Comfort in observing stereoscopic images reduced by vibration stimuli ()
1. INTRODUCTION
The development of display technologies has made the observation of stereoscopic images more accessible. In recent years, we have been able to enjoy stereoscopic images not only at home but also outdoors using portable video equipment (mobile phones, smartphones, game equipment, etc.). Here, the issue in observing stereoscopic images in such a dynamic environment is unexpected vibration given to the images. Stereoscopic imaging devices input the different images to both eyes according to the horizontal disparity. The visual information processing system in the brain fuses those images to enable the observer to perceive a stereoscopic effect. Thus it is preferable that both the projection system and the observer are in stationary condition. Even under this stationary condition, it is known that comfortable observation of stereoscopic images is inhibited by various factors including unexpected rotation, geometric distortion, vertical shift, horizontal shift, color shift, difference in brightness and so on between left and right eyes [1-11]. It is considered necessary to examine the kind of discomfort that occurs under the condition where both the display and the observer move. Some previous studies examined the comfort or discomfort that vibration stimuli themselves give to the human body [12], but there is no previous study on the effect of body vibration on observation of the stereoscopic images.
Only a few studies discussed that the biomedical effect of common 3D movies (dynamic) on observers; Speranza et al. [6] studied accommodative asthenopia using with simply oscillated objects in depth direction. Yano et al. [8] presented a common 3D movie to observers and estimated the effect of disparity changing included in the movie on the function of accommodation. In our study, on the other hand, we presented vibration stimuli irrelevant to contents of the movie for disturbing fusion right and left images to obtain 3D perception. This procedure assumed the daily life situation in which unexpected vibration stimuli possibly occur with observing stereoscopic image such as in mobile display. We also measured both psychological and physiological indexes continuously under such a successive observation, and this is another appeal point of this paper.
In this experiment we presented the observation disturbing factor, which does not exist during typical stereoscopic image observation, by placing a stereoscopic image projector on a vibration table. Two types of vibration stimuli, relatively high (20 Hz) and low (5 Hz) frequencies, were provided to the stereoscopic images. All subjects observed the images provided with two types of vibration stimuli in random order, taking approximately a one-hour break between the two observations. Two types of psychological reactions and one physiological reaction were measured while the subjects observed an approximately twenty-minute stereoscopic movie. The psychological reaction was measured using a questionnaire (SSQ, [13]) on subjective symptoms concerning motion sickness before and after observation. The other psychological measurement was conducted on subjective symptoms concerning the comfort level, which was asked every two minutes during the observation of the movie. Physiological reaction was measured based on the activity status of the autonomic nervous system derived from electrocardiograms, which were continuously measured during observation.
2. METHODS
2.1. Subjects
Forty-four male and female university students (average age: 22.36 ± 1.78 years old) participated in the experiment. All subjects received payment for their participation. The subjects did not know the purpose of the experiment. Written consent was obtained from each subject for participation in the experiment before starting the study. The details of experiment were approved by the Human Engineering Ethics Committee.
2.2. Apparatus
The experiment used commercially available Blu-ray disc software for a marine documentary movie (Deep Sea, Warner Home Video). The movie was reproduced using a Blu-ray disc player (BDP-S470, SONY) and projected shown onto a hundred-inch screen using a 3D projector (VPL-VW90ES, SONY) based on a liquid crystal shutter glasses method. Sound was provided to subjects through headphones from the Blu-ray disc player. A 3D projector was set on a vibration table (BF-50UT, IDEX, Figure 1) to provide vibration stimuli. The apparatus enables vibration stimuli to be reproduced in the directions of x, y and z axes at right angle by rotating a cylinder inside. Programmable setting by seconds is possible for the frequency, start time and stop time of vibration stimuli. Figure 2 shows the frequencies of vibration derived from frequency analysis of acceleration information during the experiment. During observation electrocardiograms were derived using Ag-Cl electrodes attached to the chest and abdomen of the subjects and recorded using BIOPAC Systems (Monte System). The sampling frequency was 1KHz. Subjective evaluations on comfort, which were asked every 2 minutes during observation, were also recorded using a digital switch of BIOPAC Systems.
Figure 1. A vibration table and a stereoscopic image projector.
Figure 2. The spectrogram of acceleration in the directions of three axes obtained by FFT of thevibration stimuli adopted in this experiment.
2.3. Procedure
The experiment was conducted in a darkened room. After attaching the electrodes for the electrocardiogram, subjects sat at a distance of 3.74 m from the screen. This distance complies with a guideline for maintaining comfort when observing stereoscopic movies [14] and it corresponds to three times the height of the screen. All subjects observed the same stereoscopic movies provided with vibration stimuli of two different frequencies (5 Hz and 20 Hz) in random order. Single observation time was approximately twenty minutes and subjects rested for approximately one hour between the first and second observations. Vibration stimuli were provided five times every two minutes and thirty seconds from five minutes after the beginning of the movie. The vibration durations were five seconds in 5 Hz and fifteen seconds in 20 Hz. These stimuli were confirmed to provide qualitatively different impressions to subjects in the preliminary study where the first author and three naive subjects participated. 5 Hz vibration stimulus slowly and greatly vibrated the whole screen. 20 Hz vibration stimulus, on the other hand, caused fine vibration, which blurred the whole screen. In both cases, the stereoscopic effect was not lost.
The subjects were asked to complete a simulator sickness questionnaire (SSQ) before and after viewing the twenty-minute movie as one of the psychological evaluations on a stereoscopic movie. The SSQ questionnaire consisted of 18 questions and four choices of answers (none, slight, moderate, severe) on movie sickness. The internal status of the observer can be outlined in three categories (nausea, eye fatigue and disorientation) and total scores by placing a specified weight on the raw score.
A direct evaluation of comfort while observing the stereoscopic movie on a five-level scale (very uncomfortable, rather uncomfortable, neither, rather comfortable, very comfortable) was conducted as the other psychological evaluation. The subjects indicated their subjective evaluation in response to a beep sound every 2 minutes and 30 seconds from 10 seconds after the start of movie using a switchbox with five channels placed on their knees.
Figure 3 shows the time schedule of these vibration stimuli and the subjective rating. The broken lines indicate the timing of the subjective rating and the width of the rectangular solid indicates the duration of the vibration stimuli.