Optimizing storage and delivery of Omnidirectional Videos in Viewport-dependent streaming

Abstract

The OMAF standard makes use of a framework called the viewport-dependent-delivery for the streaming of 360-degree videos. OMAF uses ISOBMFF for storage and MPEG-DASH as one of the delivery mechanisms. In viewport-dependent-streaming videos are spatially divided and encoded into multiple tracks and each track is further segmented for DASH delivery. Segmentation requires additional metadata which adds to bitrate overhead. The main contributor to this overhead is the track fragment run in a box with the four-character code, ‘trun’. The TRUN records the following information of each sample in a track: the size, duration, flags, and time offsets and uses a fixed byte size to record this information. To minimize the bitrate overhead of TRUN, four different representation algorithms have been explored. This paper briefly describes the four TRUN representations and discusses the benefits and drawbacks of each algorithm. For evaluation, the algorithms were implemented in the MP4BOX module of the GPAC suite. The results were evaluated for different segment durations (500ms, 1s, 2s, 4s), different tiling grids (8x4, 9x6), two videos (bip-bop, countertiles) with different packaging techniques (no encryption, encryption of Keyframes, encryption of all frames) The algorithms reduced the bitrate overhead by 59% on average as compared to the original TRUN representation.