Proceedings of the Fourteenth Australasian User Interface Conference (AUIC2013), Adelaide, Australia Making 3D Work: A Classification of Visual Depth Cues, 3D Display Technologies and Their Applications Mostafa Mehrabi, Edward M. Peek, Burkhard C. Wuensche, Christof Lutteroth Graphics Group, Department of Computer Science University of Auckland [email protected], [email protected], [email protected], [email protected] Abstract This success motivated researchers to develop new 3D 3D display technologies improve perception and display techniques with improved performance and for interaction with 3D scenes, and hence can make new application fields (Planar3D 2012). This process applications more effective and efficient. This is achieved continues as more complex and more realistic display by simulating depth cues used by the human visual techniques are being researched (Favalora 2005). system for 3D perception. The type of employed depth Different 3D display technologies are suitable for cues and the characteristics of a 3D display technology different applications depending on their characteristics affect its usability for different applications. In this paper and the depth cues that they simulate (McAllister 1993, we review, analyze and categorize 3D display Okoshi 1976). Therefore, a developer must be familiar technologies and applications, with the goal of assisting with these techniques in order to make an informed application developers in selecting and exploiting the choice about which one to use for a specific application. most suitable technology. Characterizing 3D display techniques in terms of which Our first contribution is a classification of depth cues applications they are suited for is not easy as the that incorporates their strengths and limitations. These information regarding their limitations, constraints and factors have not been considered in previous capabilities is much dispersed. contributions, but they are important considerations when Earlier contributions (Pimenta and Santos 2010) have selecting depth cues for an application. The second categorized depth cues and 3D display technologies; contribution is a classification of display technologies that however there is no information provided about the highlights their advantages and disadvantages, as well as significance of each of the depth cues, and the their requirements. We also provide examples of suitable advantages, disadvantages and constraints of display applications for each technology. This information helps techniques are not discussed. Furthermore, no guidelines system developers to select an appropriate display are provided about which display technology is most technology for their applications. suitable for a specific use-case. Keywords: classification, depth cues, stereo perception, In this paper, we address the following two research 3D display technologies, applications of 3D display questions: technologies 1. What are the limitations of the depth cues of the human visual system? 1 Introduction 2. What applications is each 3D display The first attempts for creating 3D images started in the technology suitable for? late 1880s aided by an increasing understanding of the To answer question 1, we have analysed the seminal human visual perception system. The realization that the references in that area (McAllister 1993, Okoshi 1976), in visual system uses a number of depth cues to perceive addition to references from art and psychology, and used and distinguish the distance of objects in their them to build a new classification of depth cues. To environment encouraged designers to use the same answer question 2, we have analysed the most common principles to trick the human brain into the illusion of a display technologies (Planar3D 2012, Dzignlight Studios 3D picture or animation (Limbchar 1968). 2012) and the common characteristics of the applications Moreover, the realism 3D display techniques add to they can be used for. The result is a classification of 3D images dramatically improved research, education and display technologies in terms of their depth cues, practice in a diverse range of fields including molecular advantages and disadvantages, and suitable application modelling, photogrammetry, medical imaging, remote domains. surgery, pilot training, CAD and entertainment Section 2 describes the classification of depth cues. (McAllister 1993). Section 3 describes the classification of display technologies. Section 4 establishes a link between the Copyright © 2013, Australian Computer Society, Inc. This display technologies and the applications they are paper appeared at the 14th Australasian User Interface appropriate for. Section 5 concludes the paper. Conference (AUIC 2013), Adelaide, Australia. Conferences in Research and Practice in Information Technology (CRPIT), Vol. 2 Depth Cues 139. Ross T. Smith and Burkhard Wünsche, Eds. Reproduction Depth cues are information from which the human brain for academic, not-for-profit purposes permitted provided this text is included. perceives the third visual dimension (i.e. depth or 91 CRPIT Volume 139 - User Interfaces 2013 distance of objects). Each display technique simulates artificial vision systems, e.g. in robotics, use this cue only some of the depth cues. Thus, evaluating their alone to calculate depth (Xiong and Shafer 1993). usability for a specific application requires knowing the importance of depth cues with respect to that application. 2.2 Psychological Depth Cues Visual depth cues can be classified into two major All of the psychological depth cues are monocular. In the categories: physiological and psychological depth cues. following we briefly describe all of them (McAllister Both are described in the following, and summarized in 1993, Okoshi 1976, Howard 2012, Bardel 2001). Table 1 (McAllister 1993, Okoshi 1976). Retinal Image Size. If our brain is familiar with the actual size of an object, it can estimate its distance by 2.1 Physiological Depth Cues considering its perceived size with respect to its actual The process of perceiving depth via physiological depth known size. cues can be explained using physics and mathematics. Linear Perspective. In a perspective projection parallel That is, it is possible to calculate the depth of objects if lines appear closer as they move towards the horizon and the values for some of the important physiological depth finally converge at infinity. This depth cue is one of the cues are available (e.g. using triangulation for binocular most frequently used ones to express depth in computer parallax values). For this reason, phys. depth cues are graphics renderings. used for applications that simulate human 3D perception, Texture Gradient. Details of surface textures are clearer such as in robotics to estimate the distance of obstacles when the surface is close, and fade as the surface moves (Xiong and Shafer 1993, Mather 1996). further away. Some psychologists classify linear Physiological depth cues are either binocular (i.e. perspective as a type of texture gradient (Bardel 2001, information from both the eyes is needed for perceiving Mather 2006). depth) or monocular (i.e. information from only one eye Overlapping (Occlusion). Our brain can perceive exact is sufficient to perceive depth). In the following we information about the distance order of objects, by describe different phys. depth cues. recognizing objects that overlap or cover others as closer, Accommodation. The focal lengths of the lenses of the and the ones that are overlapped as farther (Gillam and eyes change in order to focus on objects in different Borsting 1988). distances. This depth cue is normally used in combination Aerial Perspective. Very distant objects appear hazy and with convergence as it is a weak depth cue. It can only faded in the atmosphere. This happens as a result of small provide accurate information about the distance of objects particles of water and dust in the air (O’Shea and that are close to the viewer (Howard 2012). Blackburn 1994). Convergence is the angle by which our eyes converge Shadowing and Shading. Objects that cast shadow on when focusing on an object. This depth cue provides other objects are generally perceived to be closer accurate information about the distance of objects. (shadowing). Moreover, objects that are closer to a light However, the convergence angle gets close to zero as an source have a brighter surface compared to those which object moves further away, eliminating the cue for large are farther (shading). However, many psychologists do distances (i.e. convergence angle is asymptotic to not consider this as a depth cue because shadows only distance) (Howard 2012). specify the position of an object relative to the surface the Binocular Parallax. Our eyes are positioned shadow is cast on, and additional, more accurate approximately 50-60mm away from each other (Oian estimations of distance are needed from other depth cues 1997). Thus, they see images with slightly different (e.g. texture gradient) (Bardel 2001). perspectives. Two slightly different images are fused in Colour. Different wavelengths are refracted at different the brain and provide 3D perception. Every 3D display angles in the human eye. Thus, objects with different system must simulate this depth cue, as it is the most colours appear at different distances. Therefore, the important one (McAllister 1993). results obtained from this depth cue are not reliable Monocular Movement (Motion) Parallax. Objects that are (McAllister