Flexible Tiles in Adaptive Viewing Window: Enabling Bandwidth-Efficient and Quality-Oriented 360°VR Video Streaming

360° VR video offering a unique interactive experience has become a dominant streaming form for immersive multimedia applications. At the same time, remote transmission of these applications is not feasible under conventional network and rendering infrastructure. The existing viewport-based solutions employ fixed tiling patterns to stream quality variable views intending to reduce the amount of data transmission. However, they still could end up with unsatisfactory visual boundaries and feeble bandwidth utilization. For instance, employing larger resolution tiles can include redundant pixels that contribute to the waste of bandwidth resources, and smaller resolution tiles can introduce several visual distortions, e.g., flickering, floating, and blurring at the edges of the tiles. This paper proposes FlexTile, a bandwidth-efficient and quality-oriented solution that utilizes flexible tiles in an adaptive viewing window during 360° VR video streaming. FlexTile overcomes the limitations of existing works by implementing three levels of adaptation: (i) adaptive tiles selection, (ii) adaptive bitrate selection, and (iii) adaptive tiling pattern selection. FlexTile interactively selects best-fit viewing tiles for each adaptive viewing window considering a region-based visual quality metric to facilitate visual comforts and bandwidth utilization. The extensive performance evaluations show that our flexible tiling-based approach significantly improves the viewing comfort levels measured in terms of visual quality and visual smoothness while improving bandwidth utilization under real-world 4G bandwidth conditions.