DESIGN METHOD FOR WEDGE-SHAPED FILTERS

Radu Matei

Technical University of Iasi, Faculty of Electronics and Telecommunications, Bd.Carol I nr. 11, 700506 Iasi, Romania

Keywords: 2D filter design, Frequency transformations.

Abstract: We present an analytical design method for a particular class of two-dimensional filters, namely wedge

filters. The method relies on a frequency mapping which is applied to a 1D IIR low-pass prototype filter of a

desired shape. We used as prototypes a flat-top filter and a Gaussian filter. Such filters have applications in

texture analysis based on spatial filtering using various filter banks. In this paper we approached the wedge

filter design method, without actually presenting an application in texture classification or other image

processing tasks, which are extensively treated in other works.

1 INTRODUCTION

The domain of two-dimensional filters has known a

constant development, stimulated by the ever-

increasing requirements in different image processing

applications. Their design methods, both for analog

and digital implementation, are well founded

(Dudgeon, 1984). A current design technique for 2D

filters is to start with a prototype 1D filter and to

transform its impulse response in order to obtain a

filter with the desired frequency response. Generally

the existing design methods of 2D IIR filters rely to a

large extent on 1D analog filter prototypes, using

spectral transformations from s to z plane via bilinear

or Euler transformations followed by z to

(, )zz

transformations (Pendergrass, 1976), (Hirano, 1978),

(Harn, 1986).

There are several types of filters with orientation-

selective frequency response. They are useful in

some image processing tasks like edge detection,

motion analysis etc. An important class are steerable

filters, synthesized as a linear combination of a set

of basis filters (Freeman, 1991). Another important

category are Gabor filters, efficiently implemented

both in digital and analog versions (Shi, 1998). In

(Bamberger, 1991) other types of oriented filters

were approached.

A particular class of 2D filters are the so-called

wedge filters, due to their symmetric wedge-like

shape about the origin in the frequency plane. These

filters find interesting application in feature

extraction, for instance in texture classification

(Randen, 1999). In (Simoncelli and Farid, 1995),

(Simoncelli and Farid, 1996), the steerable wedge

filters were introduced, which may be used to

analyze local orientation patterns in images. In

(Coggins, 1985), a bank of four wedge oriented

filters was used.

In this work we approach the design of a class of

wedge filters in the two-dimensional frequency

domain. We will consider a general case of a wedge-

shaped filter with a given aperture angle and an

imposed orientation angle of its longitudinal axis.

For design we will use two different 1D prototype

filters, namely maximally-flat and Gaussian. We

will consider in both cases only zero-phase filters,

generally preferred in image filtering due to the

absence of phase distortions. Two ideal wedge filters

in the frequency plane are shown in Figure 1. The

filter in Figure 1(a) has its frequency response along

the axis

. The angle AOB

= will be referred

to as aperture angle. In Figure 1(b) a more general

wedge filter is shown, with aperture angle

BOD

 , oriented along an axis 'CC , forming

an angle

AOC

 with frequency axis

− .

Figure 1: Ideal wedge filters specified in the frequency

plane: (a) along the axis

; (b) oriented at an angle

(a)

(b)

A B

Matei R. (2009).

DESIGN METHOD FOR WEDGE-SHAPED FILTERS.

In Proceedings of the International Conference on Signal Processing and Multimedia Applications, pages 19-23

DOI: 10.5220/0002230100190023

 SciTePress

2 WEDGE FILTER DESIGN

USING FREQUENCY

TRANSFORMATIONS

Next we present a design method which leads to 2D

zero-phase oriented filters from 1D prototypes. Let

us consider an 1D recursive low-pass filter prototype

frequency response of second order:

01 2

()

bb b

(1)

where usually

(0) 1

bH==.

A wedge filter along the frequency axis

can be

obtained using the 1D–2D frequency transformation

(for

≠ ) :

12 1 2

(, )fa

ωω ωω

→=⋅ (2)

By a we denoted the coefficient

1tg( 2)a

= ,

where

is the aperture angle of the wedge filter, as

defined in Figure 1. Replacing in (1)

by the ratio

, we get the frequency response in

4 222 44

02 1 1 2 2 1

4 222 44

211221

(, )

bba ba

aa a a

ωω ω

ωω

ωω ω

(3)

While the frequency mapping (2) is undetermined

for

→ ,

→ , once made the substitution (2)

into the prototype function (1), the expression (3)

has no undetermination any longer.

At this point we map

(, )H

into the complex

plane

(, )

s where

= . Since

=−

we get the function

(, )

ss :

4222 44

02 1 1 2 2 1

4 222 44

211221

(, )

bs bas s bas

Hss

aa s s a a s

(4)

A little more difficult task is now to find a

mapping of

(, )

ss into the complex plane

(

z ,

z ). This can be achieved either using the

forward or backward Euler approximations, or the

bilinear transform, which gives better accuracy.

The bilinear transform is a first-order

approximation of the natural logarithm function,

which is an exact mapping of the z-plane to the s-

plane. For our purposes the sample interval takes the

value

1T = so the bilinear transform for

and

the complex plane

(, )

s has the form:

⎛⎞

−

⎜⎟

⎝⎠

⎛⎞

−

⎜⎟

⎝⎠

(5)

Substituting

in (4), we find after some algebra

a function in

z and

z written in matrix form as:

[

]

[]

(, )

Fz z

××

ZBZ

ZAZ

(6)

where

Z and

Z are the vectors:

21 2

111 11

21 2

222 22

zz zz

−−

⎡

⎤

⎣

⎦

⎡

⎤

⎣

⎦

(7)

and

denotes matrix/vector product. Throughout

the paper we will use the convenient notion of

template, borrowed from the field of cellular neural

networks (CNNs) (Chua, 1988) to denominate the

coefficient matrices corresponding to the numerator

and denominator of a 2D filter transfer function

(, )

zz . Thus, the templates B and A can be

written as a sum of three separable matrices:

01 2

=⋅ ∗ + ⋅ ∗ + ⋅ ∗

TT T

bbaba

12 33 21

BMM MM MM (8)

=∗+ ⋅∗+ ⋅∗

TT T

aa a a

12 33 21

AM M M M M M (9)

where the vectors are:

[

]

[

]

[]

1 4 6 4 1; 1 4 6 4 1;

10 201

==−−

=−

(10)

and the operator

∗

denotes outer product of vectors.

In a more general case when the wedge filter axis

has an orientation specified by an angle

(with

respect to the axis

), the oriented wedge filter

may be obtained by rotating the axes of the plane

(, )

by an angle

. The rotation is defined by

the linear transformation:

cos sin

sin cos

ϕϕ

−

⎡

⎤⎡⎤

⎡⎤

=⋅

⎢

⎥⎢⎥

⎢⎥

⎣⎦

⎣

⎦⎣⎦

(11)

where

are the original frequency variables and

the rotated ones. In this case the 1D to 2D

frequency transformation can be written as:

(tg)

(, )

(tg )

ωϕ

ωωω

ωϕω

−⋅

→=

⋅+

(12)

Using the expression above and the bilinear

transform, we finally get a mapping of the form:

(, )

Fz z a

××

→=⋅

××

zM z

(13)

where

M is a 33

matrix of the form:

22 2

222

22 2

(tg -1) 2(tg 1) (tg 1)

2(tg 1) 4(tg 1) 2(tg 1)

(tg 1) 2(tg 1) (tg -1)

⎡

⎤

−+

⎢

⎥

=− − − + − −

⎢

⎥

⎢

⎥

+−

⎢

⎥

⎣

⎦

ϕϕϕ

M (14)

and

M is the matrix

M rotated by

90 .

We apply this frequency transformation directly to

the 1D prototype function (3), for

and we

SIGMAP 2009 - International Conference on Signal Processing and Multimedia Applications

get the 2D wedge filter transfer function in

z ,

z :

(, )

Hzz

××

ZB Z

ZA Z

(15)

where the

55× matrices

A ,

B have the form:

90 2 90 4

01 2

()( )()=∗ + ∗+ ∗bbaba

ϕϕϕ ϕϕ ϕϕ

BMM MM MM

(16)

90 2 90 4

()( )()=∗ + ∗ + ∗aa a a

ϕϕ ϕϕ ϕϕ

AMM MM MM

(17)

and

Z and

Z are the vectors given in (7).

Therefore the transfer function

(, )

(15) corresponds to a wedge filter with an aperture

angle

2arctg(1/ )a

=⋅ and whose longitudinal axis

is tilted about the

axis in the frequency plane

with an angle

Even if this method is straightforward and easy

to apply once found the 1D prototype filter, the 2D

filter designed in this way, corresponding to the

transfer function in

z ,

z will present noticeable

errors towards the limits of the frequency plane and

linearity distortions as compared to the ideal

frequency response in (3). This is mainly due to the

frequency warping effect introduced by the bilinear

transform, which is expressed by the continuous-

time to discrete-time frequency mapping:

arctg

ωω

⎛⎞

⎜⎟

⎝⎠

(18)

where

is the frequency of the discrete-time filter

and

the frequency of the continuous-time filter.

In order to correct this error we will next apply a

pre-warping, using the inverse of the mapping (18).

Since for our purposes we can take

1T = , in the

frequency transformation (12) we will substitute the

following mappings:

2tg

⎛⎞

→

⎜⎟

⎝⎠

2tg

⎛⎞

→

⎜⎟

⎝⎠

(19)

In dealing with the nonlinear mappings (19), a

polynomial or rational approximation would be

more suitable. One of the most efficient rational

approximations (best tradeoff between accuracy and

approximation order) is the Chebyshev-Padé

approximation. Using it we obtain:

()

0.5 0.008439

tg ( )

10.1

ωω

⋅− ⋅

⎛⎞

≅=

⎜⎟

⎝⎠

−⋅

(20)

very accurate on a frequency range close to

[,]

−

Using (12) we obtain the frequency transformation

which includes frequency pre-warping for

(

)

()

tg( 2) tg( 2) tg

(, )

tg( 2) tg tg( 2)

ωϕ

ωωω

ωϕω

−⋅

→=

⋅+

(21)

Substituting in (21)

tg( 2)

by the rational

approximation

()g

we get a rational expression in

and

for the frequency transformation

(, )

ωω

→ . Then as previously we map

(, )

into the complex plane

(, )

s and

finally we get using bilinear transform the frequency

mapping written again in matrix form:

:F →,

(, )

→

(, )

Fz z

⎡⎤

××

⎣⎦

⎡⎤

××

⎣⎦

ZB Z

ZA Z

(22)

The templates corresponding to the numerator

and denominator, of size

, are expressed as:

90 90

11 11

tg tg=−⋅ =⋅+

ϕϕ

BM MA MM (23)

where

M is the matrix

M rotated clock-wise by

90 which is numerically given by:

0.559283 1.081434 0.559283

0.915190 1.769619 0.915190

0.559283 1.081434 0.559283

⎡

⎤

−

⎡⎤

⎢

⎥

=∗

⎢⎥

⎢

⎥

−

⎣⎦

⎢

⎥

⎣

⎦

(24)

The elements of

M result from combinations of

the coefficients occurring in the expression of

()g

in (20). Finally we obtain the 1D to 2D frequency

transformation written in the matrix form:

(, )

Fz z a

××

→=⋅

××

zBz

zAz

(25)

where matrices

∗BB B and

=∗AA A

resulted by convolution are of size

77× .

We can apply this frequency transformation directly

to the 1D prototype function (1) and we obtain the

2D wedge filter transfer function in

z and

z :

(, )

Hzz

××

ZB Z

ZA Z

(26)

where the vectors

Z and

Z have the form:

[1]

zz z

−

…

(27)

with

12N

; the 13 13

matrices

A ,

B are:

01 2

() () ()bbaba

∗+ ∗+ ∗

BAAABBB (28)

() ()aa a a

∗+ ∗ + ∗

AAA AB BB (29)

As an important remark, even if the filter templates

result relatively large, this is the price paid for

ensuring a good linearity of the wedge filter shape in

DESIGN METHOD FOR WEDGE-SHAPED FILTERS

the frequency plane. The frequency pre-warping has

therefore increased the filter order. However, the

filter large-size templates result as a discrete

convolution of small size matrices (

33× , 55

) and

consequently can be considered partially separable.

At least the numerator of the general prototype (1)

may have real roots, therefore it can be factorized,

which implies convolution of smaller size matrices.

3 FILTER PROTOTYPES

3.1 Maximally-flat Filter Prototype

Let us consider a maximally-flat 1D IIR prototype

filter with the frequency response:

0.887175 0.269975 0.018905

()

1 0.600346 5.332057

−⋅+⋅

ωω

(30)

which is plotted in Fig.2(a) in the range

[,]

ππ

∈− .

Using the method described before, let us design a

wedge filter with an aperture angle

0.2

= and

oriented at an angle

= . For these values we

get

tg( 2)=0.3249a

= and tg = 0.7265

. The

frequency response and contour plot for these

parameters are shown in Fig.4.

3.2 Gaussian Filter Prototype

Another type of wedge filters may use Gaussian-

shaped filters as 1D prototypes. Next we will find an

efficient rational approximation for a Gaussian

frequency response:

(

)

(

)

exp 2G

ωσω

=− (31)

The parameter

gives the Gaussian selectivity.

We look for a rational approximation of

()G

as a

ratio of polynomials in cos

()

() cos( ) cos( )

()

−

=≅=

∑∑

Ge bm an

ωσ

ωω

(32)

where (,)

ππ

∈− ,

. The degrees of the

numerator and denominator (M, N) may be not

necessarily equal.

One of the most efficient rational approximation

(best tradeoff between accuracy and approximation

order) is the Chebyshev-Padé rational approximation.

The coefficients are usually determined numerically

using a symbolic calculation software. In the

expression of

()G

in (31) we make the change of

frequency variable:

cos( ) arccos( )

=⇔= (33)

Then we find a Chebyshev-Padé approximation of

(

)

(

)

exp arccos ( ) 2G

ωσω

=− (34)

as a rational function of the intermediate variable x.

We return to the original variable

, then finally

obtain a rational function in

cos

. For 2

, a

second-order approximation is accurate enough:

0.018 0.02749 cos 0.01092 cos2

()

1 1.231918 cos 0.288144 cos 2

−

+⋅+⋅

=≅

−⋅+ ⋅

(35)

Using usual trigonometric identities,

()G

finally put into the factorized form:

0.03067 (cos 0.89692) (cos 0.36239)

()

(1 1.73057 cos 0.80955 (cos ) )

⋅+ ⋅+

−⋅+⋅

ωω

(36)

This frequency response is plotted in Fig.2(b).

Using a symbolic computation software

(MAPLE etc.) we can derive an accurate rational

approximation (Chebyshev- Padé) of the cosine

function in the range

[2,2]

ππ

− :

1 0.447754 0.018248

cos

1 0.041694 0.002416

−+

≅

(37)

Substituting in (36) the expression of cos

, we get:

()

(1.89692 0.41036 0.02041 )

(1.3624 0.43264 0.01912 )

( ) 0.38833

1 0.837 2.5201

⎛⎞

−⋅+⋅

⎜⎟

−⋅+⋅

⎝⎠

=⋅

+⋅+ ⋅

ωω

(38)

We can separate

()G

into two factor functions

of the form (1) and we can apply the same design

procedure as before in order to obtain either a wedge

filter oriented along one of the axis

or along

an axis tilted with a given angle

about one of the

axis. Using the frequency mapping

(, )

→

given by (25), we finally obtain a transfer function

z ,

z . This filter has a Gaussian cross-section

with every vertical plane perpendicular to its

longitudinal axis. Since the numerator and

denominator are factorized, the filter templates result

as a convolution of smaller size matrices. The

Gaussian wedge filter with the same parameters

0.2

and 0.2

is shown in Fig.5. In Fig.3 a

flat-top and a Gaussian wedge filter with

0.15

and 0

are displayed.

4 CONCLUSIONS

We proposed a design method for 2D IIR zero-phase

wedge filters, oriented along a specified direction.

They are based on a 1D low-pass prototype with an

imposed frequency response, for instance flat-top

and Gaussian. A 1D to 2D frequency mapping

function is derived which is applied to the 1D

prototype to obtain the 2D filter.

The distortions introduced by bilinear transform are

compensated through a pre-warping along both axes

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−

(a)

−

(b)

Figure 2: 1D IIR prototype filters: (a) maximally flat; (b)

Gaussian-shaped.

(a) (b)

Figure 3: (a) Flat-top wedge filter and (b) Gaussian wedge

filter with

0.15

= and

(a) (b)

Figure 4: Oriented flat-top wedge filter with

0.2

and

0.2

= : (a) frequency response; (b) contour plot.

(a) (b)

Figure 5: Oriented Gaussian wedge filter with

0.2

and

0.2

= : (a) frequency response; (b) contour plot.

. The efficient Chebyshev-Padé rational

approximation is also used. The proposed design

method is direct and does not involve any numerical

optimization techniques. Further research on the

topic may combine the analytical and numerical

methods to design more efficient filters and also to

obtain an efficient implementation of this class of

filters.

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DESIGN METHOD FOR WEDGE-SHAPED FILTERS