  Visually-induced motion sickness (VIMS), a side effect of perceived motion
caused by visual stimulation, is a major obstacle to the widespread use of
Virtual Reality (VR). Along with scene object information, visual stimulation
can be primarily indicated by optical flow, which characterizes the motion
pattern, such as the intensity and direction of the moving image. We estimated
the real time optical flow in 360-degree videos targeted at immersive user
interactive visualization based on the user's current viewport. The proposed
method allows the estimation of customized visual flow for each experience of
dynamic 360-degree videos and is an improvement over previous methods that
consider a single optical flow value for the entire equirectangular frame. We
applied our method to modulate the opacity of granulated rest frames (GRFs), a
technique consisting of visual noise-like randomly distributed visual
references that are stable to the user's body during immersive pre-recorded
360-degree video experience. We report the results of a pilot one-session
between-subject study with 18 participants, where users watched a 2-minute
high-intensity 360-degree video. The results show that our proposed method
successfully estimates optical flow, with pilot data showing that GRFs combined
with real-time optical flow estimation may improve user comfort when watching
360-degree videos. However, more data are needed for statistically significant
results.
