Publications

2009
A. Hormati, O. Roy, Y. M. Lu, and M. Vetterli, “Distributed sensing of signals under a sparse filtering model,” in Proc. 8th International Conference on Sampling Theory and Applications (SampTA), Marseille, France, 2009.Abstract

We consider the task of recovering correlated vectors at a central decoder based on fixed linear measurements ob- tained by distributed sensors. Two different scenarios are considered: In the case of universal reconstruction, we look for a sensing and recovery mechanism that works for all possible signals, whereas in the case of almost sure recon- struction, we allow to have a small set (with measure zero) of unrecoverable signals. We provide achievability bounds on the number of samples needed for both scenarios. The bounds show that only in the almost sure setup can we ef- fectively exploit the signal correlations to achieve effective gains in sampling efficiency. In addition, we propose an efficient and robust distributed sensing and reconstruction algorithm based on annihilating filters.

sparse_filtering_sampta.pdf
Y. M. Lu, C. Fredembach, M. Vetterli, and S. Süsstrunk, “Designing color filter arrays for the joint capture of visible and near-infrared images,” in Proc. IEEE International Conference on Image Processing, Cairo, Egypt, 2009.Abstract

Digital camera sensors are inherently sensitive to the near- infrared (NIR) part of the light spectrum. In this paper, we propose a general design for color filter arrays that allow the joint capture of visible/NIR images using a single sensor. We pose the CFA design as a novel spatial domain optimization problem, and provide an efficient iterative procedure that finds (locally) optimal solutions. Numerical experiments confirm the effectiveness of the proposed CFA design, which can simultane- ously capture high quality visible and NIR image pairs.

cfa_visnir.pdf
C. Carneiro, M. Karzand, F. Golay, Y. M. Lu, and M. Vetterli, “Assessment of digital surface models for the study of shadowing and radiation over the built environment using wireless sensor network data,” in Proc. 6th International Symposium on Spatial Data Quality, Newfoundland, 2009.
Y. M. Lu and M. Vetterli, “Optimal color filter array design: Quantitative conditions and an efficient search procedure,” in Proc. SPIE Electronic Imaging, Digital Photography V, 2009.Abstract

Most digital cameras employ a spatial subsampling process, implemented as a color filter array (CFA), to capture color images. The choice of CFA patterns has a great impact on the performance of subsequent reconstruction (demosaicking) algorithms. In this work, we propose a quantitative theory for optimal CFA design. We view the CFA sampling process as an encoding (low-dimensional approximation) operation and, correspondingly, demosaicking as the best decoding (reconstruction) operation. Finding the optimal CFA is thus equivalent to finding the optimal approximation scheme for the original signals with minimum information loss. We present several quantitative conditions for optimal CFA design, and propose an efficient computational procedure to search for the best CFAs that satisfy these conditions. Numerical experiments show that the optimal CFA patterns designed from the proposed procedure can effectively retain the information of the original full-color images. In particular, with the designed CFA patterns, high quality demosaicking can be achieved by using simple and efficient linear filtering operations in the polyphase domain. The visual qualities of the reconstructed images are competitive to those obtained by the state-of-the-art adaptive demosaicking algorithms based on the Bayer pattern.

optimal_cfa_ei09.pdf
Y. M. Lu, M. N. Do, and R. S. Laugesen, “A Computable Fourier Condition Generating Alias-Free Sampling Lattices,” IEEE Transactions on Signal Processing, vol. 57, no. 5, pp. 1768–1782, 2009.Abstract

We propose a Fourier analytical condition linking alias-free sampling with the Fourier transform of the indicator function defined on the given frequency support. Our discussions center around how to develop practical computation algorithms based on the proposed analytical condition. We address several issues along this line, including the derivation of simple closed-form expressions for the Fourier transforms of the indicator functions defined on arbitrary polygonal and polyhedral domains; a complete and nonredundant enumeration of all quantized sampling lattices via the Hermite normal forms of integer matrices; and a quantitative analysis of the approximation of the original infinite Fourier condition by using finite computations. Combining these results, we propose a computational testing procedure that can efficiently search for the optimal alias-free sampling lattices for a given polygonal or polyhedral shaped frequency domain. Several examples are presented to show the potential of the proposed algorithm in multidimensional filter bank design, as well as in applications involving the design of efficient sampling patterns for multidimensional band-limited signals.

sampling_lattices.pdf
Y. M. Lu and M. Vetterli, “Distributed spatio-temporal sampling of diffusion fields from sparse instantaneous sources,” in Proc. 3rd International Workshop on Computational Advances in Multi-Sensor Adaptive Processing (CAMSAP), Aruba, 2009.Abstract

We study the spatio-temporal sampling of a diffusion field driven by K unknown instantaneous source distributions. Exploiting the spatio-temporal correlation offered by the diffusion model, we show that it is possible to compensate for insufficient spatial sampling densities (i.e. sub-Nyquist sampling) by increasing the temporal sampling rate, as long as their product remains roughly a constant. Combining a distributed sparse sampling scheme and an adaptive feedback mechanism, the proposed sampling algorithm can accurately and efficiently estimate the unknown sources and reconstruct the field. The total number of samples to be transmitted through the network is roughly equal to the number of degrees of freedom of the field, plus some additional costs for in-network averaging.

sparse_diffusion.pdf
Y. M. Lu and M. Vetterli, “Spatial super-resolution of a diffusion field by temporal oversampling in sensor networks,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing, Taiwan, 2009.Abstract

We study the spatial-temporal sampling of a linear diffusion field, and show that it is possible to compensate for insufficient spatial sampling densities by oversampling in time. Our work is motivated by the following issue often encountered in sensor network sampling, namely increasing the temporal sampling density is often easier and less expensive than increasing the spatial sampling density of the network. For the case of sampling a diffusion field, we show that, to achieve trade-off between spatial and temporal sampling, the spatial arrangement of the sensors must satisfy certain conditions. We provide in this paper the precise relationships between the achievable reduction of spatial sampling density, the required temporal oversampling rate, the spatial arrangement of the sensors, and the bound for the condition numbers of the resulting sampling and reconstruction procedures.

diffusion_sampling_icassp09.pdf
O. Roy, A. Hormati, Y. M. Lu, and M. Vetterli, “Distributed sensing of signals linked by sparse filtering,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing, Taipei, 2009.Abstract

We consider the task of recovering correlated vectors at a central decoder based on fixed linear measurements obtained by distributed sensors. A general formulation of the problem is proposed, under both a universal and an almost sure reconstruction requirement. We then study a specific correlation model which involves a filter that is sparse in the time domain. While this sparsity assumption does not allow reducing the description cost in the universal case, we show that large gains can be achieved in the almost sure scenario by means of a novel distributed scheme based on annihilating filters. The robustness of the proposed method is also investigated.

sparse_filtering_icassp09.pdf
Y. M. Lu, M. Karzand, and M. Vetterli, “Iterative demosaicking accelerated: Theory and fast noniterative implementations,” in Proc. SPIE Conf. Computational Imaging VI, San Jose, USA, 2009.Abstract

Color image demosaicking is a key process in the digital imaging pipeline. In this paper, we present a rigorous treatment of a classical demosaicking algorithm based on alternating projections (AP). Since its publication, the AP algorithm has been widely cited and served as a benchmark in a flurry of papers in the demosaicking literature. Despite its impressive performances, a relative weakness of the AP algorithm is its high computational complexity. In our work, we provide a rigorous analysis of the convergence of the AP algorithm based on the concept of contraction mapping. Furthermore, we propose an efficient noniterative implementation of the AP algorithm in the polyphase domain. Numerical experiments show that the proposed noniterative implementation achieves the same results obtained by the original AP algorithm at convergence, but is about an order of magnitude faster than the latter.

iterative_demosaicking_ei09.pdf
2008
G. Barrenetxea, F. Ingelrest, Y. M. Lu, and M. Vetterli, “Assessing the challenges of environmental signal processing through the SensorScope project,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing, Las Vegas, USA, 2008, pp. 5149–5152.Abstract

SensorScope is a collaborative project between network, signal processing, and environmental researchers that aims at providing a cheap and out-of-the-box environmental monitoring system based on a wireless sensor network. It has been successfully used in a number of deployments to gather hundreds of megabytes of environmental data. With data gathering techniques well mastered, the efficient processing of the huge amounts of the acquired information to allow for useful exploitation has become an increasingly important issue. In this paper, we present a number of challenging and relevant signal processing tasks that arise from the SensorScope project. We believe the resolution of these problems will benefit from a better understanding of the underlying physical processes. We show an example to demonstrate how physical correlations between different sensing modalities can help reduce the sampling rate.

environmental_sp_icassp.pdf
Y. M. Lu and M. N. Do, “A Mapping-Based Design for Nonsubsampled Hourglass Filter Banks in Arbitrary Dimensions,” IEEE Transactions on Signal Processing, vol. 56, no. 4, pp. 1466-1478, 2008.Abstract

Multidimensional hourglass filter banks decompose the frequency spectrum of input signals into hourglass-shaped directional subbands, each aligned with one of the frequency axes. The directionality of the spectral partitioning makes these filter banks useful in separating the directional information in multi-dimensional signals. Despite the existence of various design techniques proposed for the 2-D case, to our best knowledge, the design of hourglass filter banks in 3-D and higher dimensions with finite impulse response (FIR) filters and perfect reconstruction has not been previously reported. In this paper, we propose a novel mapping-based design for the hourglass filter banks in arbitrary dimensions, featuring perfect reconstruction, FIR filters, efficient implementation using lifting/ladder structures, and a near-tight frame construction. The effectiveness of the proposed mapping-based design depends on the study of a set of conditions on the frequency supports of the mapping kernels. These conditions ensure that we can still get good frequency responses when the component filters used are nonideal. Among all feasible choices, we then propose an optimal specification for the mapping kernels, which leads to the simplest passband shapes and involves the fewest number of frequency variables. Finally, we illustrate the proposed techniques by a design example in 3-D, and an application in video denoising.

hourglassfb.pdf
Y. M. Lu and M. N. Do, “A Theory for Sampling Signals from a Union of Subspaces,” IEEE Transactions on Signal Processing, vol. 56, no. 6, pp. 2334–2345, 2008.Abstract

One of the fundamental assumptions in traditional sampling theorems is that the signals to be sampled come from a single vector space (e.g., bandlimited functions). However, in many cases of practical interest the sampled signals actually live in a union of subspaces. Examples include piecewise polynomials, sparse representations, nonuniform splines, signals with unknown spectral support, overlapping echoes with unknown delay and amplitude, and so on. For these signals, traditional sampling schemes based on the single subspace assumption can be either inapplicable or highly inefficient. In this paper, we study a general sampling framework where sampled signals come from a known union of subspaces and the sampling operator is linear. Geometrically, the sampling operator can be viewed as projecting sampled signals into a lower dimensional space, while still preserving all the information. We derive necessary and sufficient conditions for invertible and stable sampling operators in this framework and show that these conditions are applicable in many cases. Furthermore, we find the minimum sampling requirements for several classes of signals, which indicates the power of the framework. The results in this paper can serve as a guideline for designing new algorithms for various applications in signal processing and inverse problems.

sampunis.pdf
Y. M. Lu and M. N. Do, “Sampling Signals from a Union of Subspaces,” IEEE Signal Process. Mag., Special Issue on Compressive Sampling, vol. 25, 2008. sampling_union_spaces.pdf
2007
Y. M. Lu and M. N. Do, “Sampling signals from a union of shift-invariance subspaces,” in Proc. SPIE Conf. on Wavelets Applications in Signal and Image Processing XII, San Diego, CA, 2007.
Y. M. Lu and M. N. Do, “Multidimensional Directional Filter Banks and Surfacelets,” IEEE Transactions on Image Processing, vol. 16, no. 4, pp. 918-931, 2007. MATLAB and C++ codeAbstract

In 1992, Bamberger and Smith proposed the directional filter bank (DFB) for an efficient directional decomposition of 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. We propose a new family of filter banks, named NDFB, that can achieve the directional decomposition of arbitrary  N-dimensional (N >= 2) signals with a simple and efficient tree-structured construction. In 3-D, the ideal passbands of the proposed NDFB are rectangular-based pyramids radiating out from the origin at different orientations and tiling the entire frequency space. The proposed NDFB achieves perfect reconstruction via an iterated filter bank with a redundancy factor of in N-D. The angular resolution of the proposed NDFB can be iteratively refined by invoking more levels of decomposition through a simple expansion rule. By combining the NDFB with a new multiscale pyramid, we propose the surfacelet transform, which can be used to efficiently capture and represent surface-like singularities in multidimensional data.

ndfb_surf.pdf
M. Yan, et al., “Automatic detection of pelvic lymph nodes using multiple MR sequences,” in Proc. SPIE Conference on Medical Imaging, San Diego, 2007.
Y. M. Lu and M. N. Do, “Finding optimal integral sampling lattices for a given frequency support in multidimensions,” in Proc. IEEE International Conference on Image Processing, San Antonio, USA, 2007.Abstract

The search for alias-free sampling lattices for a given frequency support, in particular those lattices achieving minimum sam- pling densities, is a fundamental issue in various applications of signal and image processing. In this paper, we propose an efficient computational procedure to find all alias-free integral sampling lattices for a given frequency support with minimum sampling density. Central to this algorithm is a novel condition linking the alias-free sampling with the Fourier transform of the indicator function defined on the frequency support. We study the computation of these Fourier transforms based on the diver- gence theorem, and propose a simple closed-form formula for a fairly general class of support regions consisting of arbitrary N -dimensional polytopes, with polygons in 2-D and polyhedra in 3-D as special cases. The proposed algorithm can be useful in a variety of applications involving the design of efficient ac- quisition schemes for multidimensional bandlimited signals.

sampling_lattices_icip07.pdf

(This paper received one of the four available Student Paper Awards of ICIP.)

N. Mueller, Y. Lu, and M. N. Do, “Image interpolation using multiscale geometric representations,” in Proc. SPIE Conf. on Electronic Imaging, San Jose, USA, 2007.Abstract

With the ever increasing computational power of modern day processors, it has become feasible to use more robust and computationally complex algorithms that increase the resolution of images without distorting edges and contours. We present a novel image interpolation algorithm that uses the new contourlet transform to improve the regularity of object boundaries in the generated images. By using a simple wavelet-based linear interpolation scheme as our initial estimate, we use an iterative projection process based on two constraints to drive our solution towards an improved high-resolution image. Our experimental results show that our new algorithm significantly outperforms linear interpolation in subjective quality, and in most cases, in terms of PSNR as well.

interp_contourlet.pdf
2006
Y. Lu and M. N. Do, “Video processing using the 3-dimensional surfacelet transform,” in Fortieth Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, CA, 2006.Abstract

Motion estimation is a common ingredient in many state-of- the-art video processing algorithms, serving as an effective way to capture the spatial-temporal correlation in video signals. However, the robustness of motion estimation often suffers from problems such as ambiguities of motion trajectory (i.e. the aperture problem) and illumination variances. In this paper, we explore a new framework for video processing based on the recently proposed surfacelet transform. Instead of containing an explicit motion estimation step, the surfacelet transform provides a motion-selective subband decomposition for video signals. We demonstrate the potential of this new technique in a video denoising application.

surf_video_proc.pdf
Y. Lu and M. N. Do, “Multidimensional nonsubsampled hourglass filter banks: Geometry of passband support and filter design,” in Fortieth Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, CA, 2006, pp. 406-410.Abstract

Recently, the classical two-dimensional directional filter banks have been extended to higher dimensions. In this paper, we study one of the key components in this new construction, namely the multidimensional nonsubsampled hourglass filter banks. Starting with a rigorous analysis on the geometry of multidimensional hourglass-shaped passband supports, we propose a novel design for these filter banks in arbitrary dimensions, featuring perfect reconstruction and finite impulse response (FIR) filters. We analyze necessary and sufficient conditions for the resulting filters to achieve good frequency responses, and provide an optimal solution that satisfies these conditions using simplest filters. The proposed filter design technique is verified by a design example in 3-D.

hourglassfb_asilomar.pdf

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