Spherical Coordinates Transform-Based Motion Model for Panoramic Video Coding

Panoramic video is produced and displayed as if it is supported on sphere, but it is usually mapped to two-dimensional (2D) plane for compression purpose. The mapping from sphere to 2D plane inevitably causes deformation of video content, which shall be taken into account in panoramic video coding. In this paper, we propose a new motion model based on spherical coordinates transform to explicitly address the deformation problem. Different from the traditional 2D translational motion model, our model assumes a 3D translational motion, which is characterized by a 2D motion vector and a relative depth parameter. Based on spherical coordinates transform, we can deduce the motion compensation and motion estimation algorithms for our proposed motion model. We then integrate the algorithms into the joint exploration model (JEM) scheme for panoramic video coding, and perform experiments on some typical equirectangular projection format video sequences. Experimental results show that our proposed method leads to up to 10.7% and on average 2.6% BD-rate reduction compared to the JEM anchor on our test sequences, with no increase of decoding time.