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entropy
Article
Reliable Recurrence Algorithm for High-Order
Krawtchouk Polynomials
Khaled A. AL-Utaibi 1,*,† , Sadiq H. Abdulhussain 2,† , Basheera M. Mahmmod 2,† ,
Marwah Abdulrazzaq Naser 3,† , Muntadher Alsabah 4,† and Sadiq M. Sait 5,†
Citation: AL-Utaibi, K.A.;
Abdulhussain, S.H.; Mahmmod, B.M.;
Naser, M.A.; Alsabah, M.; Sait, S.M.
Reliable Recurrence Algorithm for
High-Order Krawtchouk
Polynomials. Entropy 2021,23, 1162.
https://doi.org/10.3390/e23091162
Academic Editor: Boris Ryabko
Received: 3 August 2021
Accepted: 1 September 2021
Published: 3 September 2021
Publisher’s Note: MDPI stays neutral
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
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Attribution (CC BY) license (https://
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4.0/).
1Department of Computer Engineering, University of Ha’il, Ha’il 682507, Saudi Arabia; alutaibi@uoh.edu.sa
2Department of Computer Engineering, University of Baghdad, Al-Jadriya, Baghdad 10071, Iraq;
sadiqhabeeb@coeng.uobaghdad.edu.iq (S.H.A.); basheera.m@coeng.uobaghdad.edu.iq (B.M.M.)
3Department of Architectural Engineering, University of Baghdad, Al-Jadriya, Baghdad 10071, Iraq;
marwahabdalkhafaji@gmail.com
4Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 4ET, UK;
mqalsabah@gmail.com
5Department of Computer Engineering, Interdisciplinary Research Center for Intelligent Secure Systems,
Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia;
sadiq@kfupm.edu.sa
*Correspondence: alutaibi@uoh.edu.sa
† These authors contributed equally to this work.
Abstract:
Krawtchouk polynomials (KPs) and their moments are promising techniques for applica-
tions of information theory, coding theory, and signal processing. This is due to the special capabilities
of KPs in feature extraction and classification processes. The main challenge in existing KPs recur-
rence algorithms is that of numerical errors, which occur during the computation of the coefficients
in large polynomial sizes, particularly when the KP parameter (
p
) values deviate away from 0.5 to 0
and 1. To this end, this paper proposes a new recurrence relation in order to compute the coefficients
of KPs in high orders. In particular, this paper discusses the development of a new algorithm and
presents a new mathematical model for computing the initial value of the KP parameter. In addition,
a new diagonal recurrence relation is introduced and used in the proposed algorithm. The diagonal
recurrence algorithm was derived from the existing
n
direction and
x
direction recurrence algorithms.
The diagonal and existing recurrence algorithms were subsequently exploited to compute the KP
coefficients. First, the KP coefficients were computed for one partition after dividing the KP plane into
four. To compute the KP coefficients in the other partitions, the symmetry relations were exploited.
The performance evaluation of the proposed recurrence algorithm was determined through different
comparisons which were carried out in state-of-the-art works in terms of reconstruction error, polyno-
mial size, and computation cost. The obtained results indicate that the proposed algorithm is reliable
and computes lesser coefficients when compared to the existing algorithms across wide ranges of
parameter values of
p
and polynomial sizes
N
. The results also show that the improvement ratio of
the computed coefficients ranges from 18.64% to 81.55% in comparison to the existing algorithms.
Besides this, the proposed algorithm can generate polynomials of an order
∼
8.5 times larger than
those generated using state-of-the-art algorithms.
Keywords:
discrete Krawtchouk polynomials; Krawtchouk moments; propagation error; energy
compaction; computation cost
1. Introduction
Digital image processing plays an essential role in several aspects of our daily lives.
Image signals are subject to several processes such as transmission [
1
], enhancement [
2
],
transformation [
3
], hiding [
4
], and compression [
5
,
6
]. Similarly to image processing, speech
signals processing is also essential [
7
], and involves several stages such as transfer [
8
], acqui-
sition [
9
], and coding [
10
]. Pattern recognition, which is considered an automated process,
Entropy 2021,23, 1162. https://doi.org/10.3390/e23091162 https://www.mdpi.com/journal/entropy
Entropy 2021,23, 1162 2 of 24
is widely used in various applications such as computer vision [
11
], statistical data analysis
[
12
], information retrieval [
13
], shot boundary detection [
14
], and bio-informatics [
15
].
However, the accuracy of extracting the significant features in these essential signal pro-
cessing approaches is crucial [
16
]. Feature extraction, in particular, is used to reduce the
dimensionality of the signals to a finite size [
17
,
18
]. Specifically, a finite number of features
can be used to represent the signals. These finite features can be considered the most
significant ones and need to be extracted using efficient methods. As such, to achieve
the best signal representation, a fast and robust feature extraction mechanism becomes
necessary. To this end, such features’ extraction mechanism needs to meet the desired
accuracy concerns by extracting the most significant features efficiently with low processing
times. Furthermore, the energy compaction and localization of the signals can also be
considered as essential factors in signal compression [
19
]. This is attributed to the fact that
using fewer effective coefficients results in a more accurate representation of the signals.
Hence, orthogonal polynomials are an effective tool that can be applied to meet these
desired requirements and features characterization.
Continuous and discrete orthogonal polynomials are commonly used in many signal
processing applications and feature characteristics. Continuous orthogonal polynomials
are used in speech and image applications, for example, in pattern recognition, robot vision,
face recognition, object classification, hiding information, data compression, template
matching, and in edge detection for image data compression [
20
–
23
]. The performance of
orthogonal polynomials is evaluated according to their ability to extract distinct features
from signals in a fast and efficient way. This special ability of feature extraction can be
quantified using properties such as a) energy compaction; b) signal representation without
redundancy; c) numerical stability; and d) localization [24–26].
Discrete orthogonal polynomials are widely used to extract features from images [
27
].
There are different types of discrete polynomials. Examples of these include discrete
Tchebichef polynomials [
28
], Chebyshev polynomials [
29
], discrete Charlier polynomials
[
26
], discrete Krawtchouk polynomials (DKPs), and discrete Meixner polynomials [
30
].
Among these polynomials, DKPs are widely exploited in image processing. This is due
to their salient characteristics, which can be used to extract local features from images.
Specifically, by exploiting the localization property of the DKPs, images can be efficiently
represented by using a finite number of features [
31
]. The localization property is carried
out by controlling the parameter value (
p
) of the DKPs. Typically, discrete orthogonal mo-
ments are generated using DKPs. Discrete orthogonal moments are extensively exploited in
image and signal processing [
11
,
32
–
34
], coding theory [
35
], and information theory [
36
,
37
].
However, reconstructing a signal using moments and maintaining the orthogonality has
to date been considered a challenging task. In addition to this, the discretization error is
another challenge that appears when reconstructing the signal, especially when moments
are used in the implementation. This discretization error, however, increases with the
moments’ order. For example, when the order of the moments increases, the discretization
error increases accordingly. As such, the accuracy of the moments’ computation is reduced,
resulting in an inaccurate representation of images [38–40].
Several studies have been performed on the discrete Krawtchouk polynomials and
methods developed to efficiently compute their coefficients, for example see [
25
,
31
,
41
–
44
].
These research works utilize a three-term recurrence algorithm [
25
]. In addition, the hy-
pergeometric series and gamma functions are widely applied in image processing [
25
].
However, the aforementioned research works use functions that require a long time to
execute and process the signals. Furthermore, these functions become numerically unstable
when the order of the moments increases. Instead, a three-terms recurrence algorithm
can be applied to come up with the aforementioned time and accuracy issues. To this
end, Yap et al. [
41
] presented a recurrence algorithm in the
n
direction to calculate the
Krawtchouk polynomial coefficients (KPCs). Due to the propagation error, this recurrence
algorithm becomes unstable—especially when the polynomial size increases. In general,
such a propagation error increases through the computation of polynomial coefficients.
Entropy 2021,23, 1162 3 of 24
This is attributed to the fact that pitfalls may happen even when small errors in floating
numbers occur. As such, there is an essential need to reduce the number of recurrences,
especially when the polynomial size is increased. Furthermore, such a reduction could also
lead to a reduction in the propagation error, thereby leading to a more stable computation
of polynomial coefficients, as desired. The work in [
31
] proposes a modified recurrence
algorithm in the
n
direction (RAN) by partitioning the KP array into two partitions. There-
fore, only 50% of the coefficients need to be computed. However, the partitioning of the
KP array generates a larger polynomial size, which is undesirable. On a similar basis, the
work in [
42
] proposes a recurrence algorithm in the
x
direction (RAX) by partitioning the
KP plane into two partitions. Specifically, the
x
direction of the recurrence algorithm is
used to compute the KPCs. The results show that the RAX algorithm outperforms the
RAN algorithm. It is worth noting that the RAN and RAX algorithms use a symmetric
property to compute the polynomial coefficients of the second portion. A novel bi-recursive
relation algorithm in the n direction (BRRN) was proposed in [
43
]. In this method, the KP
array is divided into four partitions. However, the KPC coefficients are computed for two
partitions only, i.e., 50% of the coefficients are computed. Then, a symmetric property is
used to compute the KPCs for the remaining partitions. The results indicate that the BRRN
algorithm provides higher gain than the RAX algorithm for limited values of parameter
p
,
i.e., the polynomial size. Abdulhussain et al. [
44
] developed an algorithm and presented
new properties of orthogonal polynomials such that the KP plane is divided into four por-
tions and only the KPCs for one portion are computed. For this, the size of the generated
polynomials is increased, but it is still limited, especially for parameter
p
less than 0.25 and
greater than 0.75. This is because the initial values or sets become zero as the polynomial
size increases. Recently, the work in [
25
] proposed a recurrence relation algorithm that has
the ability to compute KPCs with very large sizes. However, the proposed algorithm is
limited to the parameter value of p=0.5.
The existing algorithms suffer from the following limitations: (1) no initial value is
provided; (2) the propagation error is high; and (3) the implementation of these algorithms
is limited to a specific value of the parameter
p
. They also suffer from numerical instabilities,
especially when the polynomials orders and sizes become high. Therefore, an advanced
and reliable recurrence algorithm for high-order polynomials and large sizes is required.
Therefore, a new recurrence algorithm is presented in this paper, which handles the
numerical instabilities issue of using high orders of polynomials and large sizes. The
proposed algorithm is able to compute the KPCs for all values of the parameter
p
. In
addition to this, this paper presents the development of a new mathematical model for
computing the initial value of
p
. In particular, the initial value is accurately computed for all
values of parameter
p
. Furthermore, a new relation to compute the values of the initial sets
is derived. To this end, a diagonal recurrence relation is introduced. The proposed diagonal
recurrence algorithm is derived from the existing
n
and
x
directions of the recurrence
algorithm. The diagonal and the existing recurrence algorithm are exploited to compute
the KP coefficients. The KP coefficients are then used for one partition after dividing the KP
plane into four partitions. To compute the KP coefficients in other partitions, a symmetric
property relation is utilized.
Organization of the paper
: This paper is organized as follows. Section 2presents
the mathematical formulations of the orthogonal polynomials and moments. In Section
3, the methodology of the proposed recurrence algorithm is provided. This methodology
involves providing a discussion about the initial value selection of parameter
p
. In addition,
this section explains how the Krawtchouk polynomial’s coefficients can be computed. In
order to characterize the performance of the proposed approaches, Section 4provides the
numerical results. Finally, conclusions are discussed in Section 5.
Notation: In this paper, the operator transpose is denoted by
(·)T
and
(a
b)
denotes the
binomial coefficients.
Entropy 2021,23, 1162 4 of 24
2. Preliminaries
This section presents the Krawtchouk polynomials and their recurrence relation. To
this end, the
n
-th order of the Krawtchouk polynomials based on the hypergeometric series
is given as
ˆ
Kp
n(x) = 2F1−n−x
−N+1;1
p. (1)
The weighted function
ω(x
,
p)
and the norm function
ρ(n
,
p)
are used to generate the
weighted and normalized KP coefficients as given in [
31
]. To this end, the weighted and
normalized KP coefficients can be written as in (2) and (3), respectively:
ω(x,p) = N−1
xpx(1−p)N−x−1(2)
ρ(n,p) = (−1)n1−p
pnn!
(−N+1)n(3)
The Pochhammer symbol
(·)c
, which is known as an ascending or rising factorial function,
can be written as [45]
(a)c=Γ(a+c)
Γ(a)=a(a+1)(a+2)· · · (a+c−1), (4)
where
Γ(
.
)
denotes the Gamma function. To this end, using the weight and norm functions,
the weighted and normalized Krawtchouk polynomials of the
n
-th order for a signal of
size Nare given as [31]
Kp
n(x)=sω(n,x)
ρ(n,x)ˆ
Kp
n(x), (5)
Kp
n(x)=sN−1
nN−1
x p
1−pn+x
2F1−n−x
−N+1;1
p, (6)
n,x=0, 1, . . . , N−1; p∈(0, 1),
where 2F1describes the hypergeometric series and can be written as
2F1−n−x
−N+1;1
p=
∞
∑
k=0
(−n)k(−x)k
(−N+1)k,k!1
pk
. (7)
3. Proposed Recurrence Algorithm
This section describes the methodology of the proposed recurrence algorithm.
3.1. Computing the Initial Value
The problem with traditional approaches for computing the initial value in the
n=
0
and
x=
0 directions is the numerical instability. For example, the traditional methods
provide zero values of the initial
Kp
0(0)
—which is unstable. The initial value can be
computed as
Kp
0(0)=q(1−p)N−1. (8)
The expression in
(8)
makes the initial value (
Kp
0(0)
) decrease to zero for different
values of parameter
p
, especially for large polynomial sizes
N
. Figure 1shows the values
of
Kp
0(0)
for different values of parameter
p
and size
N
. The results show that the value of
Entropy 2021,23, 1162 5 of 24
Kp
0(0)
starts to fall to zero when
p
becomes larger than 0.1. Specifically, as the values of
parameter
p
increase, the value of
Kp
0(0)
falls to zero. For example, for
p=
0.15, the value
of
Kp
0(0)
becomes zero for
N>
5000 while for
p=
0.4, the value of
Kp
0(0)
becomes zero
earlier for
N>
2000. This makes it impossible to compute the rest of the KP coefficients’
values. Therefore, there is an essential need to find an efficient method for computing the
initial value of
p
, which prevents the initial value (
Kp
0(0)
) from dropping to zero. To this
end, this paper identifies the suitable non-zero values in the KP plane that need to be used
as an initial value. As such, there is an essential need to plot the values of coefficient
Kp
0(x)
for different values of parameter
p
. Figure 2shows the plots of the values of coefficient
Kp
0(x)
for different values of parameter
p
. Clearly, the results in Figure 2show that the
values of
Kp
0(x)
start with a small number and gradually reach the peak. Then, the values
drop to a very small number. In addition, we observe that using non-small values as an
initial set of parameter pseems useful to compute other values of the KP coefficients.
Figure 1.
The computation of the initial values (
Kp
0(0)
) as a function of different polynomial sizes
using Equation (8).
Figure 2. Plots of Kp
0(x)for a wide range of parameter pand N=500.
Figure 2demonstrates that the peak values can be located at
x=N p
. In this paper,
the value of
x=N p
is denoted by
x0
. To this end, a general formula for computing
Kp
0(x0)
can be written as
Entropy 2021,23, 1162 6 of 24
Kp
0(x0)=Kp
0(Np)×sω(N p;p)
ρ(0; p),
Kp
0(x0)=1×s(N−1
Np )pNp(1−p)−Np+N−1
1,
Kp
0(x0)=sN−1
Np pN p (1−p)−N p+N−1. (9)
Computing
Kp
0(x0)
using expression
(9)
may also lead to unstable numerical values
of coefficients with errors, especially for high-order polynomials. This is because the
binomial coefficients function tends to be very large and close to infinity. To demonstrate
this behavior, Figure 3is provided.
Figure 3.
The computation of the initial values (
Kp
0(x0)
) as a function of different polynomial sizes
using Equation (9).
Figure 3shows that the initial values (
Kp
0(x0)
) are still inaccurate where these values
record either NaN or Inf values. This is due to the nature of the polynomial coefficients
that are obtained by the expression in
(9)
. Thus, the initial values (
Kp
0(x0)
) seem difficult to
be computed for large polynomial sizes (
N
). To overcome this issue, this paper proposes
an efficient and suitable approach that makes the value of
Kp
0(x0)
commutable. It is
worth noting that the values obtained from the polynomial coefficients’ formula, especially
the Gamma function, should be reduced when the coefficients become large since their
argument value is increased. This can be achieved using
arg =exp(ln(arg)) = eln (arg)
and
the initial values can be computed as
Kp
0(x0) = e0.5×z, (10)
where zis given as
z=ln Γ(N)−log Γ(x0+1)−log Γ(N−x0)−x0ln1−p
p+ (N−1)ln(1−p).
Entropy 2021,23, 1162 7 of 24
A proof of expression (10) is presented in Appendix A.
Figure 4shows a plot of the proposed initial values of (
Kp
0(x0)
) in the developed
expression in
(10)
for various values of parameter
p
as a function of polynomials size
N
.
The results show that the proposed initial values are more computable for wide ranges of
parameter
p
and large polynomial sizes
N
, as desired. Hence, this signifies the feasibility
of the proposed formula for practical implementations compared with state-of-the-art
equations.
Figure 4.
The computation of the initial sets’ values (
Kp
0(x0)
) as a function of different polynomial
sizes using Equation (10).
3.2. The Fundamental Computation of the Initial Values
Typically, for any orthogonal polynomial, the computation of coefficients requires
the evaluation of a significant number of fundamental initials. Thus, based on the first
initial value
Kp
0(x0)
, computed using the proposed formula, the KP coefficients are ob-
tained
Kp
0(x1)
,
Kp
1(x0)
, and
Kp
1(x1)
(see Figure 5). Therefore, this section shows how the
aforementioned coefficients values are computed.
First,
Kp
0(x1)
is computed using the proposed derived formula, which provides the
two terms relation between the
Kp
0(x0)
and
Kp
0(x1)
. To this end, this relation/ratio between
the coefficients can be formulated as
Kp
0(x1)
Kp
0(x0)=v
u
u
t
(N−1
Np+1)pN p+1(1−p)−N p+N−2
(N−1
Np )pNp (1−p)−Np+N−1,
=v
u
u
u
t
(N−1)!
(Np+1)!(N−N p−2)!
(N−1)!
(Np)!(N−N p−1)!
·1−p
p,
=sN−Np −1
Np +1·p
1−p, (11)
where x1=x0+1=Np +1. Thus, the expression in (11) can be further simplified to:
Entropy 2021,23, 1162 8 of 24
Kp
0(x1) = sN−Np −1
Np +1·p
1−pKp
0(x0). (12)
Figure 5.
The fundamental computation of initial values according to the
x
and
n
directions in the
KP plane.
Then,
Kp
1(x0)
and
Kp
1(x1)
can be computed using a two-term recurrence relation
with
Kp
0(x0)
and
Kp
0(x0)
, respectively. To derive the recurrence relation of the proposed
approach, the following formulas are used:
Kp
0(x)=qωp
K(x,N), (13)
Kp
1(x)=qωp
K(x,N),p(N−1)−x
p(N−1)p(1−p). (14)
From (13) and (14), Kp
1(x)can be simplified to:
Kp
1(x)=p(N−1)−x
p(N−1)p(1−p)Kp
0(x). (15)
Using the expression in (15), Kp
1(x0)and Kp
1(x1)can be further simplified to
Kp
1(x0)=p(N−1)−Np
p(N−1)p(1−p)Kp
0(x0)
=p
p(N−1)p(1−p)Kp
0(x0)(16)
Kp
1(x1)=p(N−1)−(Np +1)
p(N−1)p(1−p)Kp
0(x1)
=p+1
p(N−1)p(1−p)Kp
0(x1). (17)
Entropy 2021,23, 1162 9 of 24
3.3. The Computation of the Initial Sets
In this section, the computation of the initial sets is discussed. These initial sets are
shown in Figure 6. Figure 6shows parts of the KP coefficients, which are covered in this
section. The initial sets are defined in the ranges of x=x0,x1and n=2, 3, . . . , x.
Figure 6. A diagram shows the location of initial sets in the KP plane.
To compute the values of the initial set, the recurrence in the
n
direction is used. To
this end, the formulation of recurrence is given as
Kp
n(x)=α1n,xKp
n−1(x)−α2n,xKp
n−2(x), (18)
α1n,x=(N−2n+1)p+n−x−1
pp(1−p)n(N−n),
α2n,x=s(n−1)(N−n+1)
n(N−n),
x=x0,x1and n=2, 3, . . . , x.
After computing the initial sets, the values in the ranges
x=x0
,
x1
and
n=
0, 1,
. . .
,
x
are used as the initials to compute the rest of the KP coefficients values.
3.4. Computation of the Coefficients Values for KP
In this section, the rest of the coefficient values are computed. These coefficients are
shown in Figure 7. As depicted in Figure 7, there are two main parts, which are located at
the left (Part 1) and the right (Part 2) sides of the initial sets. In addition, the coefficients are
located at the right side of the initial sets and can be divided into three sub-parts. These
parts are Part 2-1, Part 2-2, and Part 2-3. The detailed description of the computation of
each part is presented in the following subsections.
Entropy 2021,23, 1162 10 of 24
Figure 7.
A diagram shows the parts’ locations in the KP coefficients’ plane in the
x
and
n
directions
based on the proposed algorithm.
3.4.1. Computation of the Coefficients Located at Part 1
In this section, the values of KP coefficients in Part 1, shown in Figure 7, are computed
using a backward
x
recurrence relation. The backward recurrence relation is obtained from
the traditional recurrence relation in the xdirection as
Kp
n(x−1) = β1n,xKp
n(x)−β2n,xKp
n(x+1), (19)
β1n,x=−(N−2x−1)p−n+x
pp(1−p)x(N−x),
β2n,x=s(N−x−1)(x+1)
x(N−x),
n=0, 1, . . . , x0and x=x0,x0−1, . . . , n.
The values of KP coefficients become unstable as the index of xgoes towards n. This
is because the values of the coefficients tend to be less than 10
−7
. To overcome this issue,
the condition of a threshold value is used to stop the recurrence for each value of index
n
.
The proposed condition is given by
|Kn(x)|<10−5and |Kn(x+1)|<10−7. (20)
3.4.2. Computation of the Coefficients Located at Part 2-1
In this section, the values of KP coefficients in Part 2-1, given in Figure 7, were
computed using a forward xrecurrence relation as given in (21):
Kp
n(x+1) = γ1n,xKp
n(x)−γ2n,xKp
n(x−1)(21)
γ1n,x=(N−2x−1)p−n+x
pp(1−p)(x+1)(N−x−1)
γ2n,x=−s(N−x)x
(x+1)(N−x−1)
n=0, 1, . . . , x0and x=x0,x0+1, . . . , N−n−1
The aforementioned recurrence relation, which is used to compute the values in Part
2-1, is subject to the following condition:
|Kn(x)|<10−5and |Kn(x+1)|<10−7. (22)
Entropy 2021,23, 1162 11 of 24
3.4.3. Computation of the Coefficients Located at Part 2-2
This section presents two new recurrence relations’ approaches to compute the KP
coefficient values diagonally. This diagonal calculation is given in Part 2-2 Figure 7. The
values in the diagonal of Figure 7are then used to compute the coefficients’ values in Part
2-3 in Figure 7. This is because the recurrence relation in the
n
direction cannot be used to
compute the coefficients’ values. Consequently, some values in Part 2 become zero, which
results from the condition used to prevent the occurrence of unstable values.
This paper derives the recurrence relations provided in Figure 8. From Figure 8a, it can
be seen that the elements computed for
x0
and
x1
can be used to compute the coefficients
along the main diagonal
n=x
and
n=x−
1. Furthermore, to compute the coefficients’
values of KP
Kp
n(x+
1
)
, the coefficient value
Kp
n+1(x)
is computed using the
n
direction
recurrence algorithm. The similarity across the main diagonal (
n=x
) is exploited for
simplicity where
Kp
n(x+
1
) = Kp
n+1(x)
. To this end, the KP coefficients along
n=x−
1
are computed as
Kp
n(x+1) = δ1n,x,Kp
n(x)−δ2n,x,Kp
n−1(x), (23)
δ1n,x=(N−2x−1)p−n+x
pp(1−p)x(N−x),
δ2n,x=−s(N−x)x
(x+1)(N−x−1),
x=x0+1, x0+2, N
2−1 and n=x.
Figure 8.
A diagram shows the coefficients’ locations that are used to compute the values in Part 2-2.
To compute the values at the main diagonal where
n=x
, a new recurrence relation
approach is developed. This is achieved by combining both
n
and
x
directions recurrences.
Suppose that the values at
(n,x+1), and (n−1, x+1)
are known (see circulated values
I
and
K
in Figure 8d). Then, the value at
n+
1,
x+
1 (see circulated values
L
in Figure 8d)
can be computed using the ndirection recurrence relation as
Kp
n+1(x+1) = α1n+1,x+1,Kp
n(x+1)−α2n+1,x+1,Kp
n−1(x+1). (24)
The value at
(n−
1,
x+
1
)
can be computed using the
x
direction recurrence relation as
Kp
n−1(x+1) = γ1n−1,x,Kp
n−1(x)−γ2n−1,x,Kp
n−1(x−1). (25)
Entropy 2021,23, 1162 12 of 24
Substituting Equation
(25)
in
(24)
yields the following general expression of the recur-
rence relation:
Kp
n+1(x+1) = α1n+1,x+1Kp
n(x+1)−α2n+1,x+1γ1n−1,xKp
n−1(x)−γ2n−1,xKp
n−1(x−1)
=α1n+1,x+1Kp
n(x+1)−α2n+1,x+1γ1n−1,xKp
n−1(x) + α2n+1,x+1γ2n−1,xKp
n−1(x−1)
=η1n,x,Kp
n(x+1)−η2n,xKp
n−1(x) + η3n,xKp
n−1(x−1)(26)
η1n,x=α1n+1,x+1=(N−2n−1)p+n−x−1
pp(1−p)(n+1)(N−n−1)
η2n,x=α2n+1,x+1γ1n−1,x=sn(N−n)((N−2x−1)p+x−n+1)2
p(1−p)(n+1)(N−n−1)(x+1)(N−x−1)
η3n,x=α2n+1,x+1γ2n−1,x=sn(N−n)x(N−x)
(n+1)(N−n−1)(x+1)(N−x−1)
x=x1,x1+1, · · · ,N/2 −1; and n=x
This recurrence relation is termed as the four-term recurrence relation in the
n− −x
direction. This new development approach is used to compute the KP coefficients in the
range
x=x1+
1,
x1+
2,
. . .
,
N/
2
+
1 and
n=x−
1, and
x=x1+
1,
x1+
2,
. . .
,
N/
2 and
n=xas shown in Figure 9:
Figure 9. A diagram shows a location of the coefficients in Part 2-2.
3.4.4. Computation of the Coefficients Located at Part 2-3
This section presents the computation of the KP coefficients located at Part 2-3 in
Figure 7. These values are computed using
(21)
in the ranges
n=x1
,
x1+
1,
N/
2
−
2 and
n+2≤x≤N−n+1. However, the following condition should be met:
|Kn(x)|<10−5and |Kn(x+1)|<10−7. (27)
Entropy 2021,23, 1162 13 of 24
3.5. Computation of the Rest of the KP Coefficients
This subsection provides the computation of the rest of the KP coefficients. To this
end, the rest of the coefficients can be computed using a similarity relation of the KP. The
coefficients in the ranges
x=
0, 1,
. . .
,
N/
2
−
1 and
n=x+
1,
x+
2,
. . .
,
N−x−
1 are
given as Kp
n(x)=Kp
x(n). (28)
The coefficients in the ranges
x=
0, 1,
. . .
,
N−
1 and
n=N−x
,
N−x+
1,
. . .
,
N−
1
are computed using the following expression:
Kp
n(x)= (−1)N−n−x−1Kp
N−n(N−x). (29)
In addition, to calculate the KP coefficients for
p>
0.5, firstly the value of
p
is set to
1
−p
. Then, the KP coefficients are computed using the proposed methodology. Finally,
the following formula is applied for all coefficients [44]:
Kp
n(x)= (−1)nKp
n(N−x−1). (30)
3.6. Summary of the Proposed Algorithm
In this subsection, a summary of the proposed algorithm is presented. To this end, a
flow chart of the proposed recurrence is shown in Figure 10. In addition to this, a pseudo-
code is presented (see Algorithm 1) for more clarification. In addition, 3D plots of the KP
coefficients are given in this subsection.
Figure 10. Flowchart of the proposed algorithm.
Entropy 2021,23, 1162 14 of 24
Algorithm 1 Computation of Krawtchouk polynomials using the proposed algorithm.
Input: N,p
Nrepresents the size of the Krawtchouk polynomial,
prepresents the parameter of the Krawtchouk polynomials.
Output: Kp
n(x)
1: Flag=False
2: if p>0.5 then
3: Flag=True; p←p−1
4: end if
5: x0←N p,x1←x0+1
6: Compute Kp
0(x0)using (10)
7: Compute Kp
0(x1)using (12)
8: Compute Kp
1(x0)and Kp
1(x1)using (16) and (17)
9: .Compute initial set
10: for x=x0:x1do
11: for n=2 : xdo
12: Compute Kp
n(x)using (18)
13: end for
14: end for
15: .Compute coefficient values in Part 1
16: for n=0 : x0do
17: for x=x0:−1 : ndo .inner loop
18: Compute Kp
n(x)using (19)
19: if |Kn(x)|<10−5and |Kn(x+1)|<10−7then
20: Exit inner loop
21: end if
22: end for
23: end for
24: .Compute coefficient values in Part 2-1
25: for n=0 : x0do
26: for x=x0:N−n−1do .inner loop
27: Compute Kp
n(x)using (21)
28: if |Kn(x+1)|<10−7and |Kn(x)|<10−5then
29: Exit inner loop
30: end if
31: end for
32: end for
33: .Compute coefficient values in Part 2-2
34: for x=x0:N/2 −1do
35: n←x
36: Compute Kp
n(x)using (23)
37: end for
38: for x=x1:N/2 −1do
39: n←x
40: Compute Kp
n(x)using (26)
41: end for
42: .Compute coefficient values in Part 2-3
43: for n=x1:N/2 −2do
44: for x=n+2 : N−n−1do .inner loop
45: Compute Kp
n(x)using (21)
46: if |Kn(x)|<10−5and |Kn(x+1)|<10−7then
47: Exit inner loop
48: end if
49: end for
50: end for
51: Compute the rest of the coefficients using the similarity relations (28) and (29)
52: if Flag=True then
53: Apply (30)
54: end if
Figures 11 and 12 show a 3D plot of the KP coefficients, which are generated using
the proposed recurrence algorithm with
N=
2000 and different values of the
p
parameter
ranging between <0.5 and >0.5, respectively.
Entropy 2021,23, 1162 15 of 24
Figure 11. 3D plot of the KP coefficients computed for N=2000 and p<0.5.
Figure 12. 3D plot of the KP coefficients computed for N=2000 and p>0.5.
4. Numerical Results and Analyses
This section presents the results obtained using the proposed recurrence algorithm.
In addition, a comprehensive comparison is conducted with the existing recurrence algo-
rithms. The comparison is carried out in terms of the energy compaction, reconstruction
error, and computation cost.
Entropy 2021,23, 1162 16 of 24
4.1. Energy Compaction Analysis
The order of moments
n
impacts the process of signal reconstruction, energy com-
paction, and information retrieval. The order of the KP moments is given by
n=
0, 1, 2,
. . .
,
N−
1. The energy compaction is utilized to check the impact of using KP
to transform a large fraction of the signal energy into relatively few coefficients of moments.
To find the impact of using the KP parameter (
p
) on the energy compaction property, the
procedure given by [
46
] is employed. The stationary Markov sequence with length
N
and
zero mean is analyzed. A matrix Lwith covariance coefficients (ρ) is defined as [27]:
L=
1ρ· · · ρN−1
ρ1· · · .
.
.
.
.
........
.
.
ρN−1· · · ρ1
(31)
The matrix
L
is then transformed to the Krawtchouk domain. As such, the coefficients
in the main diagonal of the transformed matrix (
S
) are computed. The matrix
S
represents
the variance σ2
l, which can be computed as
S=RLRT, (32)
where
R
denotes the KP matrix and
(·)T
refers to the matrix transpose operation. In
addition, the normalized restriction error (Jm) can be computed using:
Jm=∑N−1
q=mσ2
q
∑N−1
q=0σ2
q
, (33)
m=0, 1, . . . , N−1, (34)
where
σ2
q
represents the order of
σ2
l
sorted in descending order. In the experiment, the
normalized restriction error is performed by considering different covariance coefficients
(ρ) and different values of the parameters MNP.
Figure 13 shows the normalized restriction error for different values of parameters (
p
)
with the covariance coefficient
ρ=
0.93. It can be observed from Figure 13 that when the
value of
p
is equal to 1
−p
, the normalized restriction error becomes equal. For example,
the normalized restriction error for
p=
0.05 is equal to
p=
1
−
0.05
=
0.95. In addition,
the energy compaction is influenced by the KP parameter
p
. For instance, as the parameter
p
increases from 0.05 to 0.45, the performance of the KP in terms of energy compaction
is changed. Furthermore, the energy compaction at parameter
p=
0.45 shows better
performance for parameter
p=
0.05 because the normalized restriction error (
Jm
) reaches
zero values. However, small values of parameter
p
shows better performance in terms of
feature extraction, as proven in [
44
]. Thus, it can be concluded that KP provides further
performance improvement as the parameter
p
reaches 0.5. Furthermore, a more accurate
result can be achieved when the parameter pis deviates from 0.5 [44].
Figure 13. Energy compaction for different values of the parameter p.
Entropy 2021,23, 1162 17 of 24
4.2. Analysis of Reconstruction Error
In this section, the proposed recurrence algorithm is evaluated by carrying out recon-
struction error analysis (REA). This REA was conducted for the proposed and the existing
works. The REA was performed using an image formed from 16 well-known images as
shown in Figure 14. In addition, the comparison was performed on an image with a large
size, i.e., (4096
×
4096). Different values of parameter
p
were considered in the analysis.
These values are p=0.1, 0.2, 0.3, and 0.4.
Figure 14. Evaluation of an image used for REA.
First, the WNKP (
R
) is generated using the proposed and existing algorithms. Then,
the KMs (
ψ
) of the image are computed. Then, the image is reconstructed from the
computed moments using a limited number of moments. Finally, the normalized mean
square error (NMSE) is calculated between the input image and the reconstructed version
of the image. Hence, the NMSE is given as [44]
NMSE(I,IRec) = ∑x,y(I(x,y)−IRec (x,y))2
∑x,y(I(x,y))2, (35)
where parameters
I
and
IRec
denote the original image and the reconstructed image, respectively.
The NMSE and the reconstructed image for
p=
0.1
and p=
0.2 are shown in
Figure 15
and Figure 16, respectively. The first row depicts the reconstructed images by utilizing the
FRK [
44
], while the second row represents the reconstructed images using the proposed
algorithm. The FRK algorithm is unable to reconstruct the image of the low-order moments.
In addition, it is unable to reconstruct the image with high orders. However, the proposed
algorithm is capable of fully reconstructing the image of different order values. In addition,
the NMSE is minimized in the proposed algorithm using a moment order of 680, which
is
∼
16%. Figure 15 shows that the proposed algorithm achieves an NMSE of 0.72 while
the FRK algorithm records a value of 0.84. This implies that the proposed algorithm
outperforms the FRK algorithm. Moreover, the NMSE reaches zero when the proposed
algorithm is used, while it records 0.64 when the FRK is used. The limitation with the FRK
algorithm is due to the initial set that was computed, which makes the KPCs values tend
towards zero, and thus, the NMSE is increased. Figure 17 provides a plot of experiments
using
p=
0.3. The results show that the proposed algorithm provides better NMSE than
the FRK starting from a moment order of 680. In addition, the performance improvement
Entropy 2021,23, 1162 18 of 24
increases when the moment order increases until it reaches the full order of (4096). At the
full order, the NMSE using the proposed algorithm reaches 0, while it reaches 0.18 using
the FRK algorithm.
Figure 15.
The NMSE performance comparing the proposed algorithm and the RAK algorithm [
44
]
with p=0.1.
Figure 16.
The NMSE performance comparing the proposed algorithm and the RAK algorithm [
44
]
with p=0.2.
Entropy 2021,23, 1162 19 of 24
Figure 17.
The NMSE performance comparing the proposed algorithm and the RAK algorithm [
44
]
with p=0.3.
Figure 18 shows a new performance evaluation using
p=
0.4. The results show that
the proposed algorithm has the ability to accurately generate the KP coefficients while
for the FRK, the KP coefficients remain inaccurate. This is attributed to the zero initial
value obtained in the FRK algorithm. It is also worth noting that the proposed algorithm is
able to generate KP coefficients for large polynomial sizes and at a high polynomial order,
which was experimentally found to be greater than 8192.
Figure 18.
The NMSE performance comparing the proposed algorithm and the RAK algorithm [
44
]
with p=0.4.
Entropy 2021,23, 1162 20 of 24
Finally, this paper provides a comparison of a maximum polynomial size between the
proposed and existing algorithms. The maximum size is obtained for different values of
the parameter
p
. For each recurrence algorithm, the polynomial (
R
) is computed first with
a size and order of N×N. Then, the following criterion is applied:
Err =sum(diagonal(R×RT)−I(N)) <Th, (36)
where
Th
is the threshold value over which 10
−5
is chosen.
I(N)
is the identity matrix with
a size of
N×N
. It is worth noting that the maximum value of
N
is set to 20,480. Table 1
lists the obtained maximum size for each algorithm for different values of parameter
p
.
This table demonstrates that the proposed algorithm is capable of generating an orthogonal
polynomial with a large size and for different values of parameter
p
. The proposed
algorithm outperforms the existing recurrence algorithms for all values of the parameters
p
considered. For example, at
p=
0.05, the proposed algorithm generates a size of 82
×
larger than the RAN algorithm, 243
×
larger than the RAX algorithm, and 16
×
larger than
the FRK algorithm. Thus, the proposed recurrence algorithm can be used to process large
signals sizes quickly and accurately.
Table 1. Maximum size of KP that is generated using the proposed and existing algorithms.
pAlgorithm pAlgorithm
RAN RAX FRK Proposed RAN RAX FRK Proposed
0.05 248 84 1236 20,480 0.55 926 932 2428 20,480
0.10 324 132 2250 20,480 0.60 808 812 2880 20,480
0.15 392 196 2252 20,480 0.65 706 708 3368 20,480
0.20 462 276 2980 20,480 0.70 618 618 3058 20,480
0.25 538 436 3400 20,480 0.75 538 490 3400 20,480
0.30 618 676 3058 20,480 0.80 462 318 2980 20,480
0.35 710 1234 3368 20,480 0.85 390 202 2252 20,480
0.40 814 1428 2880 20,480 0.90 322 140 2250 20,480
0.45 936 1220 2428 20,480 0.95 240 88 1236 20,480
4.3. Computation of the Cost Analysis
The computation cost is considered an important factor to evaluate the performance
of the proposed recurrence algorithm [
44
]. Thus, the computation cost of the proposed
algorithm is compared with the existing works. These algorithms the are recurrence
algorithm in the
x
direction (RAX), the recurrence algorithm in the
n
direction (RAN),
and FRK. The computation cost is performed using the number of computed coefficients.
Table 2shows the ratio of computed coefficients (RCCs) using the proposed algorithm for
different polynomials’ sizes. Full moments’ orders are considered. From Table 2, it can
be observed that the RCC for small values of
p
and 1
−p
achieves a low percentage. This
percentage increases as the value of
p
increases towards 0.5. The average RCC for
p=
0.05
and
p=
0.95 is 9.22% while for
p=
0.5, the average RCC is 20.35%. This is because the
effective coefficient is shaping a circle for
p=
0.5, and they are shaping a rotated ellipse as
the parameter
p
deviates towards 0 or 1, which allows the number of effective coefficients
to be reduced. Consequently, the percentage of the computed coefficients is reduced.
Entropy 2021,23, 1162 21 of 24
Table 2. The ratio comparison of the computed coefficients of the proposed recurrence algorithm.
Krawtchouk Parameter (p)
0.05,
0.95
0.1,
0.9
0.15,
0.85
0.2,
0.8
0.25,
0.75
0.3,
0.7
0.35,
0.65
0.4,
0.6
0.45,
0.55 0.5
Polynomial size (N)
1024 10.03 13.31 15.56 17.24 18.52 19.51 20.24 20.75 21.05 21.16
2048 9.48 12.74 14.99 16.68 17.97 18.96 19.69 20.20 20.50 20.60
3072 9.25 12.51 14.76 16.45 17.74 18.73 19.47 19.97 20.27 20.38
4096 9.13 12.38 14.63 16.32 17.61 18.60 19.34 19.85 20.14 20.24
5120 9.05 12.30 14.54 16.23 17.53 18.52 19.25 19.76 20.06 20.16
6144 8.99 12.24 14.48 16.17 17.47 18.46 19.19 19.70 20.00 20.10
7168 8.95 12.19 14.44 16.13 17.42 18.41 19.15 19.65 19.95 20.05
8192 8.91 12.15 14.40 16.09 17.38 18.38 19.11 19.62 19.92 20.02
Average 9.22 12.48 14.73 16.41 17.71 18.70 19.43 19.94 20.24 20.34
Table 3demonstrates the performance improvement ratio between the proposed and
the existing algorithms, namely RAN, RAX, and FRK. The RAN and RAX algorithms
compute 50% of the KPCs for all values of parameter
p
because the
nx
plane is divided
into two portions. On the other hand, the FRK algorithm computes 25% of the KPCs as it
divides the
nx
plane into four partitions. The improvement ratio between the proposed
and existing algorithms is obtained as
Improvement Ratio =1−RCC of the proposed algorithm
RCC of the existing algorithm . (37)
According to Table 3, the proposed polynomial shows an improvement ratio of 18.64%
at
p=
0.5 when compared to the FRK algorithm and 59.32% when compared to the RAN
and the RAX algorithm. The improvement ratio increases as the value of the parameter
p
deviates towards 0 and 1. For example, the results show that at parameter
p=
0.25, the
proposed method achieves an improvement ratio of 29.18% compared to the FRK algorithm,
and 64.59% compared to other algorithms. In addition, a maximum improvement ratio of
63.11% is achieved by the proposed algorithm when it compares with the FRK algorithm
while it is 81.55% when it compares with the RAN and RAX algorithms.
Table 3.
Comparing the improvement ratio of the computed coefficients of the proposed algorithm
and the existing algorithms.
Krawtchouk Polynomial Parameter (p)
0.05,
0.95
0.1,
0.9
0.15,
0.85
0.2,
0.8
0.25,
0.75
0.3,
0.7
0.35,
0.65
0.4,
0.6
0.45,
0.55 0.5
Proposed 9.22 12.48 14.73 16.41 17.71 18.70 19.43 19.94 20.24 20.34
FRK 25.00 25.00 25.00 25.00 25.00 25.00 25.00 25.00 25.00 25.00
RAN and RAX 50.00 50.00 50.00 50.00 50.00 50.00 50.00 50.00 50.00 50.00
Improvement
over
FRK
63.11 50.09 41.10 34.35 29.18 25.22 22.28 20.25 19.05 18.64
Improvement
over
RAN and RAX
81.55 75.05 70.55 67.18 64.59 62.61 61.14 60.12 59.52 59.32
5. Conclusions
This paper described that state-of-the-art algorithms suffer from high error and that
the implementation of these algorithms is limited to a specific value of parameter
p
. In
Entropy 2021,23, 1162 22 of 24
addition to this, the state-of-the-art algorithms do not provide any computation of the initial
value. Furthermore, to date, no algorithm is proposed for computing the KP coefficients
with high-order polynomial and large polynomial sizes. To address these limitations,
a new recurrence relation was proposed in this paper. To this end, a new initial value
was presented and derived. In addition to this, a new diagonal recurrence relation was
introduced. The proposed algorithm divided the KP plane into four triangles and only
the coefficients in the upper triangle are computed. The coefficients in the upper triangle
were divided into fundamental initial sets, initial sets, Part-1 and Part-2, respectively. The
n
-recurrence,
x
-recurrence, backwards
x
-recurrence, and diagonal recurrence relations
were used to compute the values of the coefficients in the upper triangle. In addition, the
identities were used to compute the values of the coefficients in the other triangles. The
proposed algorithm was evaluated and compared with the existing recurrence algorithms.
The comparison was carried out based on the reconstruction error, energy compaction, and
computation cost. The experimental results showed that the proposed algorithm achieves
a remarkable improvement over the existing algorithms in terms of the maximum size
generated and the number of coefficients computed. Although the proposed algorithm
outperforms state-of-the-art algorithms, the computational complexity is still high and can
be further reduced. This can be achieved by implementing the proposed algorithm in a
multi-processing environment (parallelizing) rather than in sequential form, as considered
in this paper. Our future work is also directed towards implementing the proposed
algorithm for KP with other orthogonal polynomials. This is expected to result in a new
OP that has the potential of using orthogonal polynomials as well as the properties of
KP coefficients.
Appendix A. Proof of Equation (10)
This section presents a proof through Equation (10):
Kp
0(x0) =sN−1
Np pN p (1−p)−N p+N−1(A1)
=N−1
Np pN p (1−p)−N p+N−11/2
(A2)
=(N−1)!
(x0)!(N−x0−1)!×px0(1−p)N−1
(1−p)x01/2
(A3)
= Γ(N)
Γ(x0+1)Γ(N−x0)×1−p
p−x0
(1−p)N−1!1/2
(A4)
(A5)
By taking the exponential and natural log (elog), then (A5) can be simplified to:
Kp
0(x0) =elogΓ(N)
Γ(x0+1)Γ(N−x0)×1−p
p−x0(1−p)N−11/2
(A6)
=e
1
2logΓ(N)
Γ(x0+1)Γ(N−x0)×1−p
p−x0(1−p)N−1(A7)
=e
1
2log Γ(N)
log Γ(x0+1)log Γ(N−x0)×1−p
p−x0(1−p)N−1(A8)
=e1
2log Γ(N)−log Γ(x0+1)−log Γ(N−x0)−x0log(1−p
p)+(N−1)log(1−p). (A9)
Author Contributions:
Conceptualization, S.H.A., K.A.A. and B.M.M.; methodology, S.H.A. and
B.M.M.; software, S.H.A., K.A.A. and M.A.N.; validation, M.A., S.M.S. and M.A.N.; formal analysis,
M.A.; investigation, M.A. and K.A.A.; writing—original draft preparation, S.H.A., M.A., M.A.N. and
B.M.M.; writing—review and editing, K.A.A., S.M.S. and M.A.; visualization, S.H.A. and B.M.M.;
Entropy 2021,23, 1162 23 of 24
supervision, S.H.A. and S.M.S.; project administration, S.H.A. All authors have read and agreed to
the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement: All data are available within the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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