Wavelets ?

Wavelets ? Raghu Machiraju Contributions: Robert Moorhead, James Fowler, David Thompson, Mississippi State University Ioannis Kakadaris, U of Houston

Simulations, scanners State-Of-Affairs • Concurrent • Presentation • Retrospective • Analysis • Representation

Why Wavelets? • We are generating and measuring larger datasets every year • We can not store all the data we create (too much, too fast) • We can not look at all the data (too busy, too hard) • We need to develop techniques to store the data in better formats

Data Analysis • Frequency spectrum correctly shows a spike at 10 Hz • Spike not narrow - significant component at between 5 and 15 Hz. • Leakage - discrete data acquisition does not stop at exactly the same phase in the sine wave as it started.

QuickFix

Windowing &Filtering

Image Example • 8x8 Blocked Window (Cosine) Transform • Each DCT basis waveform represents a fixed frequency in two orthogonal directions • frequency spacing in each direction is an integer multiple of a base frequency

Windowing & Filtering Windows – fixed in space and frequencies Cannot resolve all features at all instants

Linear Scale Space input s= 1 s= 16 s= 24 s= 32

Successive Smoothing

Sub-sampled Images • Keep 1 of 4 values from 2x2 blocks • This naive approach and introduces aliasing • Sub-samples are bad representatives of area • Little spatial correlation

Image Pyramid

Image Pyramid – MIP MAP • Average over a 2x2 block • This is a rather straight forward approach • This reduces aliasing and is a better representation • However, this produces 11% expansion in the data

Image Pyramid – Another Twist

Time Frequency Diagram

Ideally ! Create new signal G such that ||F-G|| = e

Wavelet Analysis • A1 D1 D2 D3 • D3 • D2 • D1 • A1

Why Wavelets? Because … • We need to develop techniques to analyze data better through noise discrimination • Wavelets can be used to detect features and to compare features • Wavelets can provide compressed representations • Wavelet Theory provides a unified framework for data processing

Scale-Coherent Structures • Coherent structure - frequencies at all scales • Examples - edges, peaks, ridges • Locate extent and assign saliency

Wavelets – Analysis

Wavelets – DeNoising

Wavelets – Compression Original 50:1

Wavelets – Compression

Yet Another Example 50% 7%

2% Final Example 50% 100% 1%

1.0 0.8 ) E ( n o i t a 0.6 m r o f density n u momentum i v momentum d e 0.4 w momentum z energy i l a m r o n 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 normalized rate Information Rate Curve • Energy Compaction – Few coefficients can efficiently represent functions • The Curve should be as vertical as possible near 0 rate

Filter Bank Implementation • G: High Pass Filter • H: Low Pass Filter

Synthesis Bank

Successive Approximations

Successive Details

Wavelet Representation

Coefficients

Lossey Compression

Image Example A Frame Another Frame

Image Example Average Difference

Wavelet Transform

Frequency Support

Image Example LvLh LvHh HvHh HvLh

How Does One Do This ?

Dilations • Rescaling Operation t --> 2t • Down Sampling, n --> 2n • Halve function support • Double frequency content • Octave division of spectrum- Gives rise to different scales and resolutions • Mother wavelet! - basic function gives rise to differing versions

Dilations

Successive Approximations

Translations • Covers space-frequency diagram • Versions are

Wavelet Decomposition • Induced functional Space - Wj. • Related to Vjs • Space Wj+1 is orthogonal to Vj+1 • Also • J-level wavelet decomposition -

Successive Differences

Wavelet Expansion • Wavelet expansion (Tiling- j: scale, k: translates), Synthesis • Orthogonal transformation, Coarsest level of resolution - J • Smoothing function - f, Detail function - y • Analysis: • Commonly used wavelets are Haar, Daubechies and Coiflets

Scaling Functions • Compact support • Bandlimited - cut-off frequency • Cannot achieve both • DC value (or the average) is defined • Translates of f are orthogonal

Wavelets ?

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