wavelets - nt.tuwien.ac.at
TRANSCRIPT
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Wavelets
Univ.Prof. Dr.-Ing. Markus RuppLVA 389.141 Fachvertiefung Telekommunikation
(LVA: 389.137 Image and Video Compression)
Last change: January 20, 2020
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Outline
• Remember Dyadic Filter Banks
• An approach to Wavelets via Filter Banks
• Orthonormality Properties of Wavelets
2Univ.-Prof. Dr.-Ing. Markus Rupp
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3Univ.-Prof. Dr.-Ing. Markus
Rupp
Remember Dyadic Filter Bank
• Thus:
• We find:
)()()()()(
)()()()()(
)()()()(
)()()(
42
3
42
2
2
1
0
zXzHzHzHzY
zXzHzHzHzY
zXzHzHzY
zXzHzY
HHH
LHH
LH
L
=
=
=
=
HH(z)
HL(z)
HH(z2)
HL(z2)
HH(z4)
HL(z4)
Y0(n) Y1(n) Y2(n)
Y3(n)
42 8
8x(n)
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Dyadic Filter Bank
• In principle we work only with one filter HH(z)=1-HL(z)
• and then we repeat the filter by stretchedversions: HH(z2), HH(z4), HH(z8),…
4Univ.-Prof. Dr.-Ing. Markus
Rupp
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Example 1: Haar
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Example II: Le Gall
Univ.-Prof. Dr.-Ing. Markus Rupp 6
4321
01
12
111
8
1
4
1
4
3
4
1
8
1
)()()(
−−−−
−
++−+=
+=
zzzz
zGzzGzG
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Univ.-Prof. Dr.-Ing. Markus Rupp 7
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Linear Phase
• Note that both Haar and LeGall Wavelets have symmetric (antisymmetric ) impulse responses h(t).
• This property is equivalent to having linear phase!
• Consider Fourier Transform H(ejW):
• If group delay is constant→linear phase.
Univ.-Prof. Dr.-Ing. Markus Rupp 8
g
jjj eeHeH
−=WW
= WWW
)(
)()( )(
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Wavelets
• In all examples the various filter responses are all derived from a single one, that is the coefficients are identical (just stretching or compressing)
• If this filter has few multiplications (just adds/subs), all derived filters will have!
• →cool method to reduce complexity as search room is relatively moderate.
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10 Univ.-Prof. Dr.-Ing. Markus Rupp
Wavelets
• Consider the function:
• We thus have a two-dimensional transformation with modifications in position/location and scale
(Ger: Streckung/Granularität).
( )kttp jj
kj −= −− 22)( 2/
,
ShiftStretch
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Wavelets
• For wavelets the function (t) is chosen so that
Univ.-Prof. Dr.-Ing. Markus Rupp 11
( )
ljklkj
mkmjkj
jj
kj
dttptp
dttptp
kttp
−
−
−−
=
=
−=
)()(
)()(
22)(
,,
,,
2/
,
Orthonormality in two directions
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12 Univ.-Prof. Dr.-Ing. Markus Rupp
Wavelets
• Note that, if (t) is normalized (||(t)||=1), then we also have ||pjk(t)||=1.
• We select the function (t) in such way that they build for all shifts n an orthonormal basis for a space:
The shifted functions thus build an orthonormal basis for V0.
ZnntV −= ),(span0
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13 Univ.-Prof. Dr.-Ing. Markus Rupp
Wavelets
• Example: Consider the unit pulse
• With this basis function all functions f0(t), that are constant for an integer mesh can be described exactly. Continuous functions can be approximated with the precision of integer distance.
• We write:
• integer mesh (Ger: im Raster ganzzahliger Zahlen)
ZnntV
tutut
−=
−−=
),(
)1()()(
0
( ) )()()()()()( 0,0,00 tetftpdttptftfn
nn −==
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14 Univ.-Prof. Dr.-Ing. Markus Rupp
Wavelets
• The so obtained coefficients
• Can also be interpreted as piecewise integrated areas over the function f(t).
dttptfc nn )()( ,0
)0(
=
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15 Univ.-Prof. Dr.-Ing. Markus Rupp
Wavelets
• Stretching can also be used to define new bases for other spaces.
• If these spaces are nested (Ger:Verschachtelung):
course scale fine scale→
• then we call (t) a scaling function (Ger: Skalierungsfunktion) for a Wavelet.
ZnntV
ZnntV
jj
j −=
−=
−−
−
),2(2span
),2(2span
2/
1
...... 1012 − VVVV
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16Univ.-Prof. Dr.-Ing. Markus
Rupp
Wavelets
• Example: Consider the unit impulse
)2(2)(
),2(2
)12()2()2(
,1
1
nttp
ZnntV
tutut
n −=
−=
−−=
−
−
)12()2()( −ttt
1 ½ 1
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17 Univ.-Prof. Dr.-Ing. Markus Rupp
Wavelets
• Example: Consider the unit impulse
• With this function we can resemble all functions f(t), that are constant in a half-integer (n/2) mesh. All continuous functions can be approximated by a half-integer mesh.
)2(2;),2(2
)12()2()2(
,11 ntpZnntV
tutut
n −=−=
−−=
−−
( ) )()()()()()( 1,1,11 tetftpdttptftfn
nn −−−− −==
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18 Univ.-Prof. Dr.-Ing. Markus Rupp
Wavelets
• The function f-1(t) thus is an even finer approximation of f0(t) in V0. Since V0 is a subset of V-1we have:
• With a suitable basis y0,n(t) from W0 withW0 U V0 =V-1
+=
+=
=
−
−
−
−
n
nn
n
nn
n
nn
n
nn
tdtpc
tetpc
tpctf
)()(
)()(
)()(
,0
)0(
,0
)0(
1,0,0
)0(
,1
)1(
1
y
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19Univ.-Prof. Dr.-Ing. Markus
Rupp
Wavelets
• In other words, the set Wj complements the set Vj in such a way that:
Wj U Vj =Vj-1
With Vj-1 the next finer approximation can be built.
• Hereby, Wj is the orthogonal complement of Vj:
1in −
⊥= jjj VVW
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20 Univ.-Prof. Dr.-Ing. Markus Rupp
Wavelets
• These functions yj,n(t) are called Wavelets.
• Thus, we can decompose any function at an arbitrary scaling step into two components yj,n and {pj,n}.
• Very roughly, one can be considered a high pass, the other a low pass.
• By finer scaling the function can be approximated better and better.
• The required number of coefficients is strongly dependent on the Wavelet- or the corresponding scaling function.
• Note that for coding as well as for transmission only the coefficients are required. The fewer the better…
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21Univ.-Prof. Dr.-Ing. Markus
Rupp
Wavelets
• Wavelets have also the scaling property:
• As well as orthonormal properties:
• causing that they are energy preserving!
nkljnlkj
jj
nj
dttt
ntgt
−−
−−
=
−=
yy
y
)(),(
)2(2)(
,,
2/
,
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22Univ.-Prof. Dr.-Ing. Markus
Rupp
Wavelets
• Example: Haar Wavelets
)()(2
1)(
)()(2
1)(
12,2,,1
12,2,,1
tptpt
tptptp
nmnmnm
nmnmnm
++
++
−=
+=
y
)12()2()()2(2)()()( 0,00,10,0 −−=== − tttttpttp y
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23 Univ.-Prof. Dr.-Ing. Markus Rupp
Example
f-1(t)
f0(t)
e0,-1(t) )(
)(
0,0
0,0
t
tp
y
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Modern Wavelets
• In the 80ies wavelets were just viewed as an alternative description of subband filtering.
• Ingrid Daubechies introduced new families of wavelets, some of them not having the orthogonality property but a so-called bi-orthogonal property.
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25 Univ.-Prof. Dr.-Ing. Markus Rupp
Vector Spaces
• Definition : If there are two bases,
that span the same space with the additional property:
then these bases are said to be dual or biorthogonal(biorthonormal for ki,j=1).
},...,,{};,...,,{2121 mm
qqqUpppT ==
jijijikqp −= ,,
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26 Univ.-Prof. Dr.-Ing. Markus Rupp
Vector Spaces
• Example: let:
These pairs build a dual basis in R2. Consider
then:
;1,0;1,1;1,1;0,12121
==−==TTTTqqpp
],[ bafT
=
212211
212211
)(,,
)(,,
qbaqaqpfqpff
pbpbapqfpqff
+−+=+=
++=+=
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Modern Wavelets
• Daubechies and LeGall wavelets share this biorthogonal property which makes them of linear phase.
• Unfortunately, they lose the orthogonality and thus the energy preserving property (not unitary).
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Specific Basis Functions
• Find the right basis for your problem:
• Wavelet or DCT
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Curvelet Basis
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Ridgelet Basis
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Summary
• Orthogonal wavelets can classically be interpreted as specific solutions of dyadic filter trees
• They offer further advantages in complexity when selecting the proper wavelet families.
• Biorthogonal Wavelets are the method of choice for the JPEG 2000 standard.
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