THERE ARE two primary unwanted effects in 3-D TV:

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1 482 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. 23, NO. 3, MARCH 2013 Visual Comfort Enhancement for Stereoscopic Video Based on Binocular Fusion Characteristics Donghyun Kim, Student Member, IEEE, Sunghwan Choi, Student Member, IEEE, and Kwanghoon Sohn, Member, IEEE Abstract A well-known problem in stereoscopic videos is visual fatigue. However, conventional depth adjustment methods provide little guidance in deciding the amount of depth control or determining the condition for depth control. We propose a depth adjustment method based on binocular fusion characteristics, where the fusion time is used as the parameter for adjustment. Visual comfort enhancement is implemented with the horizontal image shift approach for depth adjustment. The speeded-up robust feature is used to estimate maximum disparity and disparity distribution, while a face detection algorithm is used to estimate the viewing distance with a single web camera. Binocular fusion characteristics are acquired in advance by measuring the time required for fusion under various conditions, including foreground disparity, background disparity, and focal distance for random-dot stereograms. Finally, a subjective evaluation is conducted in fixed and free-to-move viewing conditions, and the results show that comfortable videos were generated based on the proposed depth adjustment method. Index Terms Depth adjustment, stereoscopic video, visual comfort enhancement. I. Introduction THERE ARE two primary unwanted effects in 3-D TV: depth distortion and visual fatigue. Depth distortion is a phenomenon in which watching the scene in stereoscopic display differs from viewing the scene directly. Woods and Koch analyzed various image distortions including depth plane curvature, depth nonlinearity, depth and size magnification, and shearing distortion [1]. Yamanoue et al. provided a geometric analysis of the cardboard effect and the puppet theater effect [2]. Visual fatigue is the second problem of 3-D TV. In [3], the authors stated that 3-D TV can only be a lasting success if the perceived image quality and viewing comfort are comparable to those of conventional television. The visual fatigue induced by the conflict between accommodation and convergence is reviewed in [4], where Hoffman et al. investigated the effect of vergence-accommodation conflicts by measuring the time Manuscript received January 25, 2012; revised April 28, 2012 and June 16, 2012; accepted June 19, Date of publication July 27, 2012; date of current version March 7, This work was supported by the National Research Foundation of Korea, funded by the Korean Government, Ministry of Education, Science and Technology under Grant This paper was recommended by Associate Editor G. Lafruit. The authors are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul , Korea ( kimdonghyun@yonsei.ac.kr; shch@yonsei.ac.kr; khsohn@yonsei.ac.kr). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TCSVT /$31.00 c 2012 IEEE required to identify a stereoscopic stimulus. Emoto et al. showed that repeated vergence adaptation causes decline of visual functions [5]. Shibata et al. examined the effect of the sign of the vergence-accommodation conflicts [6]. Visual fatigue can be caused by a number of other factors, including camera configuration, viewing condition, and image characteristics, such as magnitude of disparity, disparity distribution, vertical disparity, crosstalk, noise, motion, and asymmetric characteristics (luminance, color) [7] [9]. Yano investigated the factors of visual fatigue and reported that even if the image was displayed within a corresponding range of depth of focus, visual fatigue was induced if the objects in the image moved in depths [9]. A subjective evaluation was conducted to investigate the effects of manipulating camera separation, convergence distance, and focal length on perceived quality and naturalness [10]. Individual differences in 3-D perception by human visual systems (HVSs) have also been explored [11], [12]. The 3-D Consortium measured the fusional limits of 400 viewers using both crossed and uncrossed binocular disparities [11] and concluded that there are considerable individual differences among viewers. Lambooij investigated susceptibility to visual discomfort by administering the Wilkins rate of reading test and measuring vergence facility [12]. Stereoscopic film directors can control both shooting conditions and scene structure based on their target 3-D display and viewing conditions. However, when the viewing circumstances or the target display are changed, the depth and size can be distorted if angular disparities are changed. In addition, movie theater screen sizes and viewing distances are much larger than those of home 3-D TV environments, which may induce visual fatigue due to excessive disparity or eye divergence. In a previous paper, we proposed visual fatigue prediction metrics for stereoscopic images [13] and depth scaling methods [14]. In addition, we evaluated subjective preferences based on individual fusional response characteristics [14]. When stereoscopic videos cause visual fatigue, visual comfort enhancement can control the depth range using view synthesis [15] [17]; it enhances the comfort of images by reducing excessive disparity using inter-view rendering techniques. In [16], three methods for adapting stereoscopic movies to the viewing conditions were compared using the roundness of 3-D objects: baseline modification, viewpoint modification, and hybrid modification. Pan and Daly calculated the best disparity range with the HVS model and used Percival s zone and viewer preferences to determine the amount of depth

2 KIM et al.: VISUAL COMFORT ENHANCEMENT FOR STEREOSCOPIC VIDEO 483 adjustment [17]. Sun and Holliman proposed a depth mapping geometry that can adjust the camera separation according to the scene depth change so that the perceived depth stays constant [18]. However, these view synthesis approaches based on dense disparity estimation usually have high complexity that is not adequate for real-time depth adjustment. In addition, they suffer from disparity error and rendering artifacts, especially flickering artifacts in low-textured regions and repeated patterns, which significantly degrade the visual quality of rendered images. On the other hand, the horizontal image shift approach simply shifts left and right sequences in opposite directions. The horizontal image shift approach loses horizontal resolution by cropping the side regions of the images. However, it is widely used for the zero parallax setting and to prevent excessive disparity or eye divergence. Broberg provided guidance for horizontal image translation by analyzing the effectiveness and tradeoffs of several translation methods [19]. He et al. measured viewer disparity discomfort profiles and determined the amount of image shift by estimating depth histograms and predicting visual discomfort [20]. In this paper, we propose a depth adjustment method based on binocular fusion characteristics. The amount of depth adjustment is calculated based on predefined binocular fusion characteristics acquired by measuring the time required for fusion under various foreground disparity and disparity distributions, as described in a previous study [21]. The remainder of this paper is organized as follows. First, in Section II, we present a brief summary of the measurement of binocular fusion characteristics. Our proposed method for visual comfort enhancement is explained in Section III. Experimental results are presented in Section IV, and conclusions are drawn in Section V. II. Measurement of Binocular Fusion Characteristics Binocular vision enables stereopsis, in which the disparity provided by the different positions of the two eyes produces precise depth perception [22]. Such binocular vision is usually accompanied by binocular fusion, in which a single image is perceived even though each eye forms its own image [22]. Research has measured how long it takes for binocular fusion to occur when a stereoscopic stimulus is presented [4]. That study was based on the assumption that binocular fusion often fails when the conflict is large, and the authors presented vergence-accommodation conflicts by varying the disparity of the foreground object while the background was fixed to the focal plane of the display. However, it is common to place main objects near the display plane and to provide a 3-D impression from the perspective of the background scene when filming stereoscopic videos. Therefore, we conducted an experiment to measure binocular fusion time under vergence-accommodation conflicts induced by binocular disparities in stereoscopic displays [21]. We generated test images that induce conflict between vergence and accommodation by varying the foreground disparity and background disparity. We utilized an oriented corrugation pattern as a foreground pattern and the subject determined the Fig. 1. Fusion test using random dot stereogram. (a) Depth map and anaglyph of the corrugation pattern. (b) Stimulus for binocular fusion with random dot stereogram [21]. Parameters TABLE I Test Conditions of the Pattern [21] Foreground disparity Background disparity Focal distance Test Conditions 1.05, 0.75, 0.45, 0.15, 0.15, 0.45, 0.75, 1.05 (degree) 1, 0.5, 0, 0.5, 1 (degree) 0.5, 1, 2 (diopter) orientation of the pattern. The time to reach each decision by key pressing was recorded. Fig. 1 shows the fusion test using a random dot stereogram with the corrugation pattern. The depth map and anaglyph of the corrugation pattern are shown in Fig. 1(a), and the overall figure of the stimulus, which includes the foreground pattern, the background, and the aperture size, is shown in Fig. 1(b). Table I shows the parameters of the pattern. We tested the eight levels of foreground disparity and five levels of background disparity, which were all between 1 and 1 degrees. III. Visual Comfort Enhancement Based on Binocular Fusion Characteristics In this paper, we propose a depth adjustment method based on binocular fusion characteristics. The entire process for visual comfort enhancement is shown in Fig. 2. Our method enables real-time depth adjustment, and the amount of depth adjustment is calculated based on predefined binocular fusion characteristics. Binocular fusion characteristics are acquired by measuring the time required for fusion under various foreground disparity and disparity distribution conditions, as described in the previous section. While playing stereoscopic sequences, the viewing distance of the viewer and the disparity map of the sequence are estimated simultaneously, and adequate horizontal image shift is performed. The speeded-up robust feature is used to estimate the maximum disparity and the disparity distribution, and a face detection algorithm is used to estimate the viewing distance with a single web camera using OpenCV library [23]. Usually, disparities are the differences in image coordinates, as

3 484 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. 23, NO. 3, MARCH 2013 Fig. 2. Depth adjustment using feature matching and viewing distance estimation. measured in pixels. To estimate the actual perceived depth, angular disparities were used since they take into account the viewing conditions. The definition of angular disparity is the difference between the converging angles of the 3-D object and the screen, which is the difference between angles α and β in Fig. 2 by triangular calculation. We used two attributes of horizontal disparity: the maximum disparity and the disparity distribution, which are calculated by feature matching between left and right images [13]. The disparity distribution is the difference between the foreground disparity and the background disparity. The viewing distance is estimated based on the width of a detected face from the captured image of a web camera. We assumed that eye distance is constant among the viewers, and large face widths are converted to close viewing distances using a lookup table. This table is based on the focal length and the field of view of a web camera. Conventional methods for visual comfort enhancement usually maintain a consistent angular disparity based on the viewing distance. However, this is not applicable to the horizontal image shift approach because the disparity distribution is unchangeable since a decrease in crossed disparities leads directly to an increase in uncrossed disparities. Two methods for visual comfort enhancement are shown in Fig. 3. The first method sets a threshold value for disparity adjustment and applies horizontal image shift when maximum disparity exceeds the threshold. Fig. 3(a) shows the profile of maximum disparity from the sequence, and frames with light gray color exceed the predefined threshold value. Horizontal image shift can be applied to those frames to reduce the disparity. The proposed method uses binocular fusion characteristics acquired from the random dot stereogram test. According to our observations, excessive disparity distribution also interferes binocular fusion in addition to excessive maximum disparity; this results in delayed fusion time and visual fatigue. The basic idea for the proposed method is to set the threshold for depth adjustment based on the fusion time, not on a predefined angular disparity value. As shown in Fig. 3(b), the disparity characteristics are defined based on the maximum disparity and the disparity distribution of the frame, and they are depicted at a single point in the figure. Then, a horizontal image shift is applied to the frames to reduce the fusion time. This method is more suitable to model the limitation of HVS than one that only uses a single threshold value for the maximum disparity. This method is also effective for a scene with a medium level of maximum disparity and a Fig. 3. Two methods for depth adjustment. (a) Conventional method. (b) Proposed method. relatively large disparity distribution when there is a margin for background disparity. In this case, the first method may not work, whereas the proposed method applies depth adjustment based on the predicted fusion time. There are two main strengths of the proposed scheme that uses previously determined binocular fusion time. First, the algorithm can be easily modified in order to cover additional visual fatigue-related factors. For instance, the proposed scheme can be extended to account for the size of main object because the binocular fusion times for various aperture sizes of the foreground pattern are already known. To enhance visual comfort, additional modules for measuring the size of the main object is then required to adopt the size of the main object. In addition, the threshold value for visual comfort enhancement is determined without subjective evaluation. Most subjective evaluations measure either the absolute values or just noticeable values of specific visual fatigue related factors, where results with multiple combinations of these factors often tend to be ambiguous and indistinctive. One should always be careful when dealing with jittering artifacts to enhance visual comfort using either the view synthesis approach or the horizontal image shift approach. In addition to the rendering artifacts from low-quality disparity map in view synthesis approach, jittering artifacts from unstable horizontal image shift are also easily noticeable and significantly degrade the visual quality. Therefore, moving averages were used for face width, maximum and minimum disparities, and the amount of image shift needed to suppress the jittering artifacts. In addition, multithread programming was adopted so that face width and disparity can be estimated,

4 KIM et al.: VISUAL COMFORT ENHANCEMENT FOR STEREOSCOPIC VIDEO 485 Fig. 5. Estimated disparity (dashed) and adjusted disparity (solid) with 1 m of viewing distance by using (a) maximum disparity and (b) binocular fusion time. Fig. 4. Fusion time for foreground disparity and disparity distribution. (a) 3-D familiar subjects. (b) 3-D unfamiliar subjects. while stereoscopic player with horizontal image shift can be performed in high priority to prevent any video lags or buffering. We next describe a subjective evaluation of the depth adjustment method in fixed and free-to-move viewing conditions. IV. Experimental Results A. Binocular Fusion Characteristics A 55-in 240-Hz stereoscopic display was used [21]. The experiment was conducted using three subjects who were familiar with 3-D display devices because naive viewers usually exhibit large variance in fusion time. Then, we conducted additional experiments with 10 subjects who were not familiar with 3-D displays. The subjects were screened with a color vision test and had a corrected visual acuity of 20/20 or better. The disparity distribution is calculated by subtracting the background disparity from the foreground disparity. The fusion time is shown in Fig. 4, in which the x-axis and the y-axis represent the foreground disparity and the disparity distribution, respectively. The graph is modeled by second-order polynomial surface fitting, where R2 for fitting were for 3-D familiar group and for 3-D unfamiliar group, and is Fig. 6. Estimated disparity with 3 m of viewing distance. bell-shaped with a gradient increase in fusion time according to the increase in either foreground disparity or disparity distribution. This reveals that the disparity distribution also has a significant influence on fusion time and might affect visual fatigue, while conventional guidelines or studies only focus on the amount of foreground disparity. Fusion times from 3-D unfamiliar subjects were slower and more biased than those from 3-D familiar subjects, as shown in Fig. 4(a) and (b). This bias occurred because 3-D unfamiliar subjects showed a slight delay in discriminating the orientation of the pattern when the pattern and background were on the same depth plane. B. Visual Comfort Enhancement In this section, we evaluate the performance of two visual comfort enhancement methods. A 55-in 240-Hz stereoscopic display was used that offered a resolution of Ten nonexpert assessors, aged from 25 to 33 years, participated in the subjective evaluation and were screened for color vision,

5 486 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. 23, NO. 3, MARCH 2013 Fig. 7. Subjective evaluation of two visual comfort enhancement methods for 1-m viewing distance. visual acuity, and stereo acuity [24]. Subjective evaluations were conducted in both fixed and free-to-move viewing conditions. The assessors watched 10 stereoscopic sequences at three viewing distances: 1 m, 3 m, and free-to-move conditions. The duration of each sequence was 20 s. We used a five-level scale including ratings of no fatigue, slight fatigue, moderate fatigue, fatigue, and severe fatigue [25]. Test sequences were visually comfortable when viewed from 3 m, whereas some sequences had a relatively large disparity that induced visual fatigue when viewed from short viewing distances. Fig. 5 shows the estimated disparity and adjusted disparity of 10 stereoscopic sequences viewed at a distance of 1 m. The black dashed line, black solid line, gray dashed line, and gray solid line represent the original maximum disparity, the adjusted maximum disparity, the original minimum disparity, and the adjusted minimum disparity of the entire frames, respectively. Fig. 5(a) was acquired using maximum disparity as a threshold, and Fig. 5(b) was acquired by the proposed method that uses binocular fusion time as a threshold. In Fig. 5(a), frames whose maximum disparity exceed 1 were adjusted to 1, as in sequences 1, 3, 5, and 9. Accordingly, minimum disparities were adjusted for those frames while preventing eye divergence. Sequence 6 in Fig. 5(b) shows the difference between two visual comfort enhancement methods. Although the maximum disparity did not exceed 1, the sequence has relatively large disparity distribution so that the sequence was expected to induce a large fusion time. In this case, depth adjustment was performed in the proposed enhancement method. Fig. 6 shows the estimated disparity of test sequences when viewed from a 3-m distance. Depth adjustment was not performed in this viewing distance since the sequences were visually comfortable at the tested distance, and their estimated disparity did not exceed the threshold in either enhancement method. The subjective evaluations of the two enhancement methods are shown in Fig. 7 for sequences 5, 6, 9, and 10 that contain excessive angular disparities from a 1-m viewing distance, as shown in Fig. 5. The results show that both enhancement methods generated comfortable videos. In addition, the method using fusion time improved visual comfort more than the method using maximum disparity, except for sequence 9. It can be explained that similar amount of disparity was adjusted for both methods, as shown in Fig. 6. Sequences 6 and 10 show rather slight increases in visual comfort such Fig. 8. Estimated disparity (dashed), adjusted disparity (solid), and estimated viewing distance in free-to-move viewing condition by using binocular fusion time. that scores of the original sequence were even higher than those from the method using maximum disparity; however, the 95% confidence interval shown in the figure indicates the differences that are not significant. Relatively small increases in visual comfort are due to the fact that sequences 6 and 10 already consist background disparity close to 1, implying that there is a lower margin for enhancement. Fig. 8 shows the estimated disparity (dashed line), the adjusted disparity (solid line), and the estimated viewing distance in the free-to-move viewing condition. The disparity is calculated according to the viewer s movement, and the depth adjustment is applied to the frames with excessive disparity. The decrease in viewing distance at the beginning of the sequences occurs because, in order to suppress the jittering artifact, a moving average was applied to the estimated viewing distance from an initial viewing distance. V. Conclusion Depth adjustment can address excessive horizontal disparity; however, it should also provide proper depth level and preserve the viewers 3-D sensations. However, conventional depth adjustment methods provide little guidance in deciding the amount of depth adjustment or determining the depth adjustment conditions. In this paper, we proposed a depth adjustment method based on binocular fusion characteristics, where the fusion time was utilized as the parameter for enhancement, and compared it with conventional methods that use a single threshold. Subjective evaluation was conducted in fixed and free-to-move viewing conditions, and the results showed that comfortable videos were generated using the proposed depth adjustment method. In addition, compared to the method using a single threshold, the method using fusion time showed more improvement in visual comfort. The results showed that the proposed method was more effective for the scene with a medium level of foreground disparity and large disparity distribution when there was a margin for background disparity. In future work, we will extend our research to an

6 KIM et al.: VISUAL COMFORT ENHANCEMENT FOR STEREOSCOPIC VIDEO 487 optimal depth adjustment that uses face recognition techniques to apply individual preference for disparity and eye distance. However, there are difficulties with low face recognition rate due to the use of stereoscopic glasses. In addition, we are currently researching real-time disparity estimation and seamless rendering algorithm to implement view synthesis-based approach to overcome the limitations of horizontal image shift approach such as cropped side regions or less margin for enhancement. References [1] A. Woods, T. Docherty, and R. Koch, Image distortions in stereoscopic video systems, Proc. SPIE, vol. 1915, pp , Feb [2] H. Yamanoue, M. Okui, and F. Okano, Geometrical analysis of puppettheater and cardboard effects in stereoscopic HDTV images, IEEE Trans. Circuits Syst. Video Technol., vol. 16, no. 6, pp , Jun [3] L. M. J. Meesters, W. A. Jsselsteijn, and P. J. H. 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Thwaites, A study of visual fatigue and visual comfort for 3D HDTV/HDTV images, Displays, vol. 23, no. 4, pp , Donghyun Kim (S 07) received the B.S., M.S., and Ph.D. degrees in electrical and electronic engineering from Yonsei University, Seoul, Korea, in 2004, 2007, and 2012, respectively. His current research interests include 3-D video quality and visual fatigue assessment, and 3-D computer vision. Sunghwan Choi (S 10) received the B.S. degree in electronic engineering from Korea Aerospace University, Seoul, Korea, in He is currently pursuing the M.S. degree with Yonsei University, Seoul. His current research interests include 3-D image processing and disparity estimation. Kwanghoon Sohn (M 92) received the B.E. degree in electronics engineering from Yonsei University, Seoul, Korea, in 1983, the M.S.E.E. degree in electrical engineering from the University of Minnesota, Minneapolis, in 1985, and the Ph.D. degree in electrical and computer engineering from North Carolina State University, Raleigh, in From 1992 to 1993, he was a Research Staff Senior Member with the Satellite Communication Division, Electronics and Telecommunications Research Institute, Daejeon, Korea. He was also a Post-Doctoral Fellow with the MRI Center, Medical School of Georgetown University, Washington, DC. From 2002 to 2003, he was a Visiting Professor with Nanyang Technological University, Singapore. He is currently a Professor with the School of Electrical and Electronic Engineering, Yonsei University, Seoul. His current research interests include 3-D image processing, computer vision, and image communication. Dr. Sohn is a member of SPIE.