Detection of Edges in Spectral Data II. Nonlinear Enhancement

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2000

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A. Gelb & E. Tadmor (2000). Detection of Edges in Spectral Data II. Nonlinear Enhancement. SIAM Journal on Numerical Analysis 38 (2000) 1389-1408.

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We discuss a general framework for recovering edges in piecewise smooth functions with finitely many jump discontinuities, where f := f(x+)โˆ’f(xโˆ’) โ‰  0. Our approach is based on two main aspectsโ€”localization using appropriate concentration kernels and separation of scales by nonlinear enhancement. To detect such edges, one employs concentration kernels, K_๐›†(ยท), depending on the small scale ๐›†. Itis shown that odd kernels, properly scaled, and admissible (in the sense of having small Wโˆ’1,โˆž- moments of order O(๐›†)) satisfy K_๐›† โˆ— f(x) = f + O(๐›†), thus recovering both the location and amplitudes of all edges. As an example we consider general concentration kernels of the form KฯƒN (t) = ๐จฯƒ(k/N) sin kt to detect edges from the first 1/๐›† = N spectral modes of piecewise smooth fโ€™s. Here we improve in generality and simplicity over our previous study in [A. Gelb and E. Tadmor, Appl. Comput. Harmon. Anal., 7 (1999), pp. 101โ€“135]. Both periodic and nonperiodic spectral projections are considered. We identify, in particular, a new family of exponential factors, ฯƒexp(ยท), with superior localization properties. The other aspect of our edge detection involves a nonlinear enhancement procedure which is based on separation of scales between the edges, where K_๐›† โˆ— f(x) โˆผ f โ‰  0, and the smooth regions where K_๐›† โˆ— f = O(๐›†) โˆผ 0. Numerical examples demonstrate that by coupling concentration kernels with nonlinear enhancement one arrives at effective edge detectors.

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