5.1. Guidance for Hue, Size and Shape
Wolfe and Horowitz [9
] summarized previous research on the guidance from visual variables in 2D displays and found that hue and size unambiguously provide guidance, whereas the level of guidance from shape is less clear. However, the guidance strengths of these variables reveal a different trend in 3D visualization.
In our 3D-object experiment, hue provided the same strong guidance as for 2D objects [30
]. This finding occurred because hue is an associative visual variable that requires pre-attention and the addition of an extra dimension does not cause the hue to change. Although an object’s color appearance may be influenced by illumination [38
], the hues used here were quite distinct from each other; consequently, the subjects could still identify the targets.
However, the guidance from shape is stronger than that from size in 3D visualization. In 3D displays, the retinal image from the same object is smaller when it is far away. Although some researchers have stated that users can correctly perceive objects to be of the same size under this condition [57
], Exp. 2 indicated the opposite. Subjects who are aware of this effect will automatically adjust for the perceived size [58
], which requires additional time. Moreover, in a symbolic representation, which differs from real-world conditions, fewer objects are present in the background; therefore, depth cues [48
], which are important for size perception [20
], are lacking. The results of Exp. 2 support this conclusion because size cannot successfully guide visual attention when people are unable to correctly perceive size.
In 3D visualization, the shape of each symbol takes on an additional dimension compared to the 2D case; therefore, more characteristics are required to identify a particular shape. The complexity of shape guidance may also be related to the type of shape that is used. For example, shapes that use letters provide strong guidance [32
] in 2D visualization, but according to Chen [61
], the shape sensitivity for a line, which is an even simpler object, is influenced by its relationships to other shapes. In the physical world, where shape complexity can be very high, shape is also a practical visual variable for distinguishing landmarks in wayfinding [26
]. Moreover, judgment regarding the shapes of distant objects, which requires more details than hue judgment, may be difficult. Although the shapes in our stimuli were simple geometric shapes, distant objects may have been obscured by the low resolution. For example, the participants may have been unsure whether the more distant cubes were other hexahedrons; therefore, they spent more time on these objects. Generally, our guidance experiment shows that the guidance from simple geometric 3D shapes is not as strong as those from hue.
5.2. Constancy of Saturation, Size and Shape
Constancy is a persistent topic in geo-visualization and in 3D visualization in general, because it is unlike 2D visualization, in which the retinal signal of an object does not change regardless of where that object is located. As the AC results show, size constancy is the weakest and saturation constancy is the strongest. In addition, FT, AFD, and VC help to reveal the perception processes.
In this study, the FT
results show that the total time of saturation perception is strongly influenced by where the variation occurs. A shorter FT
may indicate a quick judgment of the variation in one location and a decreased judgment time for one process out of six. The data show that the subjects finished the tasks quickly when the saturation variation was located nearby or at a relatively near side position; however, when the variation in saturation was located far away or at a relatively near central position, subjects required more time to finish the task. We did not collect the FT
values for each separate location, so we can only assume that subjects may have expected that there was at least one symbol that was different from the reference object. Because the participants found changes in nearby or relatively near side locations easier to judge, they might have felt “more comfortable” judging the rest to be the same as the reference object after finding an object that was different. If this inference is correct, then this finding (i.e., that detecting changes in certain locations is more difficult than detecting changes in others) may be related to the decreased area occupied by more distant objects or to a location-related illusion, which would be consistent with the findings of Olkkonen and Brainard [63
results show that size perception is influenced most by position and requires more processing than shape and saturation do, and indicates that perception of size is more difficult, which is also supported by the poor size guidance in Exp. 1. This result is also consistent with the metacognitive theory that people supplement their perceptions with deliberate judgments when they are aware of the size-distance effect [52
], a process that requires further investigation.
However, the AFD
of size was the least influenced by position, indicating that the participants subjectively considered the difficulties at different locations to be equal, whereas the AC
results show that location actually exerts the greatest objective influence on the difficulty of size judgment. This discrepancy indicates that size perception is difficult and that the low AC
of size perception may result from subjects’ inattention to detail. Meanwhile, the AFD
results for shape indicate that subjective perception difficulty of shape increases as the object becomes more distant (location A). This observation is consistent with previous findings that a change in size can increase the effort required to identify shapes [56
shows the constancy of perception results. As for size, judgments for near objects (C and F) are less likely to be correct, whereas judgments for objects at middle distances are most accurate. Concerning orientation, the accuracy for central objects is greater than that for objects to the side. This result is consistent with the findings of Gori, Giuliana, Sandini and Burr [60
], who stated that the perception of close-to-standard objects is more precise, which indicates that judgment may be influenced by the retinal size of an object. However, in size constancy research in the physical world, the estimation bias seems to increase when the object is positioned farther away [65
], whereas our work produced a different result. This difference may be related to the integration of the haptic [60
] and assumed-size strategies [59
] that people use in the physical world. Regarding shape, the accuracy for distant objects is lower because the image may become blurred from the lower resolution of the stimulus. In addition, judgments for objects near the sides of the image are less likely to be correct. These objects are subject to stronger deformation, which results in asymmetry; therefore, subjects may judge them to be different from the target. This finding is consistent with previous research on 3D objects [19
], which revealed a greater capacity for shape discrimination for symmetrical objects. Judgments of saturation for near and far symmetrical objects are less likely to be correct, and no clear rule exists regarding orientation.
From Exp. 1, we know that subjects are not sensitive to variations in size in 3D visualization; therefore, the subjects may have relied on instinct when making size judgments, which is not always reliable. The subjects were sensitive to variations in shape (Exp. 1 did not address changes in saturation), and they can rationally identify shape variations when they occur, so the constancy of shape is stronger than that of size. However, distance may influence the visible level of detail or the visible area of an object, and orientation can influence an object’s deformation and its visible surface. Thus, the judgments regarding shape or saturation at different positions are not uniform.
In summary, Exp. 2 suggests that the constancy of shape and saturation is strong, whereas that of size is very poor. In addition, size is more difficult to perceive and judge than are shape and saturation, as reflected by a longer FT and an increased VC; consequently, size perception requires more processing but has lower accuracy. According to the same indices, judgments of shape and saturation are easier.