An important part of the input control process is displaying the user's control actions in relation to the target location (see Figure 1.1). How this display is designed can have a significant impact on input control performance. In the history of manual control research, the configuration of displays has been an inseparable part of the literature. The various studies on compensatory displays versus pursuit displays are such an example (See Poulton, 1974 for a review).
A unique challenge in designing interaction systems with 6 degrees of freedom is to effectively reveal the user's actions in relation to the target location in the depth dimension. If this relationship is not readily perceived by the user, effective input control can not be achieved, regardless of how well other parts of the interaction system are designed. In fact, much effort has been put into the design of displays used in the preceding experiments. None of those experiments could have been carried out without implementing proper 3D displays. For example, without stereoscopic displays, subjects simply could not perform the docking task, because the subject could not determine where the tetrahedron cursor (or target) was pointing and thus he/she could not decide in which direction to rotate the cursor.
Visual perception research has identified many depth cues that
allow humans to readily perceive natural 3D world. Correspondingly
many techniques, such as stereoscopic viewing, have been invented
to produce these cues in artificial ways. This chapter focuses
on a novel 3D mechanism, the partial occlusion effect, which is
implemented by the use of semi-transparent surfaces in 3D graphic
displays. The chapter starts with a brief review of various depth
cues in human perception and their exploitation in corresponding
3D display techniques. It then proceeds to an experiment to evaluate
the use of the semi-transparency effect in a 3D target acquisition
task. The experimental results are then discussed, with particular
emphasis on the semi-transparency cue characteristics for 3D interaction
and the modelling of multiple depth cues. Finally, some existing
and future potential applications of semi-transparency in 3D interaction
are presented.