Benjamin Bolte

Geometric Calibration of Head-Mounted Displays and its Effects on Distance Estimation

Head-mounted displays (HMDs) allow users to observe virtual environments (VEs) from an egocentric perspective. However, several experiments have provided evidence that egocentric distances are perceived as compressed in VEs relative to the real world. Recent experiments suggest that the virtual view frustum set for rendering the VE has an essential impact on the users estimation of distances. In this article we analyze if distance estimation can be improved by calibrating the view frustum for a given HMD and user. Unfortunately, in an immersive virtual reality (VR) environment, a full per user calibration is not trivial and manual per user adjustment often leads to mini- or magnification of the scene. Therefore, we propose a novel per user calibration approach with optical see-through displays commonly used in augmented reality (AR). This calibration takes advantage of a geometric scheme based on 2D point - 3D line correspondences, which can be used intuitively by inexperienced users and requires less than a minute to complete. The required user interaction is based on taking aim at a distant target marker with a close marker, which ensures non-planar measurements covering a large area of the interaction space while also reducing the number of required measurements to five. We found the tendency that a calibrated view frustum reduced the average distance underestimation of users in an immersive VR environment, but even the correctly calibrated view frustum could not entirely compensate for the distance underestimation effects.

Subliminal Reorientation and Repositioning in Immersive Virtual Environments using Saccadic Suppression

Virtual reality strives to provide a user with an experience of a simulated world that feels as natural as the real world. Yet, to induce this feeling, sometimes it becomes necessary for technical reasons to deviate from a one-to-one correspondence between the real and the virtual world, and to reorient or reposition the users viewpoint. Ideally, users should not notice the change of the viewpoint to avoid breaks in perceptual continuity. Saccades, the fast eye movements that we make in order to switch gaze from one object to another, produce a visual discontinuity on the retina, but this is not perceived because the visual system suppresses perception during saccades. As a consequence, our perception fails to detect rotations of the visual scene during saccades. We investigated whether saccadic suppression of image displacement (SSID) can be used in an immersive virtual environment (VE) to unconsciously rotate and translate the observers viewpoint. To do this, the scene changes have to be precisely time-locked to the saccade onset. We used electrooculography (EOG) for eye movement tracking and assessed the performance of two modified eye movement classification algorithms for the challenging task of online saccade detection that is fast enough for SSID. We investigated the sensitivity of participants to translations (forward/backward) and rotations (in the transverse plane) during trans-saccadic scene changes. We found that participants were unable to detect approximately ±0.5m translations along the line of gaze and ±5° rotations in the transverse plane during saccades with an amplitude of 15°. If the user stands still, our approach exploiting SSID thus provides the means to unconsciously change the users virtual position and/or orientation. For future research and applications, exploiting SSID has the potential to improve existing redirected walking and change blindness techniques for unlimited navigation through arbitrarily-sized VEs by real walking.

Exploiting perceptual limitations and illusions to support walking through virtual environments in confined physical spaces

Abstract Head-mounted displays (HMDs) allow users to immerse in a virtual environment (VE) in which the user’s viewpoint can be changed according to the tracked movements in real space. Because the size of the virtual world often differs from the size of the tracked lab space, a straightforward implementation of omni-directional and unlimited walking is not generally possible. In this article we review and discuss a set of techniques that use known perceptual limitations and illusions to support seemingly natural walking through a large virtual environment in a confined lab space. The concept behind these techniques is called redirected walking. With redirected walking, users are guided unnoticeably on a physical path that differs from the path the user perceives in the virtual world by manipulating the transformations from real to virtual movements. For example, virtually rotating the view in the HMD to one side with every step causes the user to unknowingly compensate by walking a circular arc in the opposite direction, while having the illusion of walking on a straight trajectory. We describe a number of perceptual illusions that exploit perceptual limitations of motion detectors to manipulate the user’s perception of the speed and direction of his motion. We describe how gains of locomotor speed, rotation, and curvature can gradually alter the physical trajectory without the users observing any discrepancy, and discuss studies that investigated perceptual thresholds for these manipulations. We discuss the potential of self-motion illusions to shift or widen the applicable ranges for gain manipulations and to compensate for over- or underestimations of speed or travel distance in VEs. Finally, we identify a number of key issues for future research on this topic.

Visual blur in immersive virtual environments: does depth of field or motion blur affect distance and speed estimation?

It is known for decades that users tend to significantly underestimate or overestimate distances or speed in immersive virtual environments (IVEs) compared to corresponding judgments in the real world. Although several factors have been identified in the past that could explain small portions of this effect, the main causes of these perceptual discrepancies still remain elusive. One of the factors that has received less attention in the literature is the amount of blur presented in the visual imagery, for example, when using a head-mounted display (HMD). In this paper, we analyze the impact of the visual blur effects depth-of-field and motion blur in terms of their effects on distance and speed estimation in IVEs. We conducted three psychophysical experiments in which we compared distance or speed estimation between the real world and IVEs with different levels of depth-of-field or motion blur. Our results indicate that the amount of blur added to the visual stimuli had no noticeable influence on distance and speed estimation even when high magnitudes of blur were shown. Our findings suggest that the human perceptual system is highly capable of extracting depth and motion information regardless of blur, and implies that blur can likely be ruled out as the main cause of these misperception effects in IVEs.

Viargo - A generic virtual reality interaction library

Traditionally, interaction techniques for virtual reality applications are implemented in a proprietary way on specific target platforms, e. g., requiring specific hardware, physics or rendering libraries, which withholds reusability and portability. Though hardware abstraction layers for numerous devices are provided by multiple virtual reality libraries, they are usually tightly bound to a particular rendering environment. In this paper we introduce Viargo - a generic virtual reality interaction library, which serves as additional software layer that is independent from the application and its linked libraries, i. e., a once developed interaction technique, such as walking with a head-mounted display or multi-touch interaction, can be ported to different hard- or software environments with minimal code adaptation. We describe the underlying concepts and present examples on how to integrate Viargo in different graphics engines, thus extending proprietary graphics libraries with a few lines of code to easy-to-use virtual reality engines.

Jumping through immersive video games

Novel user interfaces such as the Microsoft Kinect allow users to actively move their body in order to interact with immersive video games. Hence, users may navigate by using natural, multimodal methods of generating self-motions. For instance, [LaViola and Katzourin 2007] developed several body- and foot-based metaphors for hands-free navigation in IVEs, including a leaning technique for traveling short and medium distances and a floor-based world-in-miniature for traveling large distances. However, real walking is the most basic and intuitive way of moving and, therefore, keeping this ability is of great interest.