The partial-occlusion effect: utilizing semitransparency in 3D human-computer interaction

Shumin Zhai, William Buxton, Paul Milgram
1996 ACM Transactions on Computer-Human Interaction  
This study investigates user performance when using semi-transparent tools in interactive 3D computer graphics environments. We hypothesize that when the user moves a semi-transparent surface in a 3D graphic display, the partial occlusion effect introduced through semi-transparency acts as an effective cue in target localization -an essential component in many 3D manipulation tasks. This hypothesis was tested in a controlled experiment in which subjects were asked to acquire dynamic 3D targets
more » ... virtual fish) with a 3D cursor. In the experiment, cursors with and without semitransparent surfaces were compared in monoscopic and stereoscopic displays. Statistically significant effects for trial completion time, error rate and error magnitude were observed for stereopsis and partial occlusion. The partial occlusion cue was effectively used by subjects in both monoscopic and stereoscopic displays. It was no less effective than stereopsis for successful 3D target acquisition. Subjects' performance in each of the conditions improved with learning, but their relative ranking remained the same. Subjective evaluations also supported the conclusions drawn from performance measures. The experimental results and their implications are discussed, with emphasis on the relative, discrete nature of the semi-transparency cue and on interactions between depth cues. The paper concludes with a review of a number of existing and potential future applications of semi-transparency in human computer interaction. ] and entertainment. With this move to 3D, however, we see a breakdown in many of the interaction techniques that have traditionally been used in 2D direct manipulation systems. Tasks such as target acquisition, positioning, dragging, pursuit tracking, sweeping out regions, orienting, and navigating present new challenges to the interface designer. In response to these changes, a body of research is developing which is beginning to address some of these interaction issues. Representative examples are found in [Evans, Tanner, and One of the key challenges in 3D interface design is to effectively reveal spatial relationships among objects within a 3D space, particularly in the depth dimension, so that the user can perceive, locate and manipulate such objects with respect to each other effortlessly. This paper addresses one particular 3D mechanism, the partial occlusion effect, which can be introduced by the use of semitransparent surfaces as a means of improving 3D interaction performance. After a brief review of various depth cues in human perception and their exploitation in corresponding 3D display techniques, the paper presents a formal experimental study of the semitransparency effect in a 3D manipulation task. The the experimental results are discussed with particular emphasis on the semitransparency characteristics and the modeling of multiple depth cues. Finally, some existing and future potential applications of the interactive semi-transparency effect are described. PRESENTING DEPTH INFORMATION A variety of techniques are commonly used in computer interfaces for presenting 3D information. Almost all of these techniques can be linked to the depth cues identified in psychological research on human perception in the natural environment. [See Haber and Hershenson 1973; Kaufman 1974, Wickens, Todd, and Seidler 1989; McAllister 1993 for reviews of depth cue theory]. The most commonly exploited depth cues include occlusion, perspective, shadows, texture, binocular disparity, motion parallax and active movement. To put the study of semi-transparency into perspective, we briefly review these depth cues and their applications to 3D computer interfaces. Occlusion (also called interposition) is one of the most dominant cues in depth perception. Objects appearing closer to the viewer occlude other objects which are further away from the viewer. In 3D computer graphics, the importance of occlusion has long been recognized, most commonly through the use of hidden line/surface removal techniques. Stereopsis, produced from binocular disparity when viewing 3D objects in natural environments, is a strong depth cue, particularly when the perceived objects are relatively close to the viewer [Yeh 1993]. Various techniques have been devised to create stereopsis on a 2D screen [Arditi 1986; McAllister 1993] . The currently most common method uses liquid-crystal time-multiplexed shuttering glasses. The effectiveness of stereoscopic displays strongly depends on the particular experimental task to which they are applied and on technical implementation variables such as shutter frequency and the binocular geometric model. Perspective and relative size cues, which account for objects further away producing smaller retinal images than closer objects, are commonly exploited in 3D graphics [Foley, van Dam, Feiner, and Hughes 1990] . Perspective cues are particularly effective when the displayed scene has parallel lines, as noted in [Brooks 1988 ]. Operating on the same principle as in perspective and size cues, the densities of surface features (texture) increase for more distant surface elements. Texture cues are therefore also described as detail perspective [Kaufman, 1974] . The shadow of a 3D object is also often an effective depth cue. Herndon and colleagues [Herndon, Zeleznik, Robbins, Conner, Snibbe, and van Dam 1992], for example, explicitly exploit shadows for 3D interaction. In their design, shadows are projected on walls and floors of a 3D environment so that the user can control object movement in each dimension selectively by choosing and moving the shadows. The use of shadows is also an important element of the information visualization display proposed by Robertson, Mackinlay, and Card [1991] . Motion parallax . When an object moves in space relative to an observer, the resulting motion parallax produces a sensation of depth. This effect is also frequently exploited in graphical displays. For example, Sollenberger and Milgram [1993] showed the usefulness of the kinetic depth effect in graphically visualizing the connectivity of complex structures such as blood vessels in the brain. Active movement. Depth information obtained by actively altering a viewer's own viewpoint is often referred to as movement cue. Motivated by the Gibsonian ecological approach, Smets and colleagues [Smets 1992; Overbeeke and Stratmann 1988] demonstrated the advantages of the active observer, for whom images on a screen were drawn according to tracked head movements, in comparison with the passive subject, whose head movements were not coupled to the displayed image. In a path-tracing experiment, Arthur and colleagues [Arthur, Booth, and Ware 1993; Ware and Arthur 1993] found that while subjects' task completion time with an headtracking display and a stereoscopic set-up were similar, their error rates were significantly lower with the head tracking condition. As can we see, many of the depth cues have been carefully investigated and consciously applied to graphical displays. The relative strengths of various depth cues have also been studied. In one early cue conflict study, Schriever [1925] compared the relative influences of binocular disparity, perspective, shading and occlusion, and showed, among other things, the dominance of occlusion over disparity information. Edge-occlusion domination were also reported in [Braunstein, Anderson, Rouse and Tittle, 1986]. More recently, Wickens, Todd and Seidler [1989], in a review of the depth combination literature, concluded that motion, disparity and occlusion are the most powerful depth cues for computer displays. We noticed that yet another phenomenon -partial occlusion -produced by semi-transparent * surfaces can be also a strong depth cue. Whenever a semi-transparent surface overlaps another object, the viewer will see the overlapped object in lower contrast (partially occluded ) (Figure 1) . A typical example of this phenomenon in everyday life is the silk stocking; hence we also refer to the partial occlusion phenomenon as the "silk" effect.
doi:10.1145/234526.234532 fatcat:euip3ylgnnh6rezuudvfid2xd4