Fast Mode Decision And Motion Estimation For JVT/H.264 Pen Yin, Hye – Yeon Cheong Tourapis, Alexis Michael Tourapis and Jill Boyce IEEE ICIP 2003 Sep.

Outline Introduction Introduction Fast Mode Decision Fast Mode Decision Fast Motion Estimation Fast Motion Estimation Simulation Result Simulation Result Conclusion Conclusion

Introduction(1/4) High coding efficiency Minor increase in decoder complexity High encoder complexity Motion estimation (ME) and mode decision contribute most of the complexity, In H.264 In H.264 ME is separately considered from mode decision. ME is first checked all block types of inter modes, then the mode decision is made by comparing the cost of each inter mode

Introduction (2/4) Minimal the cost defined as follow (ME) The mode decision is made by minimizing The mode decision is made by minimizing

A B C D E F G H I a bc e hgf d Introduction (3/4) Assume E is best integer pel Then 1 、 2 ～ 7 are searched Assume 7 is best integer pel Then a 、 b ～ h are searched  total : 16 subpel positions are check

Introduction (4/4) This paper proposed a algorithms to alleviate the encoding complexity by jointly optimize mode decision and ME, while maintaining coding efficiency

Fast Mode Decision(1/4) Reduce the number of potential modes, eliminate ME for some block types Decreases the number of tested intra modes Modes are divided into two categories : inter modes : Skip mode 16x16 、 16x8 … 4x4 intra modes : intra 16x16 、 intra 4x4

Fast Mode Decision(2/4) Two concepts : Two concepts : High priority of SKIP mode High priority of SKIP mode Error surface : built by initial 3 mode 16x16 8x8 4x4 Error surface : built by initial 3 mode 16x16 8x8 4x4 error surface monotonic :or case 1 : case 1 : the best two modes are 16x16 and 8x8  only 16x8 and 8x16 case 2 : case 2 : the best two modes are 4x4 and 8x8  only 4x8 and 8x4 Case 3 : error surface is not monotonic  all modes

Fast Mode Decision(3/4) Step1 ： check SKIP mode if J mode (SKIP) < T1, select SKIP as best mode, stop; otherwise go to step2 step2 ： check 16x16 and 8x8 if J mode (SKIP) <J mode (16x16) && J mode (SKIP) < J mode (8x8), go to step7; otherwise, go to step3 step3 ： check 4x4 if MinJ mode = J mode (8x8) || MaxJ mode = J mode (8x8), go to step4 if MaxJ mode = J mode (4x4), go to step5 if MaxJ mode = J mode (16x16), go to step6 step4 ： check 16x8, 8x16, 8x4, 4x8; go to step7 step5 ： check 16x8, 8x16; go to step7 step6 ： check sub-macroblock partition; go to step7 step7 ： select the best inter mode if the energy of the residue for best inter mode is > T2, check intra modes; go to step8 step8 ： choose the best mode among all tested modes

Fast Mode Decision(4/4) ( In step1,T 1 = Nbitsλ mode ( Nbits equals the minimum number of bits required for non SKIP inter modes ) In step2 comparing SKIP with 16x16 and 8x8 assume that the RD cost for SKIP is the minimal, so no other modes need to be checked In step3, check the monotonic In step7, assume that inter modes have higher priority than intra modes ( T 2 = 0 )

Fast Motion Estimation(1/6) ME process involves two steps ： Fast integer pel motion search ： Based on predictive motion estimation algorithm : Enhanced Predictive Zonal Search (EPZS) Fast subpel motion refinement ： This paper mainly discuss the subpel ME Propose two patterns of fast subpel ME and early stop criterion

Fast Motion Estimation EPZS : ME around initial predictors Refinement around the best predictor using diamond or square patterns refinement is performed around the second best predictor or the median predictor Avoid being trapped into local minimum

Fast Motion Estimation First pattern First pattern : Assume that the diamond positions are more important than corner positions TOTAL 12 point need to check TOTAL 12 point need to check A B C D E F G HI a bc e hgf d

Fast Motion Estimation Second pattern : Second pattern : store the position for the best integer and its best diamond position 3 half point need to check 3 half point need to check A B C D E F G HI a bc e hgf d

Fast Motion Estimation Second pattern : Second pattern : For quarter pel search, the best position and second best position in half pel search are chosen. A B C D E F G HI a bc e hgf d

Fast Motion Estimation Three cases : Case 1 : two best pels are integer-pel  two quarter pels on the same row/column will be tested Case 2 : two best pels are at the same row/column, 3 quarter pels between these two pels will be tested. Case 3 : two best pels are in a diagonal position (ex: E 、 8)  3 quarter-pel (two diamond positions and one corner position) will be tested

Fast Motion Estimation Step1 : after integer search, examine the current minimum SAD, if SAD < T1, stop; otherwise, go to step2; Step2 : half pel positions, examine the current minimum SAD, if SAD <T2, stop; otherwise, go to step3; Step3 : examine the quarter pel positions.

Simulation Result 85% to 90% complexity reduction can be achieved versus the reference software, while coding efficiency is only slightly decreased. average PSNR gain (∆PSNR) bitrate reduction (bitrate)

Conclusion In this paper presents a new algorithm to jointly optimize motion estimation and mode decision. Simulation results demonstrate that our scheme can maintain coding efficiency at considerably lower complexity.