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Burgeoning virtual reality technology allows people to experience the content of video authentically. In order to provide an immersive experience to user, virtual reality video needs higher resolution (4K and higher) and more data than traditional video, which increases the coding complexity extraordinary. In this paper, a fast intra mode decision algorithm based on the sum of region-directional dispersion is proposed. Based on the diverse degree of horizontal stretching in different latitude area, the relationship of the optimal intra prediction mode related to latitude area for coding units is obtained. A new metric named the sum of region-directional dispersion is defined to determine the mode selection interval, and the prediction mode where located the minimum sum of dispersion is used as the center mode of the selection interval. The optimal intra prediction mode is obtained by comparing it with other adjacent modes in the interval. Compared with the original reference software HM16.20, the proposed algorithm can reduce the time in coding by 28.5%, while BD-rate only increases by 1.0%.
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