Efficient Wavefront Parallel Processing for HEVC CABAC Decoding

摘要

Context-based Adaptive Binary Arithmetic Coding (CABAC) is the only compute-intensive task in the High Efficiency Video Coding (HEVC) Standard that does not contain significant data-level parallelism. As a result, it is often a throughput bottleneck for the overall decoding process, especially for high-quality videos. Consequently, the use of high-level parallelization techniques is inevitable to reach throughput requirements for CABAC decoding. Multiple high-level parallelization tools are specified in HEVC, amongst which Wavefront Parallel Processing (WPP) has only small losses in coding efficiency. However, it lacks in parallel efficiency due to a ramp-up and -down in active parallel threads within a frame. This is a serious problem for systems that cannot process multiple frames at the same time due to performance or memory constraints (e.g. mobile devices), and also for low-delay applications such as video conferencing. To address this issue, we present three improved WPP implementations for HEVC CABAC decoding. They differ in the granularity at which dependency checks are performed. The improvement comes from increased parallel efficiency of the WPP implementation while using the same number of threads as conventional WPP. The proposed implementations allow speedups up to $1.83 imes$ with very little implementation overhead.