Content-aware Hybrid Equi-angular Cubemap Projection for Omnidirectional Video Coding

Omnidirectional video is required to be projected from the Three-Dimensional (3D) sphere to a Two-Dimensional (2D) plane before compression due to its spherical characteristics. Therefore, various projection formats have been proposed in recent years. However, these existing projection methods have problems of either oversampling or discontinuous boundary, which penalize the coding performance. Among them, Hybrid Equiangular Cubemap (HEC) projection has achieved significant coding gains by keeping boundary continuity when compared with Equi-Angular Cubemap (EAC) projection. However, the parameters of its mapping function are fixed and cannot adapt to the video contents, which results in non-uniform sampling in certain regions. To address this limitation, a projection method named Content-aware HEC (CHEC) is presented in this paper. In particular, these parameters of mapping function are adaptively achieved by minimizing the projection conversion distortion. Additionally, an omnidirectional video coding framework with adaptive parameters of mapping function is proposed to effectively improve the coding performance. Experimental results show that the proposed scheme achieves 8.57% and 0.11% bit rate reduction on average in terms of End-to-End Weighted to Spherically uniform Peak Signal to Noise Ratio (E2E WS-PSNR) when compared with Equi-Rectangular Projection (ERP) and HEC projections, respectively.