  We provide the first perceptual quantification of user's sensitivity to
radial optic flow artifacts and demonstrate a promising approach for masking
this optic flow artifact via blink suppression. Near-eye HMDs allow users to
feel immersed in virtual environments by providing visual cues, like motion
parallax and stereoscopy, that mimic how we view the physical world. However,
these systems exhibit a variety of perceptual artifacts that can limit their
usability and the user's sense of presence in VR. One well-known artifact is
the vergence-accommodation conflict (VAC). Varifocal displays can mitigate VAC,
but bring with them other artifacts such as a change in virtual image size
(radial optic flow) when the focal plane changes. We conducted a set of
psychophysical studies to measure users' ability to perceive this radial flow
artifact before, during, and after self-initiated blinks. Our results showed
that visual sensitivity was reduced by a factor of 10 at the start and for ~70
ms after a blink was detected. Pre- and post-blink sensitivity was, on average,
~0.15% image size change during normal viewing and increased to ~1.5-2.0%
during blinks. Our results imply that a rapid (under 70 ms) radial optic flow
distortion can go unnoticed during a blink. Furthermore, our results provide
empirical data that can be used to inform engineering requirements for both
hardware design and software-based graphical correction algorithms for future
varifocal near-eye displays. Our project website is available at
https://gamma.umd.edu/RoF/.
