6.4.2 Differential Wavelet Transform
and Multiscale Pointwise Product
Since edge sharpening is an essential part of image enhancement and edges can
be detected and characterized by differential operators, a particular family
of differential wavelets has been used for this purpose [11, 22]. In this case the
approximation and detail coefficients of the differential wavelet transform of an
image f are defined as S
2
j
f and W
2
j
f , and the wavelet transform is computed
using the following equations:
S
2
j
f ¼ S
2
j1
f h
"2
j1
W
2
j
f ¼ S
2
j1
f g
"2
j1
,1# j # J (6:35)
where h and g are the low-pass and high-pass filters, respectively, and "2
j1
is the
up-sampling operation by putting 2
j1
1 zeros between each pair of adjacent
samples in the filter [11]. This differential ...