Delay Impact on MPEG OMAF's Tile-Based Viewport-Dependent 360° Video Streaming

With the continuously increasing attention that 360° video streaming is drawing, several solutions have been developed lately. They can be grouped into two main categories, with the first one being the viewport-independent category that consists of encoding the whole 360° video content using a particular projection, e.g., Equirectangular Projection or Cubemap Projection, without taking any viewport orientation into account. Such approaches waste resources since content not being watched by the user is encoded with the same visual fidelity as the content actually watched. The second category, the viewport-dependent category, relies on techniques that allow for viewport adaptivity. One approach is to apply viewport-dependent projections, wherein the spherical 360° video is mapped onto a rectangular frame in such a way that a specific viewport is mapped to a comparatively larger part of the rectangular frame than the rest of the content. Another approach, as specified in MPEG OMAF, is to use tile-based streaming, with the 360° video being offered in a tiled manner at various resolutions. Thus, each user can retrieve the tiles at different resolutions so that the high-resolution tiles match its viewport. Although viewport-dependent approaches allow providing better visual quality at the viewport, the end-toend delay is critical, since it has an impact on the time needed by clients to adapt to user movements so that movements are reflected on the retrieved content. In this paper, we analyze the impact of the delay and introduce various means to reduce the impact on the observed fidelity.