Coding optimization based on weighted-to-spherically-uniform quality metric for 360 video

To optimize coding efficiency, an encoder minimizes total distortion under a constraint rate. Distortion criteria for conventional video uniformly measures coding error around picture. For non-equal area projection formats of 360 video, e.g., equirectangular projection (ERP), cube map projection (CMP), uniformly measured distortion cannot correctly represent coding loss on a watching sphere of panoramic visual content. In this paper, coding optimization based on weighted-to-spherically-uniform quality criteria is proposed to improve coding performance of 360 video, where adaptive quantization is applied in CTU level according to WS-PSNR weights. Since quantization parameter (QP) offset can be derived according to CTU position of 360 video at decoder side, no additional parameters need to be transmitted. Experimental results show that compared to coding with fixed QP, the proposed adaptive quantization method achieves 4.50%, 8.51% and 3.21% BD-rate reduction for ERP, rotated ERP and CMP on average, respectively.