Pictorial and motion-based information for depth perception.

When computer-generated objects approached the viewpoint in midair, a large far object appeared to be nearer than a small near object and appeared to hit the viewpoint before the small object, which was specified by time-to-contact information to arrive sooner. These judgements were consistent with relative size and occurred even when motion-based information was potentially above threshold. The effects of relative size persisted with higher resolution animated films of approaching objects, were weakened by ground-intercept information, and were not as robust with laterally translating objects. Although it is often asserted that the kinds of information that have traditionally been called static depth cues are superseded by motion-based depth information, this article attempts to show that the reverse also can occur in distance perception, as has been shown in form perception.     

Pictorial depth perception: Tasks levels imposed by testing instruments

Attempts to devise verbal and non-verbal instruments for measuring pictorial depth perception that are different from Hudson's (1960) have resulted in different perceptual demands. Whereas this was recognized by the constructors of non-verbal tests, it was not by those who changed Hudson's question “Which is nearer to the man…?” to “Which is/looks nearer/farther to you… ?”. While the latter question tests simple perception (or interpretation of cues) of pictorial depth, the former tests comprehension of complex pictorial space relations arising from the depth perceived. A verbal instrument that had been devised for testing simple pictorial depth interpretation, equivalent to Jahoda and McGurk's (1974) non-verbal instrument, was tested on Grade 3 Shona children against the hypothesis that the terms “nearer” and “farther” have different lexical markings likely to affect subject's responses. No support was found for the hypothesis, but Opolot's (1976) Grade 3 findings were able to be explained.     

Differential cognitive cues in pictorial depth perception among Ugandan children

Four pictures from Hudson's Depth Perception Test containing size, super-position or overlap, and linear perspective cues were administered to 240 Ugandan primary school children. The key question asked regarding the relationship of elements in each picture varied in terms of “Which (is/looks) (nearer/farther) to (man/you), the elephant or the antelope?”. Results suggested that depth in pictorial material was perceived significantly higher for the question variation “which is/looks farther” rather than “nearer.” In addition, the main effects of “grades” and “man/you” and the “grade x nearer/farther” interaction were statistically significant. Results are discussed in terms of culturally constituted experience and the effects of attentional factors and lexical markings inherent in the relational concepts used in the key question.     

Pictorial depth and the Poggendorff illusion

An informal demonstration is offered, which strongly supports previous contentions that, when the elements of a Poggendorff display appear to be arranged in pictorial space such that the two critical line segments are at different heights, an illusory impression of misalignment may occur. A second pair of demonstrations shows, however, that such a height difference is neither a necessary nor a sufficient cause of the illusion. In addition, the harmful effect of adding certain pictorial elements to the standard Poggendorff pattern requires a new understanding.     

Pictorial surface attitude and local depth comparisons

We measured local surface attitude for monocular pictorial relief and performed pairwise depthcomparison judgments on the same picture. Both measurements were subject to internal consistency checks. We found that both measurements were consistent with a relief (continuous pictorial surface) interpretation within the session-to-session scatter. We reconstructed the pictorial relief from both measurements separately, and found results that differed in detail but were quite similar in their basic structures. Formally, one expects certain geometrical identities that relate range and attitude data. Because we have independent measurements of both, we can attempt an empirical verification of such geometrical identities. Moreover, we can check whether the statistical scatter in the data indicates that, for example, the surface attitudes are derivable from a depth map or vice versa. We estimate that pairwise depth comparisons are an order of magnitude less precise than might be expected from the attitude data. Thus, the surface attitudes cannot be derived from a depth map as operationally defined by our methods, although the reverse is a possibility.     

Pictorial depth: intensity and aesthetic surface

Philosophers seldom ask questions regarding how certain phenomena occur, because such questions tend to be the province of the sciences or of technology. However, the question how pictures have depth requires philosophical reflection because it takes place on the surface of pictorial objects and involves both physical and phenomenal, i.e. aesthetic, features of those surfaces. This essay examines how pictures have depth by first separating the aesthetic question from interpretive considerations, and thereby refining the question how pictures have depth. Next it explicates two sorts of conceptual tools required to understand the question: several complex concepts needed to understand surfaces, and the concept of intensity. These are then used to understand how pictures can have depth by showing how intensities produce both an aesthetic surface and depth within it.     

Attention and depth perception

The Necker cube is a line drawing with two possible solutions in depth perception. The process of interpreting a two-dimensional line drawing as a three-dimensional object was investigated using the Necker cube. Attention was directed to a local feature of a briefly presented cube, ie an angle at a vertex. The attended angle was perceived as a front part of the cube and other parts were interpreted so as to match this interpretation. Results show that the local feature to which attention was directed was interpreted first and then global features and other local features were interpreted so as to agree with the local feature interpreted initially. This suggests that the three-dimensional interpretation of the line drawing was made sequentially from the local feature to global structures.     

Pictorial perspective: perception of size, linear, and texture perspective in children and adults

Perception of size, linear, and texture perspective was investigated in third-grade and sixth-grade children and in college adults in three separate studies. A matching task required the observer to choose from a set of four alternative real scenes the correct match for the test stimulus, which was either a picture or a real scene. Correct performance required that the subject utilize perspective information for both size and distance distance relations. Erroneous choices available to the subject indicated errors in size judgement, in distance judgment, or in both simultaneously. View was either restricted at the correct station point or was free, with head motion. There were no significant effects of grade level. For all three groups, mean percent correct was nearly 100% with the real-scene test stimuli, and significantly below the chance level with the picture test stimuli. Errors in size judgment occurred most frequently, indicating that the geometrically correct rate of perspective convergence was too rapid to be seen by the subjects as perceptually acceptable. With size-perspective information alone, the number of size plus distance errors also increased significantly. There was no significant effect of viewing condition.     

Pictorial representations in adults and children

Reaction time to make same-object judgements was measured for pairs of identical pictures, picture synonyms, identical words, word synonyms, and picture-word combinations in adults and children. At all ages, synonym comparisons took longer than identical comparisons. Adults, but not children responded no faster to picture-word pairs than to picture synonym pairs. This is taken as evidence for the use of abstract pictorial information by adults but not by children. Children seem to compare two different exemplars of the same object verbally in the absence of well-integrated abstract pictorial representations.     

Velocity gradients and relative depth perception

The effectiveness of velocity gradients in providing relative depth information was assessed using random dot patterns translating horizontally. The gradients simulated two planes meeting at a horizontal line at the center, and subjects judged whether the center was the nearest or farthest part of the display. Accuracy increased with maximum dot speed, exceeding 90% in conditions combining the highest speed (10.4^o/sec) and longer of two display durations (10 sec) with unrestricted fixation. Separate experiments examined a rotational component perceived in the motion of the planes and the latency in reporting a rigid organization of the displays. Possible reasons for the chance accuracy found by Farber and McConkie (1979) and alternative explanations of the effect of maximum dot speed on accuracy are discussed. A model is presented that accounts for the effects of dot speed and display duration on the accuracy of relative depth judgments.     

Binocular rivalry and stereoscopic depth perception

An investigation was made of stimulus factors causing retinal rivalry or allowing stereoscopic depth perception, given a requisite positional disparity. It is shown that similar colour information can be “filtered” out from both eyes; that stereopsis is not incompatible with rivalry and suppression of one aspect of the stimulus, and that the strongest cue for perception of stereoscopic depth is intensity difference at the boundaries of the figures in the same direction at each eye. Identity of colour can also act as a cue for stereopsis. The brightness of different monocular figures seen in the stereoscope in different combinations was estimated by a matching technique, and it is suggested that the perceived brightness is a compromise between the monocular brightness difference between figure and ground seen in relation to the binocular fused background, and the mean brightness of the figures. The results are discussed in terms of neurophysiological “on,” “off” and continuous response fibres.