This paper studies the construction of hexagonal tight wavelet frame filter banks which contain three “idealized” high-pass
filters. These three high-pass filters are suitable spatial shifts and frequency modulations of the associated low-pass filter,
and they are used by Simoncelli and Adelson in (Proc IEEE 78:652–664,
1990) for the design of hexagonal filter banks and by Riemenschneider and Shen in (Approximation Theory and Functional Analysis,
pp. 133–149, Academic Press, Boston
1991; J. Approx Theory 71:18–38
1992) for the construction of 2-dimensional orthogonal filter banks. For an idealized low-pass filter, these three associated
high-pass filters separate high frequency components of a hexagonal image in 3 different directions in the frequency domain.
In this paper we show that an idealized tight frame, a frame generated by a tight frame filter bank containing the “idealized”
high-pass filters, has at least 7 frame generators. We provide an approach to construct such tight frames based on the method
by Lai and Stöckler in (Appl Comput Harmon Anal 21:324–348,
2006) to decompose non-negative trigonometric polynomials as the summations of the absolute squares of other trigonometric polynomials.
In particular, we show that if the non-negative trigonometric polynomial associated with the low-pass filter
p can be written as the summation of the absolute squares of other 3 or less than 3 trigonometric polynomials, then the idealized
tight frame associated with
p requires exact 7 frame generators. We also discuss the symmetry of frame filters. In addition, we present in this paper several
examples, including that with the scaling functions to be the Courant element
B
111 and the box-spline
B
222. The tight frames constructed in this paper will have potential applications to hexagonal image processing.
Keywords Hexagonal framelets - Hexagonal tight frame filter bank - Idealized high-pass filter - Hexagonal image processing
Mathematics Subject Classifications (2000) 42C40 - 65T60
Communicated by Lixin Shen and Yuesheng Xu.
Research supported by UM Research Board 10/05 and UMSL Research Award 10/06.