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New tight frames of curvelets and optimal representations of objects with piecewise $$C ^{2}$$ singularities. (English) Zbl 1038.94502
Summary: This paper introduces new tight frames of curvelets to address the problem of finding optimally sparse representations of objects with discontinuities along piecewise $$C^2$$ edges. Conceptually, the curvelet transform is a multiscale pyramid with many directions and positions at each length scale, and needle-shaped elements at fine scales. These elements have many useful geometric multiscale features that set them apart from classical multiscale representations such as wavelets. For instance, curvelets obey a parabolic scaling relation which says that at scale $$2^{-j}$$, each element has an envelope that is aligned along a ridge of length $$2^{-j/2}$$ and width $$2^{-j}$$.
We prove that curvelets provide an essentially optimal representation of typical objects $$f$$ that are $$C^2$$ except for discontinuities along piecewise $$C^2$$ curves. Such representations are nearly as sparse as if $$f$$ were not singular and turn out to be far more sparse than the wavelet decomposition of the object. For instance, the $$n$$-term partial reconstruction $$f_n^C$$ obtained by selecting the $$n$$ largest terms in the curvelet series obeys $\| f-f_n^C \|_{L_2}^2 \leq C\cdot n^{-2} \cdot (\log n)^3,\quad n\to \infty.$ This rate of convergence holds uniformly over a class of functions that are $$C^2$$ except for discontinuities along piecewise $$C^2$$ curves and is essentially optimal. In comparison, the squared error of $$n$$-term wavelet approximations only converges as $$n^{-1}$$ as $$n\to \infty$$, which is considerably worse than the optimal behavior.

##### MSC:
 94A08 Image processing (compression, reconstruction, etc.) in information and communication theory 68P30 Coding and information theory (compaction, compression, models of communication, encoding schemes, etc.) (aspects in computer science) 68U10 Computing methodologies for image processing 92C55 Biomedical imaging and signal processing 42C40 Nontrigonometric harmonic analysis involving wavelets and other special systems 41A46 Approximation by arbitrary nonlinear expressions; widths and entropy
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