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3.2 Visual comfort and discomfort in viewing stereoscopic images

3.2.1 Discrepancies between left and right images


Here we present the results of evaluating visual comfort/discomfort in viewing stereoscopic images from the perspective of discrepancies between left and right images in the performance or characteristics of image capture and display equipment. A series of experiments was conducted on discrepancies in size, verticality, inclination, and brightness, as well as on cross-talk regarding discomfort. Natural images of HDTV were mainly used for the evaluation. The major findings of the studies are summarized in Table 8. The values represent the results of a subjective evaluation test on the five-grade impairment scale.
TABLE 8

Research on impairment caused by discrepancies and cross-talk between L/R stereo images

Factor

Image characteristic

Detection limit

Tolerance limit

Note

Reference

L/R image discrepancy

Geometric discrepancy

Size

1.2 %

2.9%

Taking size of one image as 100%

[1]

Vertical displacement

0.7%

1.5%

Taking image height
as 100%

Rotation

0.5 degrees

1.1 degrees

Angle of rotation about image centre

Brightness discrepancy

White clip level

70%

60%

Taking 100 IRE white level as 100%

[2]

Black clip level

1%

2%

Taking 100 IRE white level as 100%

Cross-talk

Contrast ratio of 100:1
in tests

1%-2%

5%-10%

Luminance ratio

[5]

TABLE 9


Research on parallax distribution, parallax range and parallax change over time

Factor

Evaluation item

Experimental results

Reference

Parallax distribution within image (frame)

Distribution pattern

Easily viewable if parallax is shaped far at top of image, near at bottom

[7]

Range of parallax

Comfortable to view if parallax range is within 60 min.

[8]

Subtitle position

Preferable if subtitle positions 10-15 min. in front of background image

[9]

Parallax change
over time

Scene cuts

Uncomfortable to view if temporal change of parallax exceeds 60 min.

[9]

When shooting and displaying stereoscopic 3DTV programmes, there can be geometrical distortions, such as size inconsistency, vertical shift, and rotation error, between left and right images [1]. Results of experiments where three kinds of distortions occurred independently from each other are shown in Table 8.

As differences between left and right images in amplitude and offset can be corrected but clipped white or black levels cannot, the degree of interference when the white or black level of one of the left and right images is clipped was evaluated. The evaluation results on detection and tolerance limits were reported to depend on image content [2]. Some of the earlier studies on differences in brightness and contrast are given by References [3] and [4].

Stereoscopic image cross-talk, in which the left and right images “leak” and can be partially seen by the opposite eye, can also result in discomfort for the viewer, and experiments evaluating it have also been done. These values on detection and tolerance limits were reported to be highly dependent on image content and display contrast, and cross talk must be reduced further still on high contrast displays [5]. One of earlier studies on cross-talk is given by Reference [6].


3.2.2 Depth range, distribution and change in parallax


Cases of extreme parallax or sudden changes in parallax cause visual discomfort, so it is important to manage parallax with special care when producing programmes with stereoscopic images. Results of experiments subjectively evaluating the relationship between visual comfort and the distribution of parallax in images, and the change in parallax before and after scene cuts are shown in Table 9. The positioning of subtitles has also been evaluated. All tests were done under standard viewing conditions for HDTV. The range of parallax for visual comfort has already been reported in § 3.1 of Annex 4.
References

[1] H. YAMANOUE, M. NAGAYAMA, M. BITOU, J. TANADA, T. MOTOKI, T. MITSUHASHI, and M. HATORI, “Tolerance for Geometrical Distortions between L/R images in 3D-HDTV,” Systems and Computers in Japan, Vo1. 29, No. 5, 1998.

[2] A. HANAZATO, M. OKUI, and F. OKANO, “Evaluation of Stereoscopic Images Impaired by the Differences in Black/White Levels,” ITE Annual Convention 2001, 4-5, pp. 49-50 (2001) (in Japanese).

[3] J. FOURNIER and T. ALPERT, “Human factor requirements for a stereoscopic television service – Admissible contrast differences between the two channels of a stereoscopic camera,” Proc. SPIE, Vol. 2177, pp. 45-54, 1994.

[4] P. BELDIE and B. KOST, “Luminance Asymmetry in Stereo TV Images, ” SPIE, Vol. 1457, pp. 242‑247, 1991.

[5] A. HANAZATO, M. OKUI, and I. YUYAMA, “Subjective Evaluation of Cross Talk Disturbance in Stereoscopic Displays,” The 20th International Display Research Conference, pp. 288-291, Sep. 2000.

[6] S. PATOOR, “Human Factors of 3D imaging: Results of Recent Research at Heinrich‑Hertz-Institut Berlin,” Proc. of the 2nd International Display Workshops (IDW’95), Vol. 3, 3D-7, pp. 69-72, 1995.

[7] S. IDE, H. YAMANOUE, M. OKUI, F. OKANO, M. BITOU, and N. TERASHIMA, “Parallax distribution for ease of viewing in stereoscopic HDTV,” Proc. SPIE, Vol. 4660, pp. 38‑45, 2002.

[8] Y. NOJIRI, H. YAMANOUE, A. HANAZATO, and F. OKANO, “Measurement of parallax distribution, and its application to the analysis of visual comfort for stereoscopic HDTV,” Proc. SPIE, Vol. 5006, pp. 195-205, 2003.

[9] Y. NOJIRI, H. YAMANOUE, A. HANAZATO, M. EMOTO, and F. OKANO, “Visual comfort/discomfort and visual fatigue caused by stereoscopic HDTV viewing,” Proc. SPIE, Vol. 5291, pp. 303-313, 2004.

[10] YAMANOUE, H., OKUI, M. and OKANO, F., “Geometrical Analysis of Puppet Theatre and Cardboard Effects in Stereoscopic HDTV Images”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 16, No. 6, p. 744-752, 2006.

[11] SPOTTISWOODE, R., SPOTTISWOODE, N.L. and SMITH, C., “Basic principles of the three‑dimensional film”, SMPTE J., Vol. 59, p. 249-286, 1952.

[12] MACADAM, D.L., “Stereoscopic perceptions of size shape distance and directions”, SMPTE J., Vol. 62, pp. 271-289, 1954.

[13] YAMANOUE, H., “The Relation between Size Distortion and Shooting Conditions for Stereoscopic Images”, SMPTE Journal, Vol. 106, No. 4, pp. 225-232, 1997.

[14] HERMAN, S., “Principles of binocular 3D displays with application to television”, SMPTE J., Vol. 80, pp. 539-544, 1971.

[15] YAMANOUE, H., OKUI, M. and YUYAMA, I., “A study on the relationship between shooting conditions and cardboard effect of stereoscopic images”, IEEE Trans. Circuits and Systems for Video Technology, Vol. 10, No. 3, pp. 411-416, 2000.

[16] NOJIRI, Y., YAMANOUE, H., HANAZATO, A. and OKANO, F., “Measurement of parallax distribution, and its application to the analysis of visual comfort for stereoscopic HDTV”, Proc. SPIE, Vol. 5006, pp. 195-205, 2003.

4 Visual fatigue in viewing stereoscopic images

4.1 Experimental results on inconsistency between vergence and accommodation


It has been confirmed through subjective evaluation that viewing stereoscopic images can result in a great degree of visual fatigue compared to viewing 2D ones. On this issue, it has been shown that changes in visual performance can be observed before and after viewing stereoscopic images, and that the fusional amplitude (the parallax range over which viewers can fuse left and right images) in particular decreases. The experimental result suggested the possibility that a fusional amplitude can be one of indices for evaluating visual fatigue [1].

Here, conditions in which vergence and accommodation are not consistent were reproduced using specialized equipment, and an objective evaluation of visual fatigue was obtained by measuring fusional amplitude before and after using these glasses for one hour. Results of these trials are shown in Figs 14 and 15. In both experiments, a common one-hour HDTV programme was used. The figures indicate mean values of ratios of the relative range of convergence (a relative value using the fusional amplitude before viewing as the basis), with small values indicating a narrower fusional amplitude.

The results from viewing a flat image in 3D for one hour with a fixed amount of parallax (such that the image is displayed in front or behind the screen) are shown in Fig. 14. The results of changing the parallax over time with the vergence and accommodation being consistent in one case, and not consistent in the other are shown in Fig. 15. Parallax was changed 16 times over a period of two minutes, and this was repeated 29 times. In both experiments, the same one-hour HDTV programme was used.

A large change in ratio value was observed when viewing images with parallax than when viewing 2D ones, and even more fatigue was caused by the time-varying images with inconsistencies between vergence and accommodation. These results indicate that inconsistencies between vergence and accommodation as well as fluctuations in time due to parallax, can be factors causing visual fatigue [2].


4.2 Experimental results on parallax amount and lateral/depth motion


A series of experiments have also been done to examine the relationship between this issue of inconsistency between vergence and accommodation, and depth-of-focus of the eyes.

First, the accommodation responses were measured [3]. Subjects viewed image content for approximately one hour on a 120-inch screen at a viewing distance of 4.5 m. The 3D image contents were two motion video sequences, and parallax for the video displayed under the conditions of the experiment was within the depth of field in almost all cases, 2D video was also used for reference.

The result was assumed significant for visual fatigue when the change of amplitude of the accommodation response in the before and after viewing was bigger than 0.5 diopters. From this aspect the results of comparing accommodation response before and after viewing showed no significant difference for the 2D images, whereas for the 3D images, three of five subjects for the first image sequence and two of five subjects for the second sequence showed visual fatigue. On the basis of these results and the fact that the video used was within the depth of focus, it is presumed that causes of visual fatigue other than depth of focus must also be considered.

Next, experiments were done to compare images in different amount of parallax, as well as 3D images with and without motion [4]. In the experiments, Japanese text with added parallax was displayed on a field-sequential 3D HD monitor (28-inch diagonal). From a viewing distance of 3H, subjects read for approximately one hour while turning pages using a mouse. This was repeated several times, changing the amount of parallax. Experiments were also done with moving text. Two types of text motion were tested: forward and backward in the depth direction, and horizontal motion. Here, the amount of parallax was limited within the depth of focus. The timing of motion was generated from an existing 3D programme.



The degree of fatigue was estimated on the basis of subjective evaluation and accommodation response. The subjective evaluation results are presented in Fig. 16. The figure shows there was a significant difference at the parallax of 1.36 degrees whereas at +1.36 degrees there was not, i.e. there was a large variance. Fatigue was also inferred when there was motion in the depth direction. These results suggest that changes in the depth direction can cause visual fatigue even when the amount of parallax remains within the depth of focus.


Figure 14

Fusional amplitude after viewing image with
large binocular parallax



Figure 15

Fusional amplitude after viewing image with time
fluctuations in binocular parallax



Figure 16

Subjective evaluation of visual fatigue while changing amount of parallax
and stationary vs. moving objects




4.3 Evaluation of fatigue caused by watching 3DTV

4.3.1 Experiment


From January to March 2011, evaluation experiments were conducted consisting of 500 adult participants watching 3D content for approximately one hour on commercially available 46 to 50‑inch 3DTVs that require the use of shutter glasses. The degree of fatigue after watching the 3DTV was evaluated under various viewing conditions.

The 3D content used in the experiment consisted of seven kinds of programmes (documentary, sports, music clip, animation, etc.) whose binocular disparities were mostly less than one degree. This content was recorded with a hard disk recorder (1 920 × 1080 60 i/10 bit/4:2:2) in the 3DTV format referred to as Side-by-Side, where the horizontal resolution of the HDTV image is reduced by a half.

Six types of viewing conditions were set, i.e. four viewing conditions in which participants watched 3D content with glasses in front of a 3DTV at three different distances (two, three and five times the screen’s height) and from an oblique position of 40 degrees, and two other control conditions in which participants watched 2D content with or without glasses in front of a 3DTV at a distance of three times the screens height.

The participants were 500 women and men aged between 20 and 69 years old. Each participant watched 3DTV in one viewing condition only (Between-Group Design).

Visual acuity, Critical Flicker Frequency (CFF) [5], and the Advanced Trail Making Test (ATMT) [6] were used as objective indexes of fatigue, whereas the Simulator Sickness Questionnaire (SSQ) [7] and Visual Analogue Scale (VAS) [8] were used as subjective indexes. Fatigue caused by watching 3DTV was evaluated by the differences between those indexes evaluated before and after watching 3DTV. In addition, the participants answered a questionnaire about their physical conditions after watching TV programmes on the day when the experiment was conducted and the following day.

4.3.2 Results


The average value of each index before and after watching 3DTV was obtained for each viewing condition. A statistical analysis was then conducted to test whether these values show significant differences between different viewing conditions.

The results of the objective indexes indicated that there was no difference between watching 3DTV and traditional TV (i.e. watching 2D content without glasses) in degree of decline of visual and cognitive functions due to fatigue.

On the other hand, the results of subjective indexes indicated that there were some differences between watching 3DTV and traditional TV in the sensation of fatigue. However, these differences may not be attributed to watching 3D content, but to wearing the 3D shutter glasses.

Although there was no difference in the sensation of fatigue between the different conditions when evaluated immediately after watching 3DTV, the results suggest that the sensation of fatigue may be persistent if 3DTV is watched at a distance closer than the standard viewing position (i.e. three times the screen’s height).

It should be noted that the results of the present study were obtained under conditions close to typical viewing situations at home, where the subjects simply watched 3D programmes whose binocular disparities were relatively small on commercially available 46 to 50-inch 3DTVs, and therefore these findings may not be applied to other viewing conditions and 3D content. Yano et al. [4], for instance, evaluated visual fatigue when subjects read the text of Japanese literature on a 28‑inch CRT and reported that changes in depth direction can cause visual fatigue even when the amount of parallax remains within the depth of focus. Emoto et al. [2] evaluated visual fatigue when subjects counted the number of characters of Japanese translations superimposed on German opera and reported that there were differences in the P100 latency [9] of the visual evoked cortical potentials (VECP) between viewing 2D and 3D content, while the subjective evaluation revealed no difference.

References


[1] M. EMOTO, Y. NOJIRI, and F. OKANO [2004] Changes in fusional vergence limit and its hysteresis after viewing stereoscopic TV, Displays, Vol. 25, pp. 67-76.

[2] M. EMOTO, T. NIIDA, and F OKANO [December 2005] Repeated Vergence Adaptation Causes the Decline of Visual Functions in Watching Stereoscopic Television, Journal of Display Technology, Vol. 1, No. 2, pp. 328‑340.

[3] S. YANO, S. IDE, T. MITSUHASHI, and H. THWAITES [2002] A study of visual fatigue and visual comfort for 3D HDTV/HDTV images, Displays, Vol. 23, pp. 191-201.

[4] S. YANO, M. EOMOTO, and T. MITSUHASHI [2004] Two factors in visual fatigue caused by stereoscopic HDTV images, Displays, Vol. 25, pp. 141-150.

[5] Critical Flicker Frequency is a psychophysical measure of visual temporal resolution. It represents the minimal number of flashes per second at which an intermittent light stimulus provides a continuous sensation, which may show some subject's visual fatigue and visual sensory sensitivity. Details of the method are described in the following paper:

T. MITSUHASHI [1996] Evaluation of Stereoscopic Picture Quality with CFF, Ergonomics, Vol. 39, No. 11.

[6] The Advanced Trail Making Test is a method to evaluate mental fatigue. In this test, circles numbered from 1 to 25 are placed randomly on the display and participants are required to use a computer mouse to click these circles in sequence. See the following reference:

O. KAJIMOTO: Development of a Method of Evaluation of Fatigue and its Economic Impacts. In Fatigue Science for Human Health. Edited by: Y. WATANABE, B. EVENGARD, B.H. NATELSON, L.A. JASON, H. KURATSUNE, New York: Springer; 2008:33-46.

[7] The Simulator Sickness Questionnaire (SSQ) has been used in many studies to measure the level of visually-induced motion sickness. The SSQ contains 16 questionnaire items with a four-point scale. See the following reference:

R.S. KENNEDY et al. [1993] Simulation Sickness Questionnaire: An Enhanced Method for Quantifying Simulator Sickness, The International Journal of Aviation Psychology, Vol. 3, No. 3, pp. 203-220.

[8] The Visual Analogue Scale measures subjective symptoms of fatigue where participants indicate the degree of fatigue on a simple visual analogue scale. "Guideline of Clinical Evaluation of Antifatigue" (in Japanese) cited this method as one of the standard evaluation methods of fatigue.

[9] P100 latency is a positive component with approximately 100-ms peak latency of the visual evoked cortical potentials (VECP) that was suggested as a fatigue index in the following study: T. YAMAZAKI, K. KAMIJO and S. FUKUZUMI [1990] Quantitative Evaluation of Visual Fatigue Encountered in Viewing Stereoscopic 3D Displays: Near-Point Distance and Visual Evoked Potential Study”, Proc. SID, Vol. 31, No. 3, pp. 245‑247.


5 Spatial distortion prediction system for 3DTV

5.1 Introduction


Spatial distortion of reproduced stereoscopic images is determined by a combination of factors including programme production techniques, display devices, 3D glasses, viewing conditions, and viewer characteristics. It is highly desirable to predict beforehand the degree and type of spatial distortion of the reproduced stereoscopic images so that more natural and more comfortable stereoscopic images can be presented to viewers.

This document describes a spatial distortion prediction system for a 3DTV (see Ref. [1]). This system calculates the spatial distortion of a reproduced stereoscopic image and predicts the extent of the puppet-theatre and cardboard effects, excessive binocular parallax, and excessive parallax distribution on the basis of the shooting, display, and viewing conditions.


5.2 Spatial distortion in 3DTV


Conditions under which images are captured, displayed, and viewed can contribute to the introduction of spatial distortions, that is, the differences between the real and the reproduced 3D spaces. Some spatial distortions might cause unnatural effects, such as the puppet-theatre effect and the cardboard effect. The puppet-theatre effect is an undesirable miniaturization effect that makes people look like animated puppets; the cardboard effect is a stereoscopic distortion causing an unnatural depth perception, where objects appear flat as if the scene is divided into discrete depth planes. When some objects are close to the camera, the entire stereoscopic image might appear to pop out from the screen and excessive binocular parallax and excessive parallax distribution may occur. Excessive parallax might also arise for background objects, which is known to cause visual discomfort or to prevent binocular fusion.

5.3 Spatial distortion prediction system for 3DTV

5.3.1 Use cases


A system capable of predicting spatial distortion and excessive parallax would be of great benefit to the industry, helping to provide more natural and more comfortable stereoscopic images.

Because a stereoscopic image can only be viewed properly under particular viewing conditions (for most stereoscopic displays), the system can be used to select appropriate shooting parameters for a particular “standard” display/viewing environment. For a programme directed at children, the system might also be used to tailor the shooting parameters to their small interpupillary distance.

When a director intends to emphasize the reproduced depth to make objects jump out from the screen to have an impact on the viewers, the director must manage the shooting conditions to avoid spatial distortion that causes the puppet-theatre effect and excessive parallax distribution. It is particularly difficult to produce the intended stereoscopic images for large displays when shooting on location. This is because a small stereoscopic display or even a 2D display is often used to monitor the stereoscopicity at a close distance or to merely measure the horizontal disparities, resulting in the director choosing the shooting conditions more by trial and error than careful selection. The system would make it possible for the director to control the stereoscopicity accurately and easily.

In a 3DTV broadcast, a broadcaster might have control of the shooting conditions but has little control over the display and viewing conditions. On the other hand, the opposite is true for the viewer. Even so, the system might help identify suitable viewing conditions to recommend to viewers.


5.3.2 System outline


The system calculates the spatial distortion of a reproduced stereoscopic image and predicts the extent of the puppet-theatre and cardboard effects, excessive binocular parallax, and excessive parallax distribution on the basis of the shooting, display, and viewing conditions listed in Table 10. The relationship between the space to be shot (real space) and the space of the reproduced stereoscopic image (reproduced space) is calculated geometrically in terms of their depth and size. The shooting conditions and the right and left images can be obtained from a stereoscopic camera system.

TABLE 10


Parameters for shooting, display, and viewing conditions

Shooting parameters

Display parameters

Viewing parameters

Camera field of view

Camera convergence distance

Camera separation


Screen width

Horizontal offset



Viewing distance

Interpupillary distance


Figure 17 shows a ground plan of real and reproduced space grids without spatial distortion; the real space grid (shown by red dots) and the reproduced space grid (shown by blue dots and texture) coincide.

It should be noted that the real space grid is always displayed as a square. The shooting, display, and viewing conditions are shown in the left pane of the window. The camera field of view and the display-viewing angle are equalized, as are the camera separation, interpupillary distance, and horizontal offset.

The system can measure the parallaxes of up to three objects, namely the object of interest, background, and foreground. Each object’s depth in real space is calculated on the basis of the measured parallax and the shooting conditions. In order to determine the depth range of the space grid for calculating the spatial distortion, the user selects two portions, one at the maximum depth (a desk lamp in this example) and another at the minimum depth (a stuffed animal).

A portion including the object of interest (a woman) should be selected to calculate the extent of the perceived puppet-theatre effect. The depth of the object of interest may be determined as the focus plane.

Figure 17



Screen shots of the spatial distortion prediction system


5.3.3 Examples of conditions and simulations


Figure 18 shows the simulation results obtained under four conditions for which some parameters were changed while other parameters remained the same as in Fig. 17. The extent of the perceived puppet-theatre and cardboard effects is expressed by changing the hue of the texture: magenta is increased in proportion to the extent of the perceived puppet-theatre effect and green is increased in proportion to the extent of the perceived cardboard effect. This system also produces an alert when excessive binocular parallax or parallax distribution is predicted.

Figure 18



Simulation results

NOTE – Magenta colour is increased in proportion to the extent of the perceived puppet-theatre effect. Green colour is increased in proportion to the extent of the perceived cardboard effect.

References

[1] K. MASAOKA, A. HANAZATO, M. EMOTO, H. YAMANOUE, Y. NOJIRI, F. OKANO, “Spatial distortion prediction system for stereoscopic images,” Journal of Electronic Imaging 15(01), 013002, 2006.

Annex 5

Results of subjective visual comfort assessment for motion and disparity magnitude of 3D content – Korea (Republic of)


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