Next, we pick a
solution from the Null Space that can learn a misclassified
point by satisfying its constraint on F1, if such a solution
exists. The learning task now reduces to updating the
foci and directrices of both the classes alternately, so
that the misclassified points are mapped into their de-
sired quadrants in ecc-Space, while the correctly classified
points remain in their respective quadrants. Note that
with such updates, our learning rate is non-decreasing. In
the next section, we construct Null Spaces for the focus
and directrix descriptors. In Section 3.4 we deal with
learning misclassified points.

3.3 Constructing Null Spaces
Assume that we have Nc classified points and a point
Xmc that is misclassified. We attempt to modify the
discriminant boundary by updating one descriptor at
a time in ]RM such that the point Xm is correctly
classified. The restriction on the update is that all the
classified points remain in their respective class quad-
rants in ecc-Space i.e, their labels not change. In this

section, we construct feasible spaces for each of the
focus and directrix descriptors within which the labeling
constraint is satisfied.

3.3.1 The Focus Null Space
Here, we consider updating F1, the focus of class 1.
For ease of readability, we drop the reference to class
from here on, unless necessary. First, we construct the
Null Space within which F adheres to the restrictions
imposed by the following N, quadratic constraints, due
to Eqn.13,:

rui < F Xill < rV Vi E 1,2,... N, (14)
where rli, and ri are lower and upper bounds on the
distance of Xi to F. In effect, each point Xi requires F to
be at a certain distance from itself, lying in the interval
(rli, rui). Next, we need to pick a solution in the Null
Space that satisfies a similar constraint on Xm, so as to
learn it, and improve the generalization capacity of the
classifier (Section 3.4). While this would otherwise have
been an NP-hard problem (like a general QP problem),
the geometric structure of these quadratic constraints
enables us to construct the Null Space in just O(N2M)
time. Note that by assumption, the number of constraints
N < M.
The Null Space of F with respect to each constraint in
Eqn.14 is the space between two concentric hyperspheres
in RPM, referred to as a shell. Hence, the Null Space for
all the constraints put together is the intersection of all
the corresponding shells in RPM. This turns out to be
a complicated object. However, we can exploit the fact
that the focus in the previous iteration, denoted as F,
satisfies all the constraints in Eqn.14 since it resulted in
Nc classified points. To that end, we first construct the
locus of all focus points, F', that satisfy the following
equality constraints:

\- F'\ FV I -F\ r, Vi e 1...Nc (15)

Note that such an F' will have the same distances to
the classified data points X, as the previous focus F, so
that the values of the discriminant function at Xi remain
unchanged, i.e., g(Xi, F') g(Xi, Fo). Later, we will
use the locus of all F' to construct a subspace of the
Null Space related to Eqn.14 that also has a much simpler

3.3.2 Intersection of Hyperspheres
The algorithm we discuss here incrementally builds the
Null Space for the equality constraints in Eqn.15; i.e., the
locus of all foci F' that are at distance ri to the respective
classified point Xi. The Null Space is initialized as the
set of feasible solutions for the first equality constraint
in Eqn.15. It can be parameterized as the hypersphere
S1 (ri, X1), centered at Xi with radius ri. Next, the
second equality constraint is introduced, the Null Space
for which, considered independently, is the hypersphere
S2 (r2, X2). Then the combined Null Space for the two


Fig. 5. (a)lntersection of two hypersphere Null Spaces,
Si(ri,Xi) and S (r2, X2) lying in a hyperplane. Any point
on the new Null Space (bright-circle) satisfies both the
hypersphere (distance) constraints. (b) Si n S2 can be
parameterized by the hypersphere S{1,2} centered at X1,2
with radius R1,2.

constraints is the intersection of the two hyperspheres,
S n S2.
As illustrated in Figure-5(a), the intersection of two
spheres in RR3 is a circle that lies on the plane of in-
tersection of the two spheres. The following solution
is based on the analogue of this fact in R1M. We make
two critical observations: the intersection of two hyper-
spheres is a hypersphere of one lower dimension, and this
hypersphere lies on the intersecting hyperplane of the original
hyperspheres. Each iteration of the algorithm involves two
steps. In the first step, we re-parameterize the combined
Null Space, Si n S2, as a hypersphere S{1,2} of one
lower dimension lying in the hyperplane of intersection
H{i,2}. Based on the geometry of the problem and the
parameterization of Si and S2, shown in Figure-5(b),
we can compute the radius and the center of the new
hypersphere S{,2} (r{1,2}, X{,2}) in O(M) time, given
by Eqns.16-18. We can also determine the intersecting
hyperplane H{12} represented as {b{i,2}, Q12}. The first
descriptor b{1,2} is the displacement of H{1,2} from the
origin and the other descriptor is the unit vector normal
to H{i,2}. In fact, Q{i,2} lies along the line joining Xi
and X2. The parameters of the new hypersphere S{i,2}
and the hyperplane H{i,2} are computed as :


(X2 Xi)/JJX2
Xi Ql,2Q ,2(Fc

- X0)

r1,2 = X1,2 F I
b Q(18)
bi,2 = -Q12Xi1,2 (18)
In the second step, the problem for the remaining
equality constraints is reposed on the hyperplane H{i,2}.
This is accomplished by intersecting each of the remain-
ing hyperspheres S3, SN that correspond to the sam-
ples X3,., XN, with H{i,2}, in O(NM) time. Once again,
based on the geometry of the problem, the new centers of
the corresponding hyperspheres can be computed using
Eqn.17 and their radii are given by:

r = ((X- F )TQi,2
In short, the intersection of the Nc hyperspheres problem
is converted into the intersection of (N,-l) hyperspheres
in the hyperplane H{12}, as summarized below:
Si nS2 -* S1,2} E H{1,2
S, nH{i,2} S'EH{1,2} Vi 3,., N
SinS2...nSNo -, S1,2n S.. nS' e H1,2 (19)
The problem is now transparently posed in the lower
dimensional hyperplane H{1,2} as a problem equivalent
to the one that we began with, except with one less
hypersphere constraint. The end result of repeating this
process (N, -1) times yields a Null Space that satisfies all
the equality constraints (Eqn.15), represented as a single
hypersphere lying in a low dimensional linear subspace
and computed in O(N2M) time. It should be observed
that all the intersections thus far are feasible and that
the successive Null Spaces have non-zero radii since the
equality constraints have a feasible solution a priori, i.e.,
Upon unfolding Eqns.16,17 over iterations, we notice
that the computation of the normal to the hyperplane
of each intersection and that of the new center can be
equivalently posed as a Gram-Schmidt orthogonalization
[19]. The process is equivalent to the QR decomposition
of the following matrix:
A [ X X2 ... X ] QR (20)
where, X, = (Xi X1)
C, w=(X) X1 + QQT(F X1) (21)
r= IF Cf11
The function 7(X) in Eqn.21 projects any point into a low
dimensional linear subspace, -t, normal to the unit vec-
tors in Q (Eqn.20) and contains F. H can be defined as
the linear subspace {X e ]RM : QT(X -F) 0}. We use
existing stable and efficient QR factorization routines to
compute S" in O(N2M) time. It is noteworthy that the final
Null Space due to the equality constraints in Eqn.15 can be
represented as a single hypersphere S" (re, Ce) C H. The
center and radius of S" can be computed using Eqn.21.
The geometry of S" enables us to generate sample focus
points that always satisfy the equality constraints. In the
next section, we pursue larger regions within which the
inequality constraints on the focus due to Eqn.14 are





Fig. 6. Cross-section of shell-shell intersection in ]R3. The
red disc in (a) is the largest disc centered at F that is
within the intersection. The thick circle passing through
F and orthogonal to the cross-section plane, is the locus
of all foci with same distance constraints as F. A product
of the disc and the thick circle results in a toroid, as in (b),
which is a tractable subregion lying within the intersection
of shells.

3.3.3 Intersection of Shells
Consider the intersection of two shells centered at X1, X2,
in ]R3, as illustrated in Figure-6. We first compute the
largest disc centered at the previous focus, F, which
is guaranteed to lie within the intersection. Next, we
revolve the disc about the line joining X1 and X2. The
resultant object is a doughnut like toroidal region, that
can be defined as a product of a circle and a disc in ]R3,
as illustrated in Figure-6(a). The circle traced by F is the
locus of all foci F that are at the same distances to points
Xi, X2, as F We pursue this idea in ]RM. The final Null
Space, say Sf, that satisfies the inequality constraints on
F (Eqn.14) can be defined as :

S -- {F = F' + U : F' e S, IU = 1,,3 e [0, a)} (22)

where F' is a point from the feasible space, S", due to
the equality constraints on F (Eqn.15), and a > 0 is the
radius of the largest solid hypersphere (ball) at all F'
that satisfies the inequality constraints. In this manner,
we intend to add a certain a thickness to the locus of
We can compute the radius, a, of the largest disc at
F from Eqns.14,22. Let ri = \ F |, F F + 3U
be a point on the ball at F. Due to triangle inequality

between F, F and any point Xi, V,3 E [0, a) we have:

Iri ,/ < II(X, F) -,.1 J < ri + P
= rtu < Iri 3 < I \ F| < + 3 < ri
=3 < ri ri, 3 S a = min {(ri ri), (r, rii)}i 1... N (23)
Here 3 < ri so as to satisfy Eqn.14. We can thus compute
a from Eqn.23 in just O(N) time given the distance
bounds rli, and ri. With very little computational ex-
pense, we can track a larger Null Space than that due to
the equality constraints (Eqn.15).

3.3.4 The Directrix Null Space
Here, we present a two-step technique to construct the
Null Space for a directrix that satisfies the learning
constraints in Eqn.12. First, we list constraints on the
directrix so that the classified points remain in their
quadrants when mapped into ecc-Space Second, we
reduce the problem of constructing the resultant feasible
space into that of a focus Null Space computation.
The constraints on the hyperplane descriptor set
{b, Q}, due to those on attributed eccentricities in Eqn.12,
Vi 1...N, are :
hi< < (b+ QTXi) < h, with QTQ 1 (24)
Shli hi < QT (X, Xi) < hi hi1
Shi < QTX_ < h, (25)
where h1i, and h,i are lower and upper bounds on the
distance of X, to the hyperplane {b, Q}. We assume every
other descriptor is fixed except the unknown {b, Q}.
Upon subtracting the constraint on X1 from the other
constraints, we obtain Eqn.25, where X, = (Xi Xi),
hi = (hli hli), and hi = (hi, hi1). We can now
convert the distances to hyperplane constraints to those
of distances to a point, by considering |Q Xill as
in Eqn.26. Let zi = (1 + |. ||-). The resultant shell
constraints on Q, Vi 1... Nc are in Eqn.27

IQ X 12 1 + |X |112 2QTX_ (26)
(zi 2,,,) < IIQ XI 12 < (zi--'i ) (27)
Thus given N, inequality constraints on point distances
to Q and given the previous Q we use the solution from
the focus update problem to construct a Null Space for Q.
The only unknown left is b which lies in an interval due
to X1 in Eqn.24, given Q. We choose b to be the center
of that interval, as b (h, + h11)/2 QTX1, so that the
margin for X1 is larger in ecc-Space.

3.4 Learning Misclassified Points
In order to keep the learning process tractable, we learn
one misclassified point, Xmc, at a time. The final Null
Space Sf, for the inequality constraints on F can be
defined as all F F' + ,U, where F' e S Cc 'H, 3 e
[0, a), and U is any normal vector. To determine if Sf
has a solution that can learn Xmc, we intersect Sf with a


.------- T-(Xmc) -- H

df n


Fig. 7. The cross-section of the toroidal Null Space, Sf,
lying in the plane spanned by {Xmc, 7(Xmc), C}. The
expressions for distances d, and df in terms of p,s, and
r, are given in Eqn.28

shell corresponding to the inequality constraint (Eqn.14)
of X ,,, say ri < lIF X,, .. < ri. This is equivalent to
checking if the range of |IF X.. I| VF e Sf intersects
with the interval (rl, rn). This range can be determined
by computing the distance to the nearest and farthest
points on Se to Xmc, as in Eqn. 28. First, we project Xmc
into the linear subspace 7', in which S' lies, using Eqn.21
to obtain w(Xmc). Let p .\.. w(Xm,)|l and s
I|Ce (Xm,)11. We then have:
d,= V(s- re) +p2
df = (s + r)2 + p2 (28)
d a< lIF X,.. where (re, C,) are the radius and center of S'. See Figure-
7 to interpret these relations. Now the intersection of Sf
and the shell corresponding to Xm is reduced to that
of the intervals: (d, a, df + a) n (rl, rn). If they don't
intersect, we can easily pick a (focus) point on Sf that is
either nearest or farthest to the shell related to Xmc, so
as to maximally learn Xmc, i.e., change r(Xmc) which in
turn maximizes ymc g(Xm,). If they do intersect, there
are only a few cases of intersections possible due to
the geometry of Sf and the shell. Each such intersection
turns out to be or contains another familiar object like
Sf, a shell, or a solid hypersphere (ball). Whenever the
intersection exists, we pick a solution in it that maxi-
mizes E yi g(Xi) for the other misclassified points, i.e., a
solution closer to satisfying all the inequality constraints
put together. In this manner, we compute a new update
for each misclassified point. Next, we describe a criterion
to pick an update, from the set of candidate updates, that
can yield simplest possible discriminant.

3.4.1 Stiffness Measure
We introduce a stiffness measure that quantifies the ex-
tent of the non-linearity of the conic section classifier. It
is defined as:

F(C, C2) =Q Q1 + |Q F ,2 + |Q2 F,2| (30)
where, F1,2 is the unit normal along the line joining the
foci. The maximum of this measure corresponds to the
configuration of conic sections that can yield a linear
discriminant as discussed in Section 2. The measure is

defined for focus and directrix descriptors. It can be
used to pick an update yielding the largest stiffness so
that the resultant discriminant can be most generalizable.
We noticed that the non-linearity of the discriminant
boundary is primarily dependent upon the angle be-
tween the directrix normals, Qi and Q2. In Figure-2,
the stiffness measures for different types of boundaries
are listed. The discriminant boundary becomes simpler
when the directrix normals are made identical. More-
over, the functional dependencies between the distance
constraints which determine the discriminant, in Eqn.4,
become simpler if the line joining the foci is parallel
to the directrix normals. It stems from the observation
that every point in the hyperplane that perpendicularly
bisects the line joining the foci, is at equal distance from
the foci (ri(X) = r2(X)). Linear discriminants can be
guaranteed when the stiffness is maximum and the class
eccentricities are either equal in magnitude or near zero.
Two configurations that yielded linear discriminants
are illustrated in Figures-2(c),2(d). From the final Null
Space of the descriptor being updated, we determine a
descriptor update favored by each misclassified point.
Among the competing updates, we choose the update
with the least empirical error and maximum stiffness.
Thus, the large stiffness pursuit incorporates a quasi
model selection into the learning algorithm.

3.5 Initialization
Our learning algorithm is equivalent to solving a highly
non-linear optimization problem that requires the final
solution to perform well on both seen and as yet un-
seen data. Naturally, the solution depends considerably
on the choice of initialization. As expected, random
initializations converged to different conic descriptors
leading to inconsistent performance. We observed that
owing to the eccentricity function (Eqn.1), the Null Spaces
become small (at times, vanishingly small) if the focus or
directrix descriptors are placed very close to the samples.
We found the following data-driven initializations to be
consistently effective in our experiments. The initializa-
tion of all descriptors were done so as to start with a
linear discriminant obtained from either linear SVM or
Fisher [7] classifier or with the hyperplane that bisects
the line joining the class means. The class eccentricities
were set to (0, 0). The foci were initialized to lie far from
their respective class samples. We initialized normals
Qi Q2 with that in the initial linear discriminant.
Now, bl, b2 were initialized to be either equal or far from
their respective class clusters. If the data was not well
separated with the current initialization, the foci and
directrices were pushed apart until they were outside
the smallest sphere containing the samples.

3.6 Discussion
One of the core characteristics of our algorithm is that
after each update any point that is correctly classified by
the earlier descriptors is not subsequently misclassified.


Details of data used in experiments.

Dataset Features Samples Class 1 Class 2
Epilepsy 216 44 19 25
Colon Tumor 2000 62 40 22
Leukemia 7129 72 47 25
CNS 7129 60 21 39
ETH-Objects 16384 20 10 10
TechTC38 1683 143 74 69
Isolet-BC 617 100 50 50

This is due to two reasons. First, we begin with an
initialization that gives a valid set of assignments for
the class attributed eccentricities. This implies that the
Null Space for the classified points is non-empty to
begin with. Second, any descriptor update chosen from
the Null Space is a feasible solution that satisfies the
distance constraints introduced by the correctly classified
points. Moreover, the current descriptor to be updated
is also a feasible solution, i.e., in the worst case the
Null Space collapses to the current descriptor. Therefore,
the learning rate of CSC in non-decreasing. The radius of
the final Null Space for each descriptor, monotonically
decreases as more points are correctly classified, as can
be deduced from Eqn.21. Also, the order of classified
samples processed does not affect the final Null Space .
A key contribution of our learning technique is the
tracking of a large set of feasible solutions as a compact
geometric object. From this Null Space we pick a set of
solutions to learn each misclassified points. From this set,
we pick a solution biased towards a simpler discriminant
using a stiffness criterion so as to improve upon general-
ization. The size of the margin, 6, in ecc-Space also gives
a modicum of control over generalization.

We evaluated the classifier on several real datasets con-
cerning cancer class discovery, epilepsy diagnosis, and
recognition of objects and spoken alphabets. We begin
with a brief overview of the datasets, and the classi-
fiers used for comparison with Conic Section classifier
(CSC). Next, we discuss implementation details of CSC
followed by a review of the results.

4.1 Datasets
We conducted experiments on a variety of datasets
involving medical diagnosis, object recognition, text-
categorization and spoken letter recognition. All the
datasets considered have the peculiar nature of being
high dimensional and sparse. This is due to the fact
that either the collection of samples is prohibitive or that
the features obtained for each sample can be too many,
especially when there is no clear way to derive a reduced
set of meaningful compound features.
The dimensionality and feature size details of the
datasets used are listed in Table-1. Epilepsy data [3] con-
sists of the shape deformation between the left and right

hippocampi for 44 epilepsy patients. First, we computed
the displacement vector field in 3D representing the non-
rigid registration that captures the asymmetry between
the left and right hippocampi. We found the joint 3D his-
togram of the (x, y, z) components of the displacement
vector field to be a better feature set for classification.
We used a 6 x 6 x 6 inning of the histograms in 3D.
The task was to categorize the localization of the focus
of epilepsy to either the left (LATL) or right temporal
lobe (RATL).
The Colon Tumor [20], the Leukemia [4], and the
CNS [21] datasets are all gene-expression datasets. The
gene-expression scores computation is explained in their
respective references. In the Colon Tumor data the task
is to discriminate between normal and tumor tissues.
The leukemia data consists of features from two dif-
ferent cancer classes, namely acute myeloid leukemia
(AML) and acute lymphoblastic leukemia (ALL). The
CNS dataset contains treatment outcomes for central
nervous system embryonal tumor on 60 patients, which
includes 21 survivors and 39 failures.
From the ETH-80 [6] object dataset, we chose 10 profile
views of a dog and a horse. The views are binary
images of size 128 x 128. From the TechTC-38 [22] text-
categorization dataset, we classify the documents related
to Alabama vs. Michigan localities (id: 38 [22]), given
word frequencies as their features. We dropped features
(words) that are either very infrequent or too common.
The Isolet-BC dataset is a part of the Isolet Spoken
Letter Recognition Database [5], [23], which consists of
features extracted from speech samples of B and C from
25 speakers.

4.2 Classifiers and Methods
We implemented a faster version of the Linear Fisher
Discriminant (LFD) [7] as described in Yu & Yang [24].
This technique exploits the fact that high-dimensional
data has singular scatter matrices and discards the sub-
space that carries no discriminative information. Support
Vector Machines (SVM) [25] and Kernel Fisher Discrim-
inants (KFD) [26] broadly represented the non-linear
category. Both employ the kernel trick of replacing inner
products with Mercer kernels. Among linear classifiers,
we chose LFD and linear SVM. We used libSVM [27], a
C++ implementation of SVM using Sequential Minimal
Optimization [28] and our own MATLAB implementa-
tion of KFD. Polynomial (PLY) and Radial Basis (RBF)
Kernels were considered for SVM and KFD.
We performed stratified 10-fold cross validation (CV)
[2] in which the samples from each class are randomly
split into 10 partitions. Partitions from each class are put
into a fold, so that the label distribution of training and
testing data is similar. In each run, one fold is withheld
for testing while the rest is used for training and the
process is repeated 10 times. The average test-error
is reported as the generalization error estimate of the
classifier. The experimental results are listed in Table-2.


Classification results for CSC, (Linear & Kernel) Fisher Discriminants and SVM.

Epilepsy 91.50 78.00 80.50 85.00 84.50 84.50
Colon Tumor 88.33 85.24 78.57 83.57 88.33 83.81
Leukemia 95.71 92.86 98.57 97.32 97.14 95.71
CNS 71.67 50.00 63.33 75.00 65.00 65.00
ETH-Objects 90.00 85.00 90.00 55.00 85.00 90.00
TechTC38 74.14 67.05 71.24 71.90 72.10 60.24
Isolet-BC 95.00 91.00 94.00 94.00 94.00 94.00

Parameters for results reported in Table. 2. *Not a parameter.

The best parameters for each classifier were empirically
explored using grid search. We searched for the best
degree for polynomial kernels from 1 ... 5, beyond which
the classifiers tend to learn noise. We computed the
largest distance between a sample pair as the base-size
for selecting the radius of the RBF kernels. The radius
was searched in an exponentially spaced grid between
10 3 to 105 times the base-size.

4.3 Conic Section Classifier
We implemented CSC in MATLAB. The computationally
intensive aspect in the learning is tracking the Null
Space for the equality constraints. Since this has been re-
duced to QR decomposition of a matrix composed from
the sample vectors, we used the MATLAB qr routine
which is numerically more stable and faster than hav-
ing to compute the hypersphere intersections separately.
Whenever possible, we cached the QR decomposition
results to avoid recomputations. We have an O(N2)
algorithm to compute the optimal class-eccentricities,
that will be used in the first iteration. This gave us
the best learning accuracy for the initial configuration
of CSC. This could result in a non-linear boundary to
start with. Then the stiffness guided choice of updates
ensured that we pursue simpler discriminants without
decreasing the learning rate. In subsequent iterations, we
searched for better eccentricity descriptors in the imme-
diate 10 x 10 neighborhood intervals of the previous class
eccentricities, in ecc-Space. This avoids jumping from the
current global optima to the next, after learning the foci
and directrix descriptors, thereby making the algorithm
more stable. We found that the descriptors converged
to a local minima typically within 75 iterations of the
learning algorithm listed in Figure-3. The parameters
of CSC involve the choice of initialization, described in
Section 3.5, and the functional margin 6. The 6 value

is fixed to be at a certain percentage of the extent of
attributed eccentricities, E*(X). We searched for the best
percentage among [1%,.1%, .01%]

4.4 Classification Results
Classification results are listed in Table-2. Conic Section
Classifier (CSC) performed significantly better than the
other classifiers for the Epilepsy data. In the gene-
expression datasets, CSC was comparable to others with
Leukemia and Colon Tumor data, but not with the CNS
data which turned out be a tougher classification task.
CSC fared slightly better than the other classifiers in
text-categorization and spoken letter recognition data.
In fact, CSC has consistently performed substantially
better than the LFD, as it is desirable by design. It is
also interesting to note that none of the SVM classifiers
actually beat CSC. This empirically proves that CSC is
able to represent more generalizable boundaries than
SVM for all the data considered.
The parameters used in the experiments are listed in
Table-3. The column related to Q, lists that the normals
to directrices were initialized from those due to either
linear SVM or LFD, or the line joining the means of
the samples from the same class. The column related to
6 denotes the margin as percentage of the range of class
attributed eccentricities. We report the average Itti.''.
of CSC over 10-folds in each experiment. Note that this
is not a parameter of CSC. We included this in the
table for comparison with the degree of the polynomial
kernels. The stiffness values for all experiments were
near its maximum (3.0). The standard deviation of the
stiffness was less that 0.02 for all the data, except for
CNS (.08), and Isolet-BC (.1) datasets. Hence, Itft-jt..
does guide descriptor updates towards yielding simpler
discriminants for the same or higher learning accuracy.
The best degree for other polynomial kernel classifiers

Q I6% I Stiffness* degree radius degree radius
Epilepsy Means 1 2.9997 4 3 5 .006
Colon Tumor LFD .01 2.9955 1 5 2 .5
Leukemia LFD 1 2.9793 1 70 1 3
CNS Means .1 2.9454 5 .07 1 .001
ETH-Objects LFD .01 2.9884 3 300 1 2
TechTC Lin-SVM 1 2.9057 1 500 1 300
Isolet-BC Lin-SVM 1 3.0 4 .3 1 .07


also turned out to be 1, in several cases. Except for the
Leukemia data, CSC performed better than linear SVM
and linear Fisher classifiers in all cases.

We performed experiments on a quad core 3.8 GHz
64-bit Linux machine. Each 10-fold CV experiment on
gene-expression datasets took under 4 minutes for all
the classifiers including CSC. For the other datasets, run
times were less than 2 minutes. Since we used a faster
variant of LFD [24], it took under a second for all the
datasets. The run times for CSC were better than the
other classifiers with RBF kernels. However, the search
for the optimal model parameters adds a factor of 5 to
10. From the experimental results in Table-2, it is evident
that Conic Section classifier out-performed or matched
the others in a majority of the datasets.


In this paper, we introduced a novel concept class based
on conic section descriptors, that can represent highly
non-linear discriminant boundaries with merely O(M)
parameters, and that subsumes linear discriminants. We
provided a tractable supervised learning algorithm in
which the set of feasible solutions, called the Null Space,
for a descriptor update is represented as a compact geo-
metric object. The computation of the Null Space related
to quadratic equality constraints on class descriptors is
reduced to that of a Gram-Schmidt orthogonalization.
We introduced a rtat;,,.- criterion that quantifies the
extent of the non-linearity in the discriminant boundary.
In each descriptor Null Space, we pick solutions that
learn misclassified points and yield simpler discriminant
boundaries, due to the stiffness criterion. Thus stiffness
enables the classifier to perform model selection in the
learning phase. As the learning problem is equivalent
to a non-linear optimization problem that is not convex,
our method is prone to local minima as well.

We tested the resultant classifier against several state-
of-the-art classifiers on many public domain datasets.
Our classifier was able to classify tougher datasets better
than others in most cases, as validated in Table-2. The
classifier in its present form uses axially symmetric conic
sections. The concept class, by definition applies to the
multi-class case as well. The learning algorithm needs to
incorporate equivalent boundary representation in ecc-
Space among other aspects. In future work, we intend
to extend this technique to multi-class classification, to
conic sections that are not necessarily axially symmetric,
and explore pursuit of conics in kernel spaces.


(Vi, X,)
{b, Q}
g (x)
Si(ri, X)
Se(r6, Ce)


Number of samples and features.
A point in RM.
A focus point.
A labeled data sample. yi e {-1, +1}
Directrix hyperplane descriptors.
Descriptors of class k: {Fk, {bk, Qk}, Ce}
The scalar valued class eccentricity.
The eccentricity function given Ck.
{X e RM : g(X) 0} (decision boundary)
Distance to focus : IIX FkII
Distance to directrix : bk + QkX
Hypersphere centered at Xi with radius ri
Space between two concentric hyperspheres.
nN ,S for N, classified points.
A linear subspace in which S6 lies.
Toroidal Null Space defined in Eqn.22.

This research was in part supported by NIH RO1
NS046812 to BCV. The authors would like to thank
Jeffrey Ho for his participation in insightful discussions.

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