傅思維

How to implement? 2 g[n]: low pass filter h[n]: high pass filter :down sampling

Fig. 2 (a) One level and (b) two level 2-D DWT. Different sub-bands: 3

Ex:  1. Localized both in time (space) and frequency domain.  2. Multiresolution analysis (MRA). 4

 Traditional Fourier transform: 5

6

7

8

9

10

1. Perform the DWT on the noisy image to obtain sub-bands. 2. Threshold all high frequency sub band coefficients using certain thresholding method. 3. Perform the inverse DWT to reconstruct the de-noised Image. 11

Hard-thresholding:  f_h(x) = x if abs(x) ≥ λ (1) = 0 otherwise Soft-thresholding:  f_s(x) = x −λ if x ≥ λ = 0 if x < λ (2) = x +λ if x ≤ −λ 12

(a) (b) Fig. 3 (a) Hard-thresholding and (b) soft-thresholding 13

14

15 D. L. Donoho and I. M. Johnstone, “Ideal spatial adaptation via wavelet shrinkage,” Biometrika, vol. 81, pp. 425–455, 1994.

 0 ->  10 ->  20 ->  30 ->

PSNR (dB) noisy image (117)30.79( 75)29.21(61) (3x3)31.26 (5x5)28.73 (7x7) Visushrink BayesShrink Table I: PSNR of test image corrupted by AWGN 1.The standard deviation of the Gaussian lowpass filter is chosen until the best result appears. 2.The window size of Wiener filter is chosen until the best result appears (shown in the parentheses). 17 Cheat!

18

19 Q & A

20