  In contrast with traditional video, omnidirectional video enables spherical
viewing direction with support for head-mounted displays, providing an
interactive and immersive experience. Unfortunately, to the best of our
knowledge, there are few visual quality assessment (VQA) methods, either
subjective or objective, for omnidirectional video coding. This paper proposes
both subjective and objective methods for assessing quality loss in encoding
omnidirectional video. Specifically, we first present a new database, which
includes the viewing direction data from several subjects watching
omnidirectional video sequences. Then, from our database, we find a high
consistency in viewing directions across different subjects. The viewing
directions are normally distributed in the center of the front regions, but
they sometimes fall into other regions, related to video content. Given this
finding, we present a subjective VQA method for measuring difference mean
opinion score (DMOS) of the whole and regional omnidirectional video, in terms
of overall DMOS (O-DMOS) and vectorized DMOS (V-DMOS), respectively. Moreover,
we propose two objective VQA methods for encoded omnidirectional video, in
light of human perception characteristics of omnidirectional video. One method
weighs the distortion of pixels with regard to their distances to the center of
front regions, which considers human preference in a panorama. The other method
predicts viewing directions according to video content, and then the predicted
viewing directions are leveraged to allocate weights to the distortion of each
pixel in our objective VQA method. Finally, our experimental results verify
that both the subjective and objective methods proposed in this paper advance
state-of-the-art VQA for omnidirectional video.
