FastIntra360: A Fast Intra-Prediction Technique for 360-Degrees Video Coding

360-degrees videos represent a whole sphere and enable the user to feel as if he is inside the scene. These videos demand more data than conventional videos to be represented, therefore they also must be compressed to be handled properly. However, current video coding standards only process rectangular videos, thus 360 videos must be represented in a flat fashion to be encoded. There are several projections to perform this and the currently most used one is the equirectangular projection (ERP), which transforms each parallel from the sphere into a row of the rectangle, resulting in a faithful representation of the equatorial area, and a stretched representation of the polar regions. This stretching in the polar regions tends to impact the behavior of intra-frame prediction, which is used to exploit the spatial redundancies in each frame. Therefore, this paper proposes FastIntra360 to accelerate the encoding of 360 videos. FastIntra360 is implemented in HEVC video coding standard [1], which is a recently established standard and poses high computational demand. During the development of FastIntra360, a set of videos were encoded and the behavior of the intra-prediction throughout the frame was extracted. Then, a statistical analysis was conducted over such data and it concluded that when encoding the polar regions of the frame, the prediction modes which exploit horizontal directions are selected more frequently than the remaining modes, whereas in the center of the frame all prediction modes present similar occurrence rates. FastIntra360 exploits this behavior to reduce the number of prediction modes evaluated in different regions of the frame to accelerate the encoding. FastIntra360 is developed in two variants: one considering three bands and other considering five bands, where each band is a horizontal stripe of the frame. Each band divides the frame samples into three or five stripes and performs the statistical analysis over these stripes individually. Both implementations were evaluated and compared against the HEVC Test Model version 16.16 (HM-16.16) according to time reduction and coding efficiency (considering BD-BR), where BD-BR represents the bitrate increase of the proposed technique. Experimental results showed that both implementations present good performance, reaching up to 16.5% complexity reduction with negligible BD-BR, that is, they present considerable complexity reduction whereas posing no harm to the video quality.