The 360-degree video is projected to 2-D formats using various projection methods for efficient compression. As a necessary part of general-video compression, rate control is also indispensable for the projected 360-degree video compression. However, the current rate control algorithm has not been optimized for the 360-degree video compression yet. The Coding Tree Unit (CTU) level bit allocation in the rate control algorithm has not taken into consideration the characteristic that various pixels in 2-D formats have different influences on the visual experiences. In this article, we first propose an optimal CTU level weight taking this characteristic into consideration. The CTU level weight is an approximation to the pixel level weight since the smallest granularity of a rate control algorithm is usually CTU. Second, based on the CTU level weight, a weighted CTU level bit allocation algorithm is proposed to achieve better coding performance. The bits of each CTU are assigned that the Lagrange multiplier $\lambda$ of a CTU is inversely proportional to its CTU level weight. This CTU level bit allocation scheme is applied to all the 360-degree video projection formats. Third, we propose a CTU row (CR) level rate control algorithm for the Equi-Rectangle Projection (ERP) format. Different CTUs in the same row in the ERP format are combined into a CR to provide more stable model parameters. The proposed algorithms are implemented in the newest video coding standard High Efficiency Video Coding (HEVC) reference software. The experimental results show that the proposed algorithm is able to achieve much better subjective and objective qualities as well as smaller bitrate errors compared with the state-of-the-art rate control algorithm.

中文翻译：

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用于 360 度视频压缩的 λ 域感知速率控制

使用各种投影方法将 360 度视频投影为 2-D 格式，以实现高效压缩。作为通用视频压缩的必要组成部分，码率控制对于投影360度视频压缩也是必不可少的。但是，目前的码率控制算法还没有针对 360 度视频压缩进行优化。码率控制算法中的编码树单元（CTU）级别的比特分配没有考虑到二维格式中各种像素对视觉体验的影响不同的特点。在本文中，我们首先提出了一个考虑到这一特性的最佳 CTU 级别权重。CTU 级别权重是像素级别权重的近似值，因为速率控制算法的最小粒度通常是 CTU。二、基于CTU级别权重，提出了一种加权CTU级比特分配算法，以获得更好的编码性能。每个 CTU 的比特被分配为一个 CTU 的拉格朗日乘数 $\lambda$ 与其 CTU 级别权重成反比。这种 CTU 级比特分配方案适用于所有 360 度视频投影格式。第三，我们为等矩形投影 (ERP) 格式提出了一种 CTU 行 (CR) 级速率控制算法。ERP 格式中同一行的不同 CTU 组合成一个 CR，以提供更稳定的模型参数。所提出的算法在最新的视频编码标准高效视频编码 (HEVC) 参考软件中实现。