Huib de Ridder

Perceived depth and the feeling of presence in 3DTV

The present study investigates the subjective feeling of presence elicited by 3DTV and its relationship to perceived depth and image content. Subjective methods of assessing presence that have been used or proposed to date do not provide a measure of temporal variation. To overcome this limitation, we have applied the continuous assessment methodology (ITU-R, BT 500-7) to the assessment of presence, perceived depth and naturalness of depth. Twelve observers continuously rated their instantaneous perception of presence, depth and naturalness of depth when viewing stereoscopic footage. The results indicate that subjective presence ratings are subject to considerable temporal variation depending on the image content and camera techniques used. The correlations between the different attributes suggest that an increase in depth may lead to an enhanced sense of presence, provided depth is perceived as natural. A qualitative analysis of the data in relation to the image content provides evidence for the hypothesis that the extent of sensory information available to an observer is a determinant of presence, as proposed by T.B. Sheridan, Musings on telepresence and virtual presence, Presence: Teleoperators and Virtual Environments 1 (1992) 120‐125. q 1998 Elsevier Science B.V.

Chroma variations and perceived quality of color images of natural scenes

Transformations of natural images in the per- ulated with respect to luminance and chromaticity in order ceptually uniform CIELUV color space have been investi- to improve the display of color. Usually, some implicit gated with respect to perceptual image quality. To this assumptions are made to limit the almost infinite number end, digitized color images of four natural scenes were of possible manipulations. One of these assumptions is described on the basis of their color point distributions that only global changes need to be considered. This stems in the CIELUV color space. A new set of images was from the notion that images are coherent in the sense that created by varying the chroma value of each pixel while different parts of the same image share luminance and the lightness and hue angle were kept constant. The spectral properties due to the fact that they belong to the chroma was changed in two different ways: (1) through same object or are illuminated by the same light source. the addition or subtraction of the same amount of chroma This assumption underlies global reproduction techniques to or from the chroma value of each pixel; (2) through in which the same color operation is applied to all pixels, multiplication of the chroma value of each pixel by a irrespective of the content of the image. 1,2 Another as- constant. In three experiments, subjects judged the per- sumption is that the optimum image equals the original ceptual quality, colorfulness, and naturalness of the im- ( real-life ) scene. Hence, all efforts should be directed ages on a ten-point numerical category scale. The results towards finding ways of approaching this equality as indicate that colorfulness is the main perceptual attribute closely as possible. underlying image quality when chroma varies. Colorful- In this article, a different constraint on color reproduc- ness itself was found to depend on both the average tion is proposed, namely, the experienced naturalness of chroma and its variability. In general, the subjects pre- an image. This choice is motivated by the argument that ferred slightly more colorful images to the original ones. constraints should be formulated not in the physical, but The perceptual quality of the images was found to be in the psychological domain, the reason being that image closely related to the naturalness of the images.q 1997 quality judgments are based on a comparison between the

pOwerball: the design of a novel mixed-reality game for children with mixed abilities

This paper presents the design of pOwerball, a novel augmented reality computer game for children aged 8-14. The pOwerball was designed to bring together children with and without a physical or learning disability and to encourage social interactions surrounding the play. The contribution of this design case is two fold. From a design perspective, pOwerball exemplifies an emerging class of computer games where the interaction style and game mechanics support social interactions amongst the players. From a methodological perspective, we describe the various ways children became involved in our design process; we highlight the related difficulties and successes in the context of relevant research literature.

Presence: Where Are We?

179 IN 1978, THE PHILOSOPHER DANIEL DENNETT presented us with an interesting and entertaining thought experiment entitled “Where am I?” as the last chapter of his Brainstorms book.1 Dennett recounts the story of a “curious episode” in his life where his brain got surgically separated from his body, with each connection between them restored by placing two “microminiaturized radio transceivers” between each input and output pathway. After the operation he, or rather his body, goes to visit his brain, which was placed, in keeping with the best philosophical traditions, in a lifesupport vat. While looking with his own eyes at his own brain he starts to wonder:

Continuous assessment of perceptual image quality

The study addresses whether subjects are able to assess the perceived quality of an image sequence continuously. To this end, a new method for assessing time-varying perceptual image quality is presented by which subjects continuously indicate the perceived strength of image quality by moving a slider along a graphical scale. The sliders position on this scale is sampled every second. In this way, temporal variations in quality can be monitored quantitatively, and a means is provided by which differences between, for example, alternative transmission systems can be analyzed in an informative way. The usability of this method is illustrated by an experiment in which, for a period of 815 s, subjects assessed the quality of still pictures comprising time-varying degrees of sharpness.

Considerate home notification systems: a field study of acceptability of notifications in the home

A field study in ten homes was conducted to understand what influences users’ acceptability of notifications in the home environment. The key finding is that perceived message urgency is the primary indicator of acceptability of notifications in the home—if people think a message is urgent, they want the message to be shown immediately, regardless of what they are doing at the time of notification. The study also shows that the acceptability of low-urgent and medium-urgent messages could be improved by taking into account mental activity load at the time of notification. No effect of physical activity was found on acceptability. The results suggest that to improve the scheduling of notifications in the home, notification systems need a mechanism assessing both the message urgency and the mental activity load, whereas physical activity can be ignored. From a methodological point of view, it is difficult to measure acceptability of notifications in a realistic setting, given the need to balance experimental control with realistic context. The present paper suggests a way to introduce controlled notifications and subjective measurements of acceptability in homes.

Considerate home notification systems: A user study of acceptability of notifications in a living-room laboratory

Recent field experiments on acceptability of notifications in the home showed that people generally want to be informed of urgent messages as soon as possible, whereas non-urgent messages should not be presented at all. A possible way to improve the acceptability of a notification might be to adjust the presentation mode and the timing of notifications to the message content and to the state of the user. For example, acceptability might be improved by considering user activities when selecting the best time to present the message. The relation between acceptability, presentation mode and timing has not been formally studied in a controlled home setting before. This paper presents the results of a user study, in which 10 participant couples were asked to engage in everyday home activities, and to subjectively rate factors that were expected to influence acceptability. The study was situated in a living-room laboratory in which the user activities and the timing of notifications were controlled. Questionnaire data was evaluated using cluster analysis in order to construct a semantic model that describes the relationship between user, system and environment. The key findings in the present study are: (1) acceptability could be improved by adjusting the level of intrusiveness of the presentation to message urgency: urgent messages should be presented intrusively, medium-urgent messages unobtrusively, and (2) non-urgent messages should be postponed until the message urgency has increased, or skipped if the message urgency never exceeds the predefined presentation threshold. Surprisingly, the user activities at the time of notification were not found to influence acceptability. These findings have resulted in a model of acceptability of notifications for the design of future home notification systems.

Training of basic laparoscopy skills on SimSurgery SEP.

Abstract The aim of this study was to assess the performance curve for novices training in bimanual tissue manipulation and angled laparoscope navigation, and compare those performances with the performances of experienced laparoscopic surgeons. The Camera Navigation task with a 30° angled laparoscope and the Place Arrow task of the new SimSurgery SEP virtual reality simulator were used. Fourteen medical trainees (no laparoscopy experience) performed four training sessions within one week, including 15 repetitions of each task in total. The experienced participants (>50 procedures & familiar with angled laparoscope) performed each task twice. The performance on both tasks by the novices improved significantly over the training sessions. The experienced participants performed both tasks significantly better than the novices in repetition 3. After repetition 15, the performances of the novices on both tasks were of the same level as the performances of the experienced participants. By training on SimSurgery SEP, medical trainees can extensively improve their skills in navigation with 30° angled laparoscope and bimanual tissue manipulation. Further research should focus on the transfer of skills acquired on the simulator to the clinical setting. Knowledge on proficiency thresholds and training end-points for pre-clinical criterion-based training of different laparoscopic tasks also needs to be extended.

Cognitive issues in image quality measurement

Designers of imaging systems, image processing algorithms, etc., usually take for granted that methods for assessing perceived image quality produce unbiased estimates of the viewers’ quality impression. Quality judgments, however, are affected by the judgment strategies induced by the experimental procedures. In this paper the results of two experiments are presented illustrating the influence judgment strategies can have on quality judgments. The first experiment concerns contextual effects due to the composition of the stimulus set. Subjects assessed the sharpness of two differently composed sets of blurred versions of one static image. The sharpness judgments for the blurred images present in both stimulus sets were found to be dependent on the composition of the set as well as the scaling technique employed. In the second experiment subjects assessed either the overall quality or the overall impairment of manipulated and standard JPEG-coded images containing two main artifacts. The results indicate a systematic difference between the quality and impairment judgments that could be interpreted as instruction-based different weighting of the two artifacts. Again, some influence of scaling technique was observed. The results of both experiments underscore the important role judgment strategies play in the psychophysical evaluation of image quality. Ignoring this influence on quality judgments may lead to invalid conclusions about the viewers’ impression of image quality.

Camera navigation and tissue manipulation; are these laparoscopic skills related?

BackgroundIt is a tacit assumption that clinically based expertise in laparoscopic tissue manipulation entails skilfulness in angled laparoscope navigation. The main objective of this study was to investigate the relation between these skills. To this end, face and construct validity had to be established for the place arrow (PA) and camera navigation (CN) tasks on the SimSurgery SEP.MethodsThirty-three novices (no laparoscopy experience) and 33 experienced participants (>50 laparoscopic procedures and familiar with angled laparoscopy) performed both tasks twice, on one of two hardware platforms (SimSurgery SimPack or Xitact/Mentice IHP), and rated the realism and didactic value of SimSurgery SEP on five-point scales.ResultsBoth tasks were rated by the experienced participants as realistic (CN: 3.7; PA: 4.1) and SimSurgery SEP as a user-friendly environment to train basic skills (4.1). Both tasks were performed in less time by the experienced group, with shorter tip trajectories. For both groups jointly, the time to accomplish each task correlated with the tip trajectory and also with the time and tip trajectories of the opposite task (Spearman’s correlation, p ≤ 0.05). Within the groups however, the performances on both tasks did not always correlate.ConclusionsA correlation was not always found between the performances on the two tasks, which suggests that clinically based expertise in tissue manipulation does not automatically entail skilfulness in angled laparoscope navigation, and vice versa. Training and assessment of basic laparoscopic skills should focus on these tasks independently. More research is needed to better identify the skills and required proficiency levels for different laparoscopic tasks.