Conventional stereoscopic displays present images on a single focal plane. The resulting mismatch between the stimuli to the eyes' focusing response (accommodation) and to convergence causes fatigue and poor stereo performance. One solution is to distribute image intensity across a number of widely spaced image planes-a technique referred to as depth filtering. Previously, we found this elicits accurate, continuous monocular accommodation responses with image-plane separations as large as 1.1 Diopters (D, the reciprocal of distance in meters), suggesting that a small number of image planes could eliminate vergence-accommodation conflicts over a large range of simulated distances. Evidence exists, however, of systematic differences between accommodation responses to binocular and monocular stimuli when the stimulus to accommodation is degraded, or at an incorrect distance. We examined the minimum image-plane spacing required for accurate accommodation to binocular depth-filtered images. We compared accommodation and vergence responses to changes in depth specified by depth filtering, using image-plane separations of 0.6 to 1.2 D, and equivalent real stimuli. Accommodation responses to real and depth-filtered stimuli were equivalent for image-plane separations of ∼ 0.6 to 0.9 D, but differed thereafter. We conclude that depth filtering can be used to precisely match accommodation and vergence demand in a practical stereoscopic display.
Conventional stereoscopic displays present conflicting stimuli to vergence and accommodation, causing fatigue,
discomfort, and poor stereo depth perception. One promising solution is 'depth filtering', in which continuous variations
in focal distance are simulated by distributing image intensity across multiple focal planes. The required image-plane
spacing is a critical parameter, because there are constraints on the total number that can be used. Depth-filtered images
have been shown to support continuous and reasonably accurate accommodation responses with 1.1 dioptre (D) image-plane
spacings. However, retinal contrast is increasingly attenuated with increasing image-plane separation. Thus, while
such stimuli may eliminate the vergence-accommodation conflict, they may also unacceptably degrade stereoscopic
depth perception. Here we measured stereoacuity, and the time needed for stereoscopic fusion, for real targets and depth-filtered
approximations to the same stimuli (image-plane spacings of 0.6, 0.9 and 1.2 D). Stereo fusion time was
reasonably consistent across conditions. Stereoacuity for depth-filtered stimuli was only slightly poorer than for real
targets with 0.6 D image-plane separation, but deteriorated rapidly thereafter. Our results suggest that stereoscopic depth
perception, not accommodation and vergence responses, is the limiting factor in determining acceptable image-plane
spacing for depth-filtered images. We suggest that image-plane spacing should ideally not exceed ~0.6 D.
Conventional stereoscopic displays present images on a single focal plane. The resulting mismatch between the stimuli
to the eyes' focusing response (accommodation) and to convergence causes fatigue and poor stereo performance. One
promising solution is to distribute image intensity across a number of relatively widely spaced image planes - a
technique referred to as depth filtering. Previously, we found this elicits accurate, continuous monocular accommodation
responses with image-plane separations as large as 1.1 Diopters, suggesting that a relatively small (i.e. practical)
number of image planes is sufficient to eliminate vergence-accommodation conflicts over a large range of simulated
distances. However, accommodation responses have been found to overshoot systematically when the same stimuli are
viewed binocularly. Here, we examined the minimum image-plane spacing required for accurate accommodation to
binocular depth-filtered images. We compared accommodation and vergence responses to step changes in depth for
depth-filtered stimuli, using image-plane separations of 0.6-1.2 D, and equivalent real stimuli. Accommodation
responses to real and depth-filtered stimuli were equivalent for image-plane separations of ~0.6-0.9 D, but inaccurate
thereafter. We conclude that depth filtering can be used to precisely match accommodation and vergence demand in a
practical stereoscopic display, using a relatively small number of image planes.
Conventional stereoscopic displays present images at a fixed focal distance. Depth variations in the depicted scene
therefore result in conflicts between the stimuli to vergence and to accommodation. The resulting decoupling of
accommodation and vergence responses can cause adverse consequences, including reduced stereo performance,
difficulty fusing binocular images, and fatigue and discomfort. These problems could be eliminated if stereo displays
could present correct focus cues. A promising approach to achieving this is to present each eye with a sum of images
presented at multiple focal planes, and to approximate continuous variations in focal distance by distributing light energy
across image planes - a technique referred to as depth-filtering1. Here we describe a novel multi-plane display in which
we can measure accommodation and vergence responses. We report an experiment in which we compare these
oculomotor responses to real stimuli and depth-filtered simulations of the same distance. Vergence responses were
generally similar across conditions. Accommodation responses to depth-filtered images were inaccurate, however,
showing an overshoot of the target, particularly in response to a small step-change in stimulus distance. This is
surprising because we have previously shown that blur-driven accommodation to the same stimuli, viewed monocularly,
is accurate and reliable. We speculate that an initial convergence-driven accommodation response, in combination with a
weaker accommodative stimulus from depth-filtered images, leads to this overshoot. Our results suggest that
stereoscopic multi-plane displays can be effective, but require smaller image-plane separations than monocular
accommodation responses suggest.
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