Fast Context-Adaptive Mode Decision Algorithm for Scalable Video Coding With Combined Coarse-Grain Quality Scalability (CGS) and Temporal Scalability

To speed up the H.264/MPEG scalable video coding (SVC) encoder, we propose a layer-adaptive intra/inter mode decision algorithm and a motion search scheme for the hierarchical B-frames in SVC with combined coarse-grain quality scalability (CGS) and temporal scalability. To reduce computation but maintain the same level of coding efficiency, we examine the rate-distortion (R-D) performance contributed by different coding modes at the enhancement layers (EL) and the mode conditional probabilities at different temporal layers. For the intra prediction on inter frames, we can reduce the number of Intra4×4/Intra 8×8 prediction modes by 50% or more, based on the reference/base layer intra prediction directions. For the EL inter prediction, the look-up tables containing inter prediction candidate modes are designed to use the macroblock (MB) coding mode dependence and the reference/base layer quantization parameters (Qp). In addition, to avoid checking all motion estimation (ME) reference frames, the base layer (BL) reference frame index is selectively reused. And according to the EL MB partition, the BL motion vector can be used as the initial search point for the EL ME. Compared with Joint Scalable Video Model 9.11, our proposed algorithm provides a 20× speedup on encoding the EL and an 85% time saving on the entire encoding process with negligible loss in coding efficiency. Moreover, compared with other fast mode decision algorithms, our scheme can demonstrate a 7-41% complexity reduction on the overall encoding process.

I. Introduction

In response to the increasing demand for scalability features in many applications, the Joint Video Team has recently, based upon H.264/advanced video coding (AVC) [1], standardized a scalable video coding standard (referred hereafter to as SVC) [2], [3] that furnishes spatial, temporal, signal-to-noise ratio (SNR) and their combined scalabilities within a fully scalable bit stream. By employing multilayer coding along with hierarchical temporal prediction [4], [5], the SVC encodes a video sequence into an inter-dependent set of scalable layers, allowing a variety of viewing devices to perform discretionary layer extraction and partial decoding according to their playback capability, processing power, and/or network quality. As a scalable extension to H.264/AVC, the SVC inherits all the coding tools of H.264/AVC and additionally it incorporates an adaptive inter-layer prediction mechanism for reducing the coding efficiency loss relative to the single-layer coding. A superior coding efficiency is achieved with little increase in decoding complexity by means of the so-called single-loop decoding. These key features distinguish the SVC from the scalable systems in the prior video coding standards.