Advanced Spherical Motion Model and Local Padding for 360-Degree Video Compression

The 360-degree video compression has two main challenges due to projection distortions namely the geometry distortion and the face boundary discontinuity. There are some trade-offs between selecting Equi-rectangular and polyhedron projections. In the Equi-rectangular projection, the geometry distortion is severer than the face boundary discontinuity, while for the polyhedron projections the face boundary discontinuity is severer than the geometry distortion. These two distortions will have side effects on the motion compensation and undermine the compression efficiency of the 360-degree video. In this paper, an integrated framework is developed to handle these two problems to improve the coding efficiency. The proposed framework mainly has two key contributions. First, we derive a unified advanced spherical motion model to handle the geometry distortion of different projection formats for the 360-degree video. When fitting the projection between the various projection formats and the sphere into the unified framework, a specific solution can be obtained for each projection format. Second, we propose a local 3-D padding method to handle the face boundary discontinuity between the neighboring faces in various projection formats of the 360-degree video. The local 3-D padding method can be applied to different projection formats through setting different angles between neighboring faces. These two methods are independent of each other and can also be combined into an integrated framework to achieve a better rate-distortion performance. The proposed framework can be seamlessly integrated into the latest video coding standard High Efficiency Video Coding. The experimental results demonstrate introducing proposed coding tools can achieve significant bitrate savings compared with the current state-of-the-art method.