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Panoramic video undergoes projection onto a two-dimensional plane for compression and subsequent back-projection onto a sphere for display. This process introduces inconsistency between compression distortion and perceived spherical distortion, which causes a serious loss in coding efficiency. Meanwhile, the existing independent rate-distortion optimization (RDO) model for spherical distortion solely accounts for the current coding frame and neglects its influence on subsequent frames, which leads to sub-optimal coding performance. To this end, we propose a spherical distortion temporal propagation and spatial mapping model for efficient panoramic video coding. First, a zero-delay spherical distortion backward propagation chain is established in the temporal domain, and distortion impact factors are computed. Then, an accurate spatial mapping relationship between spherical distortion and coding distortion is constructed, along with the calculation of spatial mapping weights. Finally, these components are integrated into spherical RDO. The experimental results demonstrated the effectiveness of the proposed algorithm. Compared to the versatile video coding test model (VTM-14.0) with a 360Lib extension under low-delay P frame and B frame configurations, the proposed algorithm achieves bitrate savings of 9.4% (up to 19.4%) and 8.5% (up to 19.0%) by using WSPSNR as the distortion evaluation index, respectively. Additionally, the coding time was reduced by 14.53% and 15.65%, respectively.
Yang et al. (Mon,) studied this question.