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Posts Tagged ‘wavelets’

Which one is the fake?

December 7, 2012 5 comments
“Crab on its back” “Willows at sunset” “Still life: Potatoes in a yellow dish”
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So you want to be an Applied Mathematician

September 16, 2011 10 comments

The way of the Applied Mathematician is one full of challenging and interesting problems. We thrive by association with the Pure Mathematician, and at the same time with the no-nonsense, hands-in, hard-core Engineer. But not everything is happy in Applied Mathematician land: every now and then, we receive the disregard of other professionals that mistake either our background, or our efficiency at attacking real-life problems.

I heard from a colleague (an Algebrist) complains that Applied Mathematicians did nothing but code solutions of partial differential equations in Fortran—his skewed view came up after a naïve observation of a few graduate students working on a project. The truth could not be further from this claim: we do indeed occasionally solve PDEs in Fortran—I give you that—and we are not ashamed to admit it. But before that job has to be addressed, we have gone through a great deal of thinking on how to better code this simple problem. And you would not believe the huge amount of deep Mathematics that are involved in this journey: everything from high-level Linear Algebra, Calculus of Variations, Harmonic Analysis, Differential Geometry, Microlocal Analysis, Functional Analysis, Dynamical Systems, the Theory of Distributions, etc. Not only are we familiar with the basic background on all those fields, but also we are supposed to be able to perform serious research on any of them at a given time.

My soon-to-be-converted Algebrist friend challenged me—not without a hint of smugness in his voice—to illustrate what was my last project at that time. This was one revolving around the idea of frames (think of it as redundant bases if you please), and needed proving a couple of inequalities involving sequences of functions in L_p—spaces, which we attacked using a beautiful technique: Bellman functions. About ninety minutes later he conceded defeat in front of the board where the math was displayed. He promptly admitted that this was no Fortran code, and showed a newfound respect and reverence for the trade.

It doesn’t hurt either that the kind of problems that we attack are more likely to attract funding. And collaboration. And to be noticed in the press.

Alright, so some of you are sold already. What is the next step? I am assuming that at his point you own your Calculus, Analysis, Probability and Statistics, Linear Programming, Topology, Geometry, Physics and you are able to solve most known ODEs. From here, as with any other field, my recommendation is to slowly build a Batman belt: acquire and devour a sequence of books and scientific articles, until you are very familiar with their contents. When facing a new problem, you should be able to recall from your Batman belt what technique could work best, in which book(s) you could get some references, and how it has been used in the past for related problems.

Following these lines, I have included below an interesting collection with the absolutely essential books that, in my opinion, every Applied Mathematician should start studying:

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Wavelets in sage

January 23, 2011 2 comments

There are no native wavelet packages in sage. But there is a great module in python that contains, among other things, forward and inverse discrete wavelet transforms (for one and two dimensions). It comes bundled with seventy-six wavelet filters, and allows support to build your own! The name is PyWavelets, written by Tariq Rashid, and can be retrieved from pypi.python.org/pypi/PyWavelets. In order to install it in sage, take the following steps:

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Presentation: Hilbert Transform Pairs of Wavelets

September 19, 2007 Leave a comment

Now in the stage of the Approximation Theory Seminar, I presented a general overview of the work of Selesnick and others towards the design of pairs of wavelet bases with the “Hilbert Transform Pair property”. Click on the image below to retrieve a pdf file with the slides.

Presentation: The Dual-Tree Complex Wavelet Transform

September 6, 2007 Leave a comment

In the first IMI seminar, I presented an introduction to the survey paper “The Dual-Tree Complex Wavelet Transform“, by Selesnick, Baraniuk and Kingsbury. It was meant to be a (very) basic overview of the usual techniques of signal processing with an emphasis on wavelet coding, an exposition on the shortcomings of real-valued wavelets that affect the work we do at the IMI, and the solutions proposed by the three previous authors. In a subsequent talk, I will give a more mathematical (and more detailed) account on filter design for the dual-tree \mathbb{C}WT. Click on the image below to retrieve a pdf version of the presentation.

Poster: Curvelets vs. Wavelets (Mathematical Models of Natural Images)

August 5, 2007 Leave a comment

Together with Professor Bradley J. Lucier, we presented a poster in the Workshop on Natural Images during the thematic year on Mathematical Imaging at the IMA. We experimented with wavelet and curvelet decompositions of 24 high quality photos from a CD that Kodak® distributed in the late 90s. All the experiment details and results can be read in the file Curvelets/talk.pdf.

The computations concerning curvelet coefficients were carried out in Matlab, with the Curvelab 2.0.1 toolbox developed by Candès, Demanet, Donoho and Ying. The computations concerning wavelet coefficients were performed by Professor Lucier’s own codes.

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Wavelet Coefficients

August 5, 2007 Leave a comment

To aid in my understanding of wavelets, during the first months I started studying this subject I wrote a couple of scripts to both compute wavelet coefficients of a given pgm gray-scale image (the decoding script), and recover an approximation to the original image from a subset of those coefficients (the coding script). I used OCaml, a multi-paradigm language: imperative, functional and object-oriented.

The decoding script uses the easiest wavelets possible: the Haar functions. As it was suggested in the article “Fast wavelet techniques for near-optimal image processing“, by R. DeVore and B.J. Lucier, rather than computing the actual raw wavelet coefficients, one computes instead a related integer value (a code). The coding script interprets those integer values and modifies them appropriately to obtain the actual coefficients. The storage of the integers is performed using Huffman trees, but I used a very simple one, not designed for speed or optimization in any way.

Following a paper by A.Chambolle, R.DeVore, N.Y.Lee and B.Lucier, “Non-linear wavelet image processing: Variational problems, compression and noise removal through wavelet shrinkage“, these scripts were used in two experiments later on: computation of the smoothness of an image, and removal of Gaussian white noise by the wavelet shrinkage method proposed by Donoho and Johnstone in the early 90’s.

Progressive reconstruction of a grey-scale image of size 1024 \times 1024 with the largest (in absolute value) 2^n coefficients, n = 1,2, \dotsc ,8.

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