Spatial Cues in 3D Visualization - Ambient Intelligence for Scientific Discovery - page 108

that the benefits of stereo viewing are task-specific [21, 30]. However, there is
largely a consensus that stereo viewing is a powerful, and perhaps dominant,
depth cue for engaging in a variety of tasks involving the spatial manipulation
of objects perceived in depth.
Shadows. The application of lighting techniques to promote realism and to
improve the perception and understanding of computer visualizations has many
approaches, including luminance, ambient and diffuse lighting, specular high-
lights, object surface shading, and attached and cast shadows, to name just a
few. Because contemporary hardware has just recently become powerful enough
to easily compute and render realtime, moving objects casting shadows cre-
ated by different (unseen) light sources on various background surfaces, this
issue has not been fully explored in contemporary human factors visualiza-
tion research. (For a thorough online summary of research in this area, see
http://artis.imag.fr/Recherche/RealTimeShadows/index.html).
In [31], he distinguished between two types of object shadows: the primary
or attached shadow created when the shadow of an object is visible on that same
object (sometimes called 'self-shadowing'); and the derived or cast shadow, cre-
ated when the shadow of one object is visible on a different object or background
surface. Attached shadows are created by the surface anomalies of an object that
self-shade other surface areas of that same object relative to some light source.
In contrast, cast shadows are created by a light-occluding object positioned be-
tween the light source and another detached, but otherwise illuminated, object
surface. Yonas [31] was one of the first to demonstrate that both attached and
cast shadows can affect the perceived relative size, shape, elevation, and depth of
a object viewed within a scene. However, because shadows cast by moving objects
'cascade' over objects and surfaces that are displaced at various distances and
orientations from the light-occluding object, cast shadows are potentially more
useful than attached shadows for extracting depth information about objects
viewed in the scene.
Scene Background Complexity. It has been demonstrated that complex
scene backgrounds impede spatial task performances, especially in the presence
of objects casting shadows [19]. However, in that experiment, background scene
complexity was manipulated by varying the entire configuration of the back-
ground (i.e. flat floor with no walls, 'stair-step' floor with no walls, or room with
walls). In a subsequent experiment [20], they manipulated the (simple or com-
plex) characteristics of room walls (as textured and/or 'zig-zag') that contained
the visual worlds. They again reported that more complex background scenery
can impair understanding the spatial characteristics and relationships of objects
depicted in 2D and 3D space. Studies [32, 33] have discussed forces that can help
or hinder task performances that require the integration of depth cues that are
in close proximity. Specifically, they state that increased perceptual information
access cost (IAC) can disrupt performance. They state [33]: “when noise or vi-
sual clutter is close to relevant indicators, it will disrupt the movement of visual
attention to the indicator, often imposing greater uncertainty as to where the
target is located.”