To deliver ultra-high resolution 360-degree video (such as 8K, 12K, or even higher) across the internet, viewport-dependent streaming becomes necessary to save bandwidth. During viewport switches, clients and servers will instantly exchange coordination info and contents for the given viewports. However, those viewport switches pose a serious challenge for video encoding because the temporal dependency between contents within changing viewports is unpredictable. In existing practices, it is commonly noted that GOP (Group of Pictures) size in a bitstream intrinsically prohibits the reduction of the viewport switch latency, such as Motion-to-photon (MTP) latency, or motion-to-high-quality (MTHQ) latency. In this paper, we presented a Scalable Video Coding (SVC) based bitstream schema, which can structurally remove the impacts of GOP in viewport-dependent streaming and provide instant viewport switches within one-frame time (the best possible). In addition, combined with tiling, this new coding schema allows an efficient packing of the non-adjacent regions within a viewport of 360-degree video. Our experiments also show that the overall encoding with this SVC-based approach is faster than with multi-stream approaches. Compared with current 360-degree video streaming solutions based on MPEG-I OMAF, our approach is superior in terms of viewport switch latency, simplicity of viewport packing, and encoding performance.

翻译：为了在互联网上传输超高分辨率360度视频（如8K、12K甚至更高），视口相关流传输成为节省带宽的必要手段。在视口切换过程中，客户端与服务器会即时交换与特定视口相关的协调信息与内容。然而，这些视口切换对视频编码构成了严峻挑战，因为切换视口内内容之间的时间依赖性是不可预测的。现有实践中普遍注意到，比特流中GOP（图像组）尺寸从根本上限制了视口切换延迟的降低，例如运动到光子（MTP）延迟或运动到高质量（MTHQ）延迟。本文提出了一种基于可伸缩视频编码（SVC）的比特流方案，该方案在结构上消除了GOP对视口相关流传输的影响，并能在单帧时间内（最理想情况）实现即时视口切换。此外，结合分片技术，这种新型编码方案允许高效打包360度视频视口内的非相邻区域。实验表明，该基于SVC的总体编码速度优于多流方案。与当前基于MPEG-I OMAF的360度视频流传输解决方案相比，本方法在视口切换延迟、视口打包简便性及编码性能方面均更具优势。