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Digital Image Processing 11/11/20 DIGITAL IMAGE PROCESSING Lecture 4 Frequency domain Tammy Riklin Raviv Electrical and Computer Engineering Ben-Gurion University of the Negev Fourier Domain 1 11/11/20 2D Fourier transform and its applications Jean Baptiste Joseph Fourier (1768-1830) ...the manner in which the author arrives at these A bold idea (1807): equations is not exempt of difficulties and...his Any univariate function can analysis to integrate them still leaves something be rewritten as a weighted to be desired on the score of generality and even sum of sines and cosines of rigour. different frequencies. • Don’t believe it? – Neither did Lagrange, Laplace, Poisson and Laplace other big wigs – Not translated into English until 1878! • But it’s (mostly) true! – called Fourier Series Legendre Lagrange – there are some subtle restrictions Hays 2 11/11/20 A sum of sines and cosines Our building block: Asin(wx) + B cos(wx) Add enough of them to get any signal g(x) you want! Hays Reminder: 1D Fourier Series 3 11/11/20 Fourier Series of a Square Wave Fourier Series: Just a change of basis 4 11/11/20 Inverse FT: Just a change of basis 1D Fourier Transform 5 11/11/20 1D Fourier Transform Fourier Transform 6 11/11/20 Example: Music • We think of music in terms of frequencies at different magnitudes Slide: Hoiem Fourier Analysis of a Piano https://www.youtube.com/watch?v=6SR81Wh2cqU 7 11/11/20 Fourier Analysis of a Piano https://www.youtube.com/watch?v=6SR81Wh2cqU Discrete Fourier Transform Demo http://madebyevan.com/dft/ Evan Wallace 8 11/11/20 2D Fourier Transform Do not take in for granted! I switched one pair of FT –can you guess which? 2D Fourier Transform 9 11/11/20 2D Fourier Transform 2D Fourier Transform 10 11/11/20 Sinusoidal Waves Sinusoidal Waves 11 11/11/20 Sinusoidal Waves Sinusoidal Waves 12 11/11/20 Sinusoidal Waves Fourier analysis in images Intensity images Fourier decomposition images http://sharp.bu.edu/~slehar/fourier/fourier.html#filtering 13 11/11/20 Signals can be composed + = http://sharp.bu.edu/~slehar/fourier/fourier.html#filtering More: http://www.cs.unm.edu/~brayer/vision/fourier.html Fourier Transform • Stores the amplitude and phase at each frequency: • For mathematical convenience, this is often notated in terms of real and complex numbers • Related by Euler’s formula Hays 14 11/11/20 Euler’s formula Wikipedia Fourier Transform • Stores the amplitude and phase at each frequency: • For mathematical convenience, this is often notated in terms of real and complex numbers • Related by Euler’s formula • Amplitude encodes how much signal there is at a particular frequency Amplitude: A = ± Re(w)2 + Im(w)2 • Phase encodes spatial information (indirectly) -1 Im(w) Phase: f = tan Re(w) Hays 15 11/11/20 Fourier Bases Teases away ‘fast vs. slow’ changes in the image. Blue = sine Green = cosine This change of basis is the Fourier Transform Hays Fourier Bases Hays 16 11/11/20 Important Fourier Transform Pairs Important Fourier Transform Pairs 17 11/11/20 Important Fourier Transform Pairs Important Fourier Transform Pairs 18 11/11/20 Important Fourier Transform Pairs Important Fourier Transform Pairs 19 11/11/20 2D Fourier Decomposition Basis reconstruction Danny Alexander 20 11/11/20 2D Fourier Transform of Real Images What does it mean to be at pixel x,y? What does it mean to be more or less bright in the Fourier decomposition image? 2D Fourier Transform of Real Images 21 11/11/20 Low/High Pass Filters Low and High Pass filtering 22 11/11/20 Removing frequency bands Brayer Removing frequency bands Brayer 23 11/11/20 Removing frequency bands Removing frequency bands 24 11/11/20 Editting frequencies Magnitude vs. Phase 25 11/11/20 Magnitude vs. Phase The Importance of Phase ? ? 26 11/11/20 The Importance of Phase Phase and Magnitude- Another Example ? ? 27 11/11/20 The Importance of Phase Phase and Magnitude- Yet Another Example Efros 28 11/11/20 Phase and Magnitude- Yet Another Example Amplitude Phase Efros Phase and Magnitude- Yet Another Example Efros 29 11/11/20 What about phase? Amplitude Phase Efros Cheebra Zebra phase, cheetah Cheetah phase, zebra amplitude amplitude Efros 30 11/11/20 Phase and Frequency Rotation by 900 Efros Bonus question for next week • Surprise us with hybrid images that are mixture of phase and amplitude of different images. • Manipulate phase and frequency (e.g., by rotation) of the same image to generate interesting artifacts. • There will be a competition! • Extra bonus point to the winner! 31 11/11/20 Phase and Frequency • The frequency amplitude of natural images are quite similar • Heavy in low frequencies, falling off in high frequencies • Will any image be like that, or is it a property of the world we live in? • Most information in the image is carried in the phase, not the amplitude • Not quite clear why Efros Properties of the Fourier Transform 32 11/11/20 Properties of Fourier Transforms Properties of Fourier Transforms See Szeliski Book (3.4) 33 11/11/20 The Convolution Theorem http://jclahr.com/science/psn/wielandt/node8.html Filtering Vs. Convolution in 1D 34 11/11/20 Filtering Vs. Convolution in 1D Filtering Vs. Convolution in 1D 35 11/11/20 Filtering Vs. Convolution in 1D Filtering Vs. Convolution in 2D 36 11/11/20 Filtering Vs. Convolution in 2D Convolution 37 11/11/20 Filtering vs. Convolution in 2D Matlab Convolution Theorem 38 11/11/20 The Importance of Convolution Theorem The Importance of Convolution Theorem 39 11/11/20 The Importance of Convolution Theorem The Importance of Convolution Theorem 40 11/11/20 Filtering: Spatial Domain vs. Frequency Domain More on Filtering in spatial domain 1 0 -1 2 0 -2 1 0 -1 * = Hays 41 11/11/20 Filtering in frequency domain FFT FFT = Inverse FFT Slide: Hoiem Fast Fourier Transform in Matlab 42 11/11/20 Fast Fourier Transform in Matlab row &cols Standard Deviation pointwise multiplication Fast Fourier Transform in Matlab 43 11/11/20 Fast Fourier Transform in Matlab Fast Fourier Transform in Matlab 44 11/11/20 Fast Fourier Transform in Matlab Class Work • Read cameraman image: I = imread('cameraman.tif'); • Calculate its frequency spectrum with fft2 • Display the absolute value of its spectrum with and w/o fftshift • It is recommended to present the spectral image using logarithmic scale. 45 11/11/20 Sampling Theorem 1D Sampling 46 11/11/20 1D Sampling 1D Sampling 47 11/11/20 The Sampling Theorem and Aliasing 2D Sampling 48 11/11/20 2D Sampling Sampling Theorem in 2D 49 11/11/20 Sampling Theorem in 2D Aliasing: 1D Example 50 11/11/20 Aliasing in video Aliasing in video 51 11/11/20 Aliasing in 2D: under-sampling reconstruction Aliasing in Images 52 11/11/20 What’s happening Anti-aliasing 53 11/11/20 Anti-aliasing Aliasing in MRI MRI 54 11/11/20 Aliasing in MRI Aliasing in MRI MRI K-Space K-Space 55 11/11/20 Aliasing in MRI Mask MRI K-Space Reconstructed MRI Ideas for final Projects Solitaire Recognition Chess recognition Real time Human Activity recognition Face Recognition Flaying Object Detection Ball detection (in soccer game) 56 11/11/20 Ideas for final Projects EENG 512/CSCI 512 - Final Projects Hoch, Garrett, Solitaire Recognition Ideas for Final Projects EENG 512/CSCI 512 - Final Projects Xiao, Ke, Chess Recognition 57 11/11/20 Ideas for Final Projects BGU – ECE 2013: Topaz, Ohad, TsacHi, Nadav Ideas for Final Projects BGU – ECE 2015:Doron, Boris, Alex 58 11/11/20 Ideas for Final Projects BGU – ECE 2015:Nir, Tal & Shay – Interactive temple Run Ideas for Final Projects BGU – ECE 2013:Ariel, Tomer, Oren – Virtual Keyboard 59.
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