Graph Spectral Image Processing. Gene Cheung

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I.6. References

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      1 1 If a graph node represents a pixel in an image, each pixel would typically have three color components: red, green and blue. For simplicity, one can treat each color component separately as a different graph signal.

      2 2 One can prove that a graph G with positive edge weights has PSD graph Laplacian L via the Gershgorin circle theorem: each Gershgorin disc corresponding to a row in L is located in the non-negative half-space, and since all eigenvalues reside inside the union of all discs, they are non-negative.

PART 1 Fundamentals of Graph Signal Processing

      1

      Graph Spectral Filtering

       Yuichi TANAKA

       Tokyo University of Agriculture and Technology, Japan

1.1. Introduction

      The filtering of time- and spatial-domain signals is one of the fundamental techniques for image processing and has been studied extensively to date. GSP can treat signals with irregular structures that are mathematically represented as graphs. Theories and methodologies for the filtering of graph signals are studied using spectral graph theory. In image processing, graphs are strong tools for representing structures formed by pixels, like edges and textures.

      The filtering of graph signals is not only an extension of that for standard time- and spatial-domain signals, but it also has its own interesting properties. For example, GSP can represent traditional pixel-dependent image filtering methods as graph spectral domain filters. Furthermore, theory and design methods for wavelets and filter banks, which are studied extensively in signal and image processing, are also updated to treat graph signals.

      In this chapter, the spectral-domain filtering of graph signals is introduced. In section 1.2, the filtering of time-domain signals is briefly described as a starting point. The filtering of graph signals, both in the vertex and spectral domains, is detailed in section 1.3, in addition to its relationship with classical filtering. Edge-preserving image smoothing is represented as a graph filter in section 1.4. Furthermore, a framework of filtering by multiple graph filters, i.e. graph wavelets and filter banks, is presented in section 1.5. Eventually, section 1.6 introduces several fast computation methods of graph filtering. Finally, the concluding remarks of this chapter are discussed in section 1.7.

1.2. Review: filtering of time-domain signals

      We start by reviewing the filtering in discrete-time linear time-invariant (LTI) systems, which has been extensively studied in literature. Suppose that a one-dimensional discrete-time signal x_n is obtained by sampling its continuous-time counterpart x(t), with a fixed sampling period T, i.e. x_n = x(nT). A two-dimensional image signal can be similarly obtained by performing sampling in both the horizontal and vertical directions. In this case, the spatial sampling period usually corresponds to the spacing between an array of photosensors.

      Suppose that an impulse response of a filter h_n is given a priori. The discrete-time filtered signal y_n in the LTI system is calculated from x_n and h_n by convolution as follows:

[1.1]

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