Texture-Aware Spherical Rotation for High Efficiency Omnidirectional Intra Video Coding

To adapt to the existing video coding standards, omnidirectional videos are usually projected from Three-Dimensional (3D) sphere to Two-Dimensional (2D) plane. However, this projection will cause geometrical stretching distortion and boundary discontinuity, which may degrade coding efficiency. In this paper, we present a Spherical Rotation based Omnidirectional Video Coding (SROVC) method, which exploits the textural properties of omnidirectional videos with spherical rotation. Firstly, SROVC framework is presented and Full-traversal Spherical Rotation (FSR) is developed to derive the optimal rotation angle with frame-level Rate Distortion Optimization (RDO). Secondly, to achieve comparable coding gains and lower computational complexity when compared with FSR, a Texture-aware Spherical Rotation (TSR) method is proposed to predict the rotation angle. Finally, to further reduce complexity and maintain coding efficiency, a Group-oriented TSR (G-TSR) approach is presented, in which the group length is statistically determined. Extensive experiments demonstrate that the proposed TSR and G-TSR schemes can achieve bit rate reductions up to 4.38%, 0.94% and 0.89% on average for CubeMap Projection (CMP) based high efficiency omnidirectional video coding. Additionally, the TSR scheme achieves bit rate saving from 0.91% to 1.19% on average under three more CMP-based projection formats, and 1.89% for joint rotation of X, Y, and Z axes.