Human Performance in Six Degree of Freedom Input Control

Shumin Zhai, Ph.D.

5.2 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, binocular disparity, perspective, shadows, texture, motion parallax and active movement. To put the study of semi-transparency into perspective, this section briefly reviews these depth cues and their applications to 3D interaction systems.

Occlusion  is one of the most dominant cues in depth perception (Schriever, 1925) . 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 recognised, 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 equivalent binocular perspective.

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 by Brooks (1988) .

Operating on the same principle as for 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, 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 visualisation 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 visualising 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 a passive subject, whose head movements were not coupled to the displayed image. In a path-tracing experiment, Arthur, Booth, and Ware (1993, also Ware and Arthur, 1993) found that while subjects' task completion times with an active head-tracking display and a stereoscopic set-up were similar, their error rates were significantly lower with the head tracking condition.

Figure 5.1The Partial occlusion effect; Portions of an object appearing in front of or behind the semi-transparent "silk" surface are perceived as such according to different levels of contrast.

As we can see, many of these depth cues have been carefully investigated and consciously applied to graphical displays. There is yet another phenomenon, produced by semi-transparent  surfaces, which can be a strong depth cue. Whenever a semi-transparent surface overlaps another object, the viewer will see not only the proximal occluding object, but also the overlapped or partially occluded  object, or portions of that object, in lower contrast (Figure 5.1). One example of this phenomenon in everyday life is the silk stocking; hence the semi-transparency* cue is referred as the "silk" effect in this chapter.

The effectiveness of semi-transparency as a depth cue has not been the subject of the same extensive research as other depth cues, possibly because semi-transparency is not experienced very commonly in the natural environment. There are many reasons to hypothesise that the partial-occlusion effect introduced through semi-transparency would be no less powerful than some of the more common depth cues used in 3D interactive systems. First, the literature has well concluded that interposition is one of the strongest depth cues. However, it is difficult to utilise the total interposition cue in 3D interaction systems, since objects further away are completely obscured. A semi-transparent surface, on the other hand, does not completely occlude distal objects. Such partially occluded objects only appear with lowered contrast, which should assist the viewer in perceiving the relative location of the distal object relative to the semi-transparent surface. Second, in a graphical environment, the partial occlusion effect can be enhanced interactively.  When a "silk" surface is gradually moved through  an object, the resulting immediate changes in the object's appearance (see Figure 5.1) are continuous. This suggests a potentially powerful mechanism for displaying users' input actions in relation to targets in 3D space.