Outline Introduction Introduction Fast Inter Prediction Mode Decision for H.264 – –Pre-encoding An Efficient Inter Mode Decision Approach for H.264 Video Coding

Introduction The H.264 shows significant performance gain over contemporary video coding standards. The performance improvement is at the expense of computational complexity increment. During the encoding, the encoder examine all modes to achieve the best prediction. Mode selection results in significant performance improvement as well as heavy computational load at the encoder.

Inter mode selection

Fast Inter Prediction Mode Decision for H.264 Dongdong Zhu, Qionghai Dai and Rong Ding Tsinghua University, Beijing ICME 2004

Fast Inter Mode Decision Down Sampling Original image Small images Small ref images Pre- Encoding Modes Motion vectors Encoding with original image Ref. image Output

Down Sampling Down sample the original image to a small image of 1/2 low resolution Extend to a multiple of 16 by using pixels on the edge, if the width or height of the small image is not a multiple of 16

Pre-Encoding Pre-encode on 8x8 block level – –do not use 16x16, 16x8 and 8x16 blocks in pre- encoding Search range should be scaled to 1/2 of the original one Store the top 2 best modes – –In case of the best prediction mode mismatch between pre-encoding and real encoding

Encoding Modes obtained from pre-encoding Candidate modes Macro block mode 8x8 sub- partition mode SKIP/DIRE CT SKIP/DIRECT, 16x16 I16x16 I4x4I4x4, I16x16 P 8x8 DIRCECTDERICT, 16x16 8x88x8, 16x16 8x48x4, 16x8 4x84x8, 8x16 4x4Following

Encoding By By edge direction histogram ( Histo(K) ) for horizontal and vertical direction K = 1 for horizontal direction, 2 for vertical direction and 3 for other.

Edge direction histogram

Early termination S1 ： S1 ： vector (0,0) and MVs of neighbor blocks S2 ： S2 ： the MVs obtained from pre-encoding Firstly, early termination test is performed on S2. – –For each initial motion vector in S2, SAD is calculated and min SAD is gotten as: SAD min = min ( SAD(V i ) ), V i =(yi,xi )  S2 If SADmin is smaller than a threshold T1, terminate the motion estimation

Result

Result Mobile

Result Foreman

Conclusion Pre-encoding process can reduce 50% of encoding time with PSNR decrease less than 0.2db. Pre-encoding process can reduce 50% of encoding time with PSNR decrease less than 0.2db.

An Efficient Inter Mode Decision Approach for H.264 Video Coding Xuan Jing and Lap-Pui Chau Nanyang Technological University,Singapore ICME 2004

Efficient inter mode decision The main concept is to use large blocks for homogeneous areas and small blocks for areas containing complex motions.

Efficient inter mode decision The amplitudes in a MB are small, it is most likely that this MB belongs to a homogeneous region and using only larger block sizes. Otherwise, this MB may contain complex motions and using more block types.

Efficient inter mode decision We first subtract the current frame by its previous frame. The sum of absolute difference for the current MB can be calculated using.

Efficient inter mode decision Step 1: – –(current frame - previous frame) and calculate the SAD for the current MB Step 2: – –If SAD<TH go to step 3; – –Otherwise go to step 4; Step 3: – –mode decision using 16×16, 16×8, 8×16 block types. Step 4: – –mode decision using all 7 block types 16×16, 168, 8××16, 8×8, 8×4, 4×8 and 4×4.

Simulation result

Conclusion On average, the proposed scheme achieved 0.2% bitrate saving at the cost of 0.04 dB PSNR drop. On On averagely about 36% computations are saved.