1. Reduction of blocking artifacts in DCT-coded images
Presenters :
梁速清
莊令誼
Date :

2. Outline Introduction Luo’s Method [1] Shi’s Method [2] Comparison
Conclusion 1

3. Introduction
DCT & Quantize
IDCT
Original Lena
Lena after DCT 2

4. Luo’s Method (1/4)
一. Search two adjacent blocks(8x8),Block A and Block B.
二. Assume Block A and Block B have the same pixel value, a, b
respectively ,and a ≠ b.
三. Block C include right half of Block A and left half of Block B 3

5. Luo’s Method (2/4) Block A Block B Block C
* DCT coefficient of Block A , Block B, Block C
480
720
Block A
Block B
654 34 23 45 11
Block C 4

6. Luo’s Method (3/4)
* Ying proposed three constraint : (a, b exist strong edge. c means low frequency)
* T1 = 350, T2 = 120 , T3 = 60 .
* The choice of thresholds are image and compression ratio dependent. 5

7. Luo’s Method (4/4)
* The blockiness reduction in the DCT domain by modifying the five relevant
coefficients of Fc(u,v).
* Block C is modified by the weight average of block A,B, and C.
* The pixels of C(k,l), are replace by CMc(k,l), the inverse DCT of FMc(u,v) 6

8. Reduction of Blocking artifacts
Shi’s Method
Propose a visibility function of blocking artifacts based on HVS (Human Visual System )
Reduction of Blocking artifacts
Smooth regions : modify parameter, use linear blocks 7

9. Model for Blocking Artifacts
New shifted block c
Step function 8

10. : average value of block c : the magnitude of s
Model for Blocking Artifacts (cont.)
Model of c
: average value of block c
: the magnitude of s
: the activity inside c
C, S represent the DCTs of c, s 9

11. Visible Function Visible function
falls below a threshold, no further process need
Mean squared difference of slopes (MSDS)
c( i , j ) : luminance value of pixel ( i , j ) 10

12. Visible Function (cont.)
Activity Masking
Ah : horizontal activity
Av : vertical activity
Brightness Masking
b0 =150, r =2
b : average luminance value 11

13. Reduction of Blocking Artifacts
Edges should be detected to avoid blurring information-bearing edges in the image
Edge detection : satisfy the two conditions, represent no edges
QP : quantization parameter 12

14. Reduction of Blocking Artifacts (cont.)13

15. Reduction of Blocking Artifacts (cont.)
For smooth region
Replace step block s with linear block in the shifted block 14

16. Reduction of Blocking Artifacts (cont.)
Use information of the left block a and the right block b 15

17. Reduction of Blocking Artifacts (cont.)
A DCT-domain post-filtering method is applied to eliminate newly-created artifact
For edge region
The same post-filtering method is applied 16

18. PSNR at different bit rate with different algorithms
Shi’s Result
PSNR at different bit rate with different algorithms
Image
Bitrate (bpp)
zero
Liu
Shi
Lena
0.167
0.183
0.217 17

19. Shi’s Result (cont.)
Bitrate = 0.183
Zero
Liu
Shi 18

20. Comparison (Luo’s & Shi’s)
PSNR
(dB)
Image
Bitrate (bpp)
Luo
Shi
Lena
0.167
0.183
0.217 19

21. Conclusion Luo’s method Shi’s method Effective Good
Preserve edge information
Less
More (HVS)
Computing time
More
Different deblocking methods are applied to different regions 20

22. Reference
[1]Ying Luo and Rabab Kward, "Removing the blocking artifacts of block-based DCT compressed images," IEEE Trans on Image and Processing, vol. 12, pp , July, 2003
[2]M. Shi, Q. Yi, and J. Gong, “Blocking Effect Reduction Based On Human Visual System For Highly Compressed Images,” Electrical and Computer Engineering, Canadian Conference on, May 2006
[3]S. Z. Liu and C. B. Alan, "Efficient DCT-domain blind measurement and reduction of blocking artifacts," IEEE Transactions on Circuits and Systems for Video Technology, vol. 12, pp , Dec., 2002 21