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Chapter 4 Image Enhancement in the Frequency Domain.

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Presentation on theme: "Chapter 4 Image Enhancement in the Frequency Domain."— Presentation transcript:

1 Chapter 4 Image Enhancement in the Frequency Domain

2 Fourier Transform 1-D Fourier Transform 1-D Discrete Fourier Transform (DFT) Magnitude Phase Power spectrum

3 2D DFT Definition: if f(x,y) is real

4 Centered Fourier Spectrum It can be shown that:

5 Example SEM Image

6 Filtering in the Frequency Domain 1. Multiply the input image by (-1)^x+y to center the transform 2. Compute F(u,v), the DFT of input 3. Multiply F(u,v) by a filter H(u,v) 4. Computer the inverse DFT of 3 5. Obtain the real part of 4 6. Multiply the result in 5 by (-1)^(x+y)

7 Fourier Domain Filtering

8 Some Basic Filters Notch filter:

9 Lowpass and Highpass Filters

10 Convolution Theorem Definition Theorem Need to define the discrete version of impulse function to prove these results.

11 Gaussian Filters Difference of Gaussians (DoG)

12 Illustration

13 Smoothing Filters Ideal lowpass filters Butterworth lowpass filters Gaussian lowpass filters

14 Ideal Lowpass Filters

15 Example

16 Ringing Effect

17 Butterworth Lowpass Filters Definition:

18 Example

19 Ringing Effect

20 Gaussian Lowpass Filters Definition:

21 Example

22 More example

23 Sharpening Filters High-pass filters In general, Ideal highpass filter Butterworth highpass filter: Gaussian highpass filters

24 Relationship between Lowpass and Highpass Filters

25 Spatial Domain Representation

26 Ideal Highpass Example

27 Butterworth Highpass Example

28 Gaussian Highpass Example

29 Laplacian in the Frequency Domain It can be shown that: Therefore,

30 Illustration

31 Other Filters Unsharp masking High-boost filtering High-frequency emphasis filtering

32 Homomorphic Filtering

33 DFT: Implementation Issues Rotation Periodicity and conjugate symmetry Separability Need for padding Circular convolution FFT

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