Y. Lu and M. N. Do, “A new contourlet transform with sharp frequency localization,” in Proc. IEEE International Conference on Image Processing, Atlanta, USA, 2006, pp. 1629-1632.Abstract

The contourlet transform was proposed as a directional multiresolution image representation that can efficiently capture and represent singularities along smooth object boundaries in natural images. Its efficient filter bank construction as well as low redundancy make it an attractive computational framework for various image processing applications. However, a major drawback of the original contourlet construction is that its basis images are not localized in the frequency domain. In this paper, we analyze the cause of this problem, and propose a new contourlet construction as a solution. Instead of using the Laplacian pyramid, we employ a new multiscale decomposition defined in the frequency domain. The resulting basis images are sharply localized in the frequency domain and exhibit smoothness along their main ridges in the spatial domain. Numerical experiments on image denoising show that the proposed new contourlet transform can significantly outperform the original transform both in terms of PSNR (by several dB’s) and in visual quality, while with similar computational complexity.


(This paper received one of the two available Most Innovative Paper Awards of ICIP.)


Y. Lu and M. N. Do, “3-D directional filter banks and surfacelets,” in Proc. of SPIE Conf. on Wavelet Applications in Signal and Image Processing XI, San Diego, 2005, pp. 591-601.Abstract

In 1992, Bamberger and Smith proposed the directional filter bank (DFB) for an efficient directional decompo- sition of two-dimensional (2-D) signals. Due to the nonseparable nature of the system, extending the DFB to higher dimensions while still retaining its attractive features is a challenging and previously unsolved problem. This paper proposes a new family of filter banks, named 3DDFB, that can achieve the directional decomposition of 3-D signals with a simple and efficient tree-structured construction. The ideal passbands of the proposed 3DDFB are rectangular-based pyramids radiating out from the origin at different orientations and tiling the whole frequency space. The proposed 3DDFB achieves perfect reconstruction. Moreover, the angular resolution of the proposed 3DDFB can be iteratively refined by invoking more levels of decomposition through a simple expansion rule. We also introduce a 3-D directional multiresolution decomposition, named the surfacelet transform, by combining the proposed 3DDFB with the Laplacian pyramid. The 3DDFB has a redundancy factor of 3 and the surfacelet transform has a redundancy factor up to 24/7.

Y. Lu and M. N. Do, “The finer directional wavelet transform,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing, Philadelphia, 2005.Abstract

Directional information is an important and unique feature of multidimensional signals. As a result of a separable ex- tension from one-dimensional (1-D) bases, the multidimen- sional wavelet transform has very limited directionality. Furthermore, different directions are mixed in certain wavelet subbands. In this paper, we propose a new transform that fixes this frequency mixing problem by using a simple “add- on” to the wavelet transform. In the 2-D case, it provides one lowpass subband and six directional highpass subbands at each scale. Just like the wavelet transform, the proposed transform is nonredundant, and can be easily extended to higher dimensions. Though nonseparable in essence, the proposed transform has an efficient implementation based on 1-D operations only.

Y. Lu and M. N. Do, “A geometrical approach to sampling signals with finite rate of innovation,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing, Montreal, Canada, 2004.Abstract

Many signals of interest can be characterized by a finite number of parameters per unit of time. Instead of span- ning a single linear space, these signals often lie on a union of spaces. Under this setting, traditional sampling schemes are either inapplicable or very inefficient. We present a framework for sampling these signals based on an injec- tive projection operator, which "flattens" the signals down to a common low dimensional representation space while still preserves all the information. Standard sampling procedures can then be applied on that space. We show the necessary and sufficient conditions for such operators to exist and provide the minimum sampling rate for the representation space, which indicates the efficiency of this framework. These results provide a new perspective on the sampling of signals with finite rate of innovation and can serve as a guideline for designing new algorithms for a class of problems in signal processing and communications.

Y. Lu and M. N. Do, “CRISP-contourlets: A critically-sampled directional multiresultion image representation,” in Proc. of SPIE Conference on Wavelet Applications in Signal and Image Processing X, San Diego, USA, 2003, pp. 655-665.Abstract

Directional multiresolution image representations have lately attracted much attention. A number of new systems, such as the curvelet transform and the more recent contourlet transform, have been proposed. A common issue of these transforms is the redundancy in representation, an undesirable feature for certain applications (e.g. compression). Though some critically sampled transforms have also been proposed in the past, they can only provide limited directionality or limited flexibility in the frequency decomposition. In this paper, we propose a filter bank structure achieving a nonredundant multiresolution and multidirectional expansion of images. It can be seen as a critically sampled version of the original contourlet transform (hence the name CRISP-contourets) in the sense that the corresponding frequency decomposition is similar to that of contourlets, which divides the whole spectrum both angularly and radially. However, instead of performing the multiscale and directional decomposition steps separately as is done in contourlets, the key idea here is to use a combined iterated non- separable filter bank for both steps. Aside from critical sampling, the proposed transform possesses other useful properties including perfect reconstruction, flexible configuration of the number of directions at each scale, and an efficient tree-structured implementation.