Intra prediction based on geometry padding for omnidirectional video coding
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Intra prediction based on geometry padding for omnidirectional video coding Ning Li 1 & Shuai Wan 1 Received: 28 October 2019 / Revised: 30 July 2020 / Accepted: 4 August 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Omnidirectional videos or 360 degree videos play an important role in virtual reality (VR) applications. In order to employ the existing video coding standards, omnidirectional videos are firstly projected onto the 2-Dimension (2D) plane, which generates the discontinuity at boundaries and may result in unexpected artifacts when lossy coding is applied. In this paper, we propose a new intra prediction method to deal with the above coding artefacts in omnidirectional videos. Different from the conventional intra prediction using the left and top reference samples, the right reference samples are derived from the reconstructed samples on the left boundary and padded for intra prediction. The proposed method applies to the planar, DC and partial angular prediction modes in intra prediction. The experimental results demonstrate a Bjøntegaard-Delta-rate reduction of up to 2.97% using weighted spherical peak-signal to noise ratio (WSPSNR) quality metric for the coding tree units (CTUs) at the right boundary. Keywords Omnidirectional video . Video coding . ERP . Intra prediction
1 Introduction Omnidirectional video plays an important role in virtual reality. Providing users with immersive and interactive experience, omnidirectional videos are usually in ultrahigh resolutions, such as 4 K and 8 K, and require high transmission bandwidths. Therefore, efficient coding is in crucial need. Existing video coding standards [30, 33, 37, 42], however, are mostly designed for the planar video. Omnidirectional videos have to be mapped onto the plane to accommodate these coding standards. Many projection formats, including the EquiRectangular Projection (ERP) [29, 39],
* Shuai Wan [email protected] Ning Li [email protected]
1
School of Electronics and Information, Northwestern Polytechnical University, Xi’an, China
Multimedia Tools and Applications
Cube-map Projection (CMP) [26, 39], Rhombic Dodecahedron Projection (RDP) [9], etc. have been studied. Among these projection formats, the ERP is the most common projection format because of its simplicity. An example of ERP projection is given in Fig. 1. Different coding schemes have been proposed in the literature for omnidirectional videos. In general, these schemes can be classified into two categories. One is to adjust the projection format while keeping the encoding method unchanged. Another is to adjust the encoding method according to the characteristics of projection formats or application scenes. In the first category [17, 18, 24, 40], the ERP is widely studied, in which the redundant pixels in north and south poles are reduced as much as possible. In [17, 18, 24, 40], an omnidirectional video frame is divided into many tiles. The tiles at the polar are down-sampled to reduce the bitrates. In the second category [4,
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