Reduction of blocking artifacts in DCT-coded images Presenters : 695410076 梁速清 695410122 莊令誼 Date : 2007.6.20

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

Introduction DCT & Quantize IDCT Original Lena Lena after DCT 2

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

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

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

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

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

Model for Blocking Artifacts New shifted block c Step function 8

: 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

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

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

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

Reduction of Blocking Artifacts (cont.) 13

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

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

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

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 28.9774 29.3625 29.5578 0.183 30.1689 30.5002 30.7532 0.217 30.2573 30.2362 30.4607 17

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

Comparison (Luo’s & Shi’s) PSNR (dB) Image Bitrate (bpp) Luo Shi Lena 0.167 29.3544 29.5578 0.183 30.5014 30.7532 0.217 30.3071 30.4607 19

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

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.838-842, 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.1139 - 1149, Dec., 2002 21