In optics in general, a sharp aberration-free image is normally the desired goal, and the whole field of adaptive optics has developed with the aim of producing blur-free images. Likewise, in ophthalmic optics we normally aim for a sharp image on the retina. But even with an emmetropic, or well-corrected eye, chromatic and high order aberrations affect the image. We describe two different areas where it is important to take these effects into account and why creating blur correctly via rendering can be advantageous. Firstly we show how rendering chromatic aberration correctly can drive accommodation in the eye and secondly report on matching defocus-l generated using rendering with conventional optical defocus.
Stereoscopic 3D (S3D) displays provide an enhanced sense of depth by sending different images to the two eyes. But these displays do not reproduce focus cues (blur and accommodation) correctly. Specifically, the eyes must accommodate to the display screen to create sharp retinal images even when binocular disparity drives the eyes to converge to other distances. This mismatch causes discomfort, reduces performance, and distorts 3D percepts. We developed two techniques designed to reduce vergence-accommodation conflicts and thereby improve comfort, performance, and perception. One uses focus-tunable lenses between the display and viewer’s eyes. Lens power is yoked to expected vergence distance creating a stimulus to accommodation that is consistent with the stimulus to vergence. This yoking should reduce the vergence-accommodation mismatch. The other technique uses a fixed lens before one eye and relies on binocularly fused percepts being determined by one eye and then the other, depending on simulated distance. This is meant to drive accommodation with one eye when simulated distance is far and with the other eye when simulated distance is near. We conducted performance tests and discomfort assessments with both techniques and with conventional S3D displays. We also measured accommodation. The focus-tunable technique, but not the fixed-lens technique, produced appropriate stimulus-driven accommodation thereby minimizing the vergence-accommodation conflict. Because of this, the tunable technique yielded clear improvements in comfort and performance while the fixed technique did not. The focus-tunable lens technique therefore offers a relatively easy means for reducing the vergence-accommodation conflict and thereby improving viewer experience.
Common stereoscopic 3D (S3D) displays utilize either spatial or temporal interlacing to send different images to each
eye. Temporal interlacing sends content to the left and right eyes alternatingly in time, and is prone to artifacts such as
flicker, unsmooth motion, and depth distortion. Spatial interlacing sends even pixel rows to one eye and odd rows to the
other eye, and has a lower effective spatial resolution than temporal interlacing unless the viewing distance is large. We
propose a spatiotemporal hybrid protocol that interlaces the left- and right-eye views spatially, but the rows
corresponding to each eye alternate every frame. We performed psychophysical experiments to compare this novel
stereoscopic display protocol to existing methods in terms of spatial and temporal properties. Using a haploscope to
simulate the three protocols, we determined perceptual thresholds for flicker, motion artifacts, and depth distortion, and
we measured the effective spatial resolution. Spatial resolution is improved, flicker and motion artifacts are reduced, and
depth distortion is eliminated. These results suggest that the hybrid protocol maintains the benefits of spatial and
temporal interlacing while eliminating the artifacts, thus creating a more realistic viewing experience.
Prolonged use of conventional stereo displays causes viewer discomfort and fatigue because of the vergenceaccommodation
conflict. We used a novel volumetric display to examine how viewing distance, the sign of the vergenceaccommodation
conflict, and the temporal properties of the conflict affect discomfort and fatigue. In the first experiment,
we presented a fixed conflict at short, medium, and long viewing distances. We compared subjects’ symptoms in that
condition and one in which there was no conflict. We observed more discomfort and fatigue with a given vergenceaccommodation conflict at the longer distances. The second experiment compared symptoms when the conflict had one sign compared to when it had the opposite sign at short, medium, and long distances. We observed greater symptoms with uncrossed disparities at long distances and with crossed disparities at short distances. The third experiment compared symptoms when the conflict changed rapidly as opposed to slowly. We observed more serious symptoms when the conflict changed rapidly. These findings help define comfortable viewing conditions for stereo displays.
In the conventional temporally interlaced S3D protocol, red, green, and blue are presented simultaneously to one eye and then to the other eye. Thus, images are presented in alternating fashion to the two eyes. Moving objects presented with this protocol are often perceived at incorrect depth relative to stationary parts of the scene. We implemented a colorinterlaced protocol that could in principle minimize or even eliminate such depth distortions. In this protocol, green is presented to one eye and red and blue to the other eye at the same time. Then red and blue are presented to the first eye and green to the second. Using a stereoscope, we emulated the color-interlaced protocol and measured the magnitude of depth distortions as a function of object speed. The results showed that color interlacing yields smaller depth distortions than temporal interlacing in most cases and never yields larger distortions. Indeed, when color interlacing produces no brightness change within sub-frames, the distortions are eliminated altogether. The results also show that the visual system’s calculation of depth from disparity is based on luminance, not chromatic signals. In conclusion, color interlacing provides great potential for improved stereo presentation.
Properly constructed stereoscopic images are aligned vertically on the display screen, so on-screen binocular
disparities are strictly horizontal. If the viewer's inter-ocular axis is also horizontal, he/she makes horizontal
vergence eye movements to fuse the stereoscopic image. However, if the viewer's head is rolled to the side, the onscreen
disparities now have horizontal and vertical components at the eyes. Thus, the viewer must make horizontal
and vertical vergence movements to binocularly fuse the two images. Vertical vergence movements occur naturally,
but they are usually quite small. Much larger movements are required when viewing stereoscopic images with the
head rotated to the side. We asked whether the vertical vergence eye movements required to fuse stereoscopic
images when the head is rolled cause visual discomfort. We also asked whether the ability to see stereoscopic depth
is compromised with head roll. To answer these questions, we conducted behavioral experiments in which we
simulated head roll by rotating the stereo display clockwise or counter-clockwise while the viewer's head remained
upright relative to gravity. While viewing the stimulus, subjects performed a psychophysical task. Visual discomfort
increased significantly with the amount of stimulus roll and with the magnitude of on-screen horizontal disparity.
The ability to perceive stereoscopic depth also declined with increasing roll and on-screen disparity. The magnitude
of both effects was proportional to the magnitude of the induced vertical disparity. We conclude that head roll is a
significant cause of viewer discomfort and that it also adversely affects the perception of depth from stereoscopic
The vergence-accommodation conflict associated with viewing stereoscopic 3D (S3D) content can cause visual
discomfort. Previous studies of vergence and accommodation have shown that the coupling between the two responses is
driven by a fast, phasic component. We investigated how the temporal properties of vergence-accommodation conflicts
affect discomfort. Using a unique volumetric display, we manipulated the stimulus to vergence and the stimulus to
accommodation independently. There were two experimental conditions: 1) natural viewing in which the stimulus to
vergence was perfectly correlated with the stimulus to accommodation; and 2) conflict viewing in which the stimulus to
vergence varied while the stimulus to accommodation remained constant (thereby mimicking S3D viewing). The
stimulus to vergence (and accommodation in natural viewing) varied at one of three temporal frequencies in those
conditions. The magnitude of the conflict was the same for all three frequencies. The young adult subjects reported more
visual discomfort when vergence changes were faster, particularly in the conflict condition. Thus, the temporal
properties of the vergence-accommodation conflict in S3D media affect visual discomfort. The results can help content
creators minimize discomfort by making conflict changes sufficiently slow.
Prolonged use of conventional stereo displays causes viewer discomfort and fatigue because of the vergenceaccommodation
conflict. We used a novel volumetric display to examine how viewing distance and the sign of the
vergence-accommodation conflict affect discomfort and fatigue. In the first experiment, we presented a fixed conflict at
short, medium, and long viewing distances. We compared subjects' symptoms in that condition and one in which there
was no conflict. We observed more discomfort and fatigue with a given vergence-accommodation conflict at the longer
distances. The second experiment compared symptoms when the conflict had one sign compared to when it had the
opposite sign at short, medium, and long distances. We observed greater symptoms with uncrossed disparities at long
distances and with crossed disparities at short distances. These findings help define comfortable viewing conditions for
In stereo displays, binocular disparity creates a striking impression of depth. However, such displays
present focus cues-blur and accommodation-that specify a different depth than disparity, thereby
causing a conflict. This conflict causes several problems including misperception of the 3D layout,
difficulty fusing binocular images, and visual fatigue. To address these problems, we developed a
display that preserves the advantages of conventional stereo displays, while presenting correct or
nearly correct focus cues. In our new stereo display each eye views a display through a lens that
switches between four focal distances at very high rate. The switches are synchronized to the
display, so focal distance and the distance being simulated on the display are consistent or nearly
consistent with one another. Focus cues for points in--between the four focal planes are simulated by
using a depth-weighted blending technique. We will describe the design of the new display, discuss
the retinal images it forms under various conditions, and describe an experiment that illustrates the
effectiveness of the display in maximizing visual performance while minimizing visual fatigue.
When a picture is viewed from positions other than its center of projection, there can be large changes specified in the retinal image, yet the perceived spatial layout and shape of objects do not seem to change. We have shown that compensation for oblique viewing occurs provided that the viewer can estimate the slant and tilt of the picture surface accurately (Vishwanath, Girshick, & Banks, 2004). Compensation is nearly veridical with binocular viewing at close range. Compensation generally does not occur with monocular viewing through a small aperture; instead, the percept is dictated by the shape of the retinal image. The mechanism for compensation appears to operate locally; that is, separately for each part of the picture. Our findings help to explain invariance for incorrect viewing positions, and other phenomena like perceived distortions with wide fields of view and the anamorphic effect. Our findings also have relevance to the design of displays. We will discuss, for example, how the viewer’s position ought to affect percepts depending on the shape of the display surface.
As the disparity gradient of a stimulus increases, human observers’ ability to solve the correspondence problem and thereby estimate the disparities becomes poorer. It finally fails altogether when a critical gradient - the disparity-gradient limit (Burt & Julesz, 1980)- is reached. We investigated the cause of the disparity-gradient limit. As part of this work, we developed a local cross-correlator similar to ones proposed in the computer vision literature and similar to the disparity-energy model of neurons in area V1. Like humans, the cross-correlator exhibits poorer performance as the disparity gradient increases. We also conducted a psychophysical experiment in which observers were presented sawtooth waveforms defined by disparity. They made spatial phase discriminations. We presented different corrugation spatial frequencies and amplitudes, and measured observers’ ability to discriminate the two phases. Coherence thresholds (the proportion of signal dots at threshold relative to the total number of dots in the stimulus) were well predicted by the disparity gradient and not by either the spatial frequency or amplitude of the corrugation waveform. Thus, human observers and a local cross-correlator exhibit similar behavior, which suggests that humans use such an algorithm to estimate disparity. As a consequence, disparity estimation is done with local estimates of constant disparity (piecewise frontal), which places a constraint on the highest possible stereo resolution.
Focus cues specify inappropriate 3-D scene parameters in conventional displays because the light comes from a single surface, independent of the depth relations in the portrayed scene. This can lead to distortions in perceived depth, as well as discomfort and fatigue due to the differing demands on accommodation and vergence. Here we examine the efficacy of a stereo-display prototype designed to minimize these problems by using multiple image planes to present near-correct focus cues. Each eye’s view is the sum of several images presented at different focal distances. Image intensities are assigned based on the dioptric distance of each image plane from the portrayed object, determined along visual lines. The stimulus to accommodation is more consistent with the portrayed depth than with conventional displays, but it still differs from the stimulus in equivalent real scenes. Compared to a normal, fixed-distance display, observers showed improved stereoscopic performance in different psychophysical tasks including speed of fusing stereoscopic images, precision of depth discrimination, and accuracy of perceived depth estimates. The multiple image-planes approach provides a practical solution for some shortcomings of conventional displays.