A new tile boundary artifact removal method for tile-based viewport-adaptive streaming in 360∘ videos

In recent years, 360∘ video is becoming popular with the development of the virtual reality technology. However, the 360∘ video streaming faces many challenging issues since such video contains a large volume of data and requires streaming omnidirectional view. The Tile-based viewport-adaptive streaming method is used to deliver 360∘ videos more efficiently. In this method, a set of high and low-resolution tiles, corresponding to the viewport and non-viewport parts of the scene, is generated. These tiles are encoded and decoded independently using the Motion Constrained Tile Set (MCTS) technique. However, coding and decoding each tile independently, may create artifacts around the tile boundaries. In this paper, a novel method, called Resizing, is proposed to reduce the tile boundary artifact in tile-based viewport-adaptive streaming. In this method, the adjacent tiles are overlapped before encoding. Then, the overlapping tiles are downsampled to keep the resolution of video constant. This way, the mixed-resolution packing of 6K equirectangular (ERP) content, defined in MPEG Omnidirectional Media Format (OMAF) standard Annex D, can be used so that the output bitstream complies with the 4K-decoding constraint and the High Efficiency Video Coding (HEVC) standard. Finally, the video is upsampled after decoding and the overlapping parts are removed. Then, a novel method to remove the overlapping areas at the decoder side is proposed that could increase the perceived quality considerably. Simulation results indicate that our proposed approach achieves a 14.14% reduction in bandwidth in the Bjøntegaard-Bitrate scale, compared to the state-of-the-art methods that use variants of filtering for tile boundary artifact removal.