Scalable Fast Rate-Distortion Optimization for H.264/AVC
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Scalable Fast Rate-Distortion Optimization for H.264/AVC Feng Pan,1, 2 Hongtao Yu,3 and Zhiping Lin3 1 Media
Processing Department, Institute for Infocomm Research, 21 Heng Mui Keng Terrace, Singapore 119613 Systems Inc., 245 Consumers Road, Toronto, ON, Canada M2J 1R3 3 School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 2 ViXS
Received 6 August 2005; Revised 17 March 2006; Accepted 27 May 2006 The latest H.264/AVC video coding standard aims at significantly improving compression performance compared to all existing video coding standards. In order to achieve this, variable block-size inter- and intra-coding, with block sizes as large as 16 × 16 and as small as 4 × 4, is used to enable very precise depiction of motion and texture details. The Lagrangian rate-distortion optimization (RDO) can be employed to select the best coding mode. However, exhaustively searching through all coding modes is computationally expensive. This paper proposes a scalable fast RDO algorithm to effectively choose the best coding mode without exhaustively searching through all the coding modes. The statistical properties of MBs are analyzed to determine the order of coding modes in the mode decision priority queue such that the most probable mode will be checked first, followed by the second most probable mode, and so forth. The process will be terminated as soon as the computed rate-distortion (RD) cost is below a threshold which is content adaptive and is also dependent on the RD cost of the previous MBs. By adjusting the threshold we can choose a good tradeoff between timesaving and peak signal-to-noise (PSNR) ratio. Experimental results show that the proposed fast RDO algorithm can drastically reduce the encoding time up to 50% with negligible loss of coding efficiency. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
H.264/AVC [1] is the newest international video coding standard developed by the joint video team (JVT), which consists of experts from VCEG and MPEG. It has achieved a significant improvement in coding efficiency compared to all the existing standards [2–4]. As in other video coding standards, H.264/AVC employs hybrid block-based motion compensated predictive coding. One of the novel features of H.264/AVC video coding is the use of different MB coding modes such as SKIP, INTER16 × 16, INTER16 × 8, INTER8 × 16, INTER8 × 8, INTRA16 × 16, and INTRA4 × 4, so that the temporal and spatial details in an MB are best presented. Note that in INTER8 × 8 mode, each block can be further divided independently into 8 × 8, 8 × 4, 4 × 8, or 4 × 4 subpartitions. To select the best coding mode, RDO is employed so that for each MB, all the MB coding modes are tried and the one that leads to the least RD cost is selected. This is to achieve the best tradeoff between the rate and distortion performance. Unfortunately, the computational burden of this type of exhaustively full searching algorithm is far more demanding than any other existing vid
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