Shadows and Reflections

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Shadows and reflections

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The design was identical to that of Experiment 4. Unlike in Experiment 4, these results suggest that subjects have some ability to identify consistent and inconsistent reflections. Perhaps, then, subjects make use of the reflection vanishing point to objectively judge the consistency of the reflections in a scene, but only in instances where the vanishing point is relatively easy to determine.

This finding offers some support for our suggestion that people might only rely on perceptual biases to make judgements about reflections when there is a lack of information available to make a more informed decision. Experiment 5. Response time, confidence, and reflection vanishing point distance were added as continuous variables. The reflection position predictor variable was not applicable in the consistent reflection scenes. Considering the consistent reflection scenes, the GEE analysis revealed that only one variable had an effect on accuracy—the distance of the reflection vanishing point.

As with the inconsistent scenes, when the reflection vanishing point was closer to the center of the image the scenes were more likely to be identified as consistent compared with when it was further from the center. It appears, then, that people might be able to make use of the geometrical information provided in the scenes to objectively judge the validity of the reflections when the reflection vanishing point is closer to the center of the image. That said, another possibility is that moving the inconsistent reflections further from the consistent position in Experiment 5 than in Experiment 4 made it more visually apparent when the reflections were consistent versus inconsistent.

So why did subjects perform better on the reflection task in Experiment 5 than in Experiment 4? Given that we made two changes to the stimuli between Experiments 4 and 5, there are two possible reasons. One possibility is that creating scenes with the reflection vanishing point closer to the center of the image made it easier for people to use geometric analysis to work out the answer. A second possibility is that the bigger physical distance between the consistent and inconsistent reflection position made it easier to make a subjective judgement about the consistency or inconsistency of the reflections in the scene.

In Experiment 6, the scene reflection vanishing point remained the same as in Experiment 5, but we decreased the distance between the inconsistent and the consistent reflection position to match the distance in Experiment 4. A further 10 subjects were excluded from the analyses because they experienced technical difficulties. The design was identical to that of Experiments 4 and 5.

The stimuli remained the same as in Experiment 5 with one exception: We decreased the distance that we moved the street sign reflection from its consistent position when creating the inconsistent scenes. In Experiment 5, we moved the street sign 10 m on the z axis relative to the original street sign position; this time we moved it the same distance as in Experiment 4 7 m. The resulting mean angle difference between the reflection vanishing point for the inconsistent reflections and the reflection vanishing point for the rest of the scene was 2.

Experiment 5: 3. In line with this suggestion, in Experiment 5 the inconsistent reflections were positioned further from the consistent position than in Experiment 6, and we did not find evidence of a response bias. Taken together, these results suggest that people might have a relatively conservative criterion for judging that the reflections in a scene are inconsistent.

As such, it is possible that people have a perceptual threshold for detecting reflection inconsistencies—that is, there is a point at which the inconsistent reflections are close enough to the consistent position that people will find it extremely difficult to detect the inconsistency; instead, they simply accept the reflection as consistent. Experiment 6. Our finding that there was an influence of reflection position in Experiments 4 and 6, but not in Experiment 5, suggests a perceptual threshold for detecting when reflections in a scene are inconsistent.

Shadows and Reflections: Florida's Lost People

Yet in Experiments 4 and 6 when the inconsistent reflections were moved a smaller distance 7 m from the consistent position, we did find a reliable effect of reflection position on performance. These findings suggest that there might be a point at which the inconsistent reflection becomes different enough from its consistent position to make the inconsistency noticeable.

That said, it is important to note that adjusting the distance of the inconsistent reflections from the original position also changes the angle difference between the scene reflection vanishing point and the reflection vanishing point for the inconsistent reflection—as the distance increases, so does the angle difference. Thus, we are not able to isolate the two factors and test them individually. For each subject, we calculated the number of correct responses on the inconsistent trials—0, 1, or 2.

Our results revealed that both of these variables had an effect on the likelihood of responding correctly. At first glance this effect of vanishing point distance seems somewhat surprising—intuitively, a vanishing point closer to the center of the image would be easier to determine than one that is located further away.

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Therefore, perhaps subjects did not make use of the reflection vanishing point when deciding whether the reflections in each scene were consistent or inconsistent. An accurate reconstruction of the reflection vanishing point offers an objective technique for verifying the in consistency of the reflections in a scene. Nonetheless, this reconstruction is a perceptually challenging task that requires accurately locating numerous object and corresponding reflection points as well as mapping these corresponding points as lines that extend beyond the image plane.

Accordingly, our findings suggest that when deciding whether the reflections in a scene were consistent or inconsistent, subjects tended to rely on the appearance of the in-plane image cues rather than attempting to determine the reflection vanishing point. The results of Experiment 4 suggest that people have an extremely limited ability to identify when reflections in a scene are consistent or inconsistent.

Moreover, in Experiment 6 we kept the vanishing point position the same as in Experiment 5, but decreased the distance between the inconsistent and the consistent reflection position to match the distance in Experiment 4. Subjects then correctly classified fewer of the consistent and inconsistent scenes than in Experiment 5, but more than in Experiment 4. Across seven experiments, we found that people had a limited ability to determine whether the shadows or reflections within a scene were consistent or inconsistent, and this ability depended to some degree on the size of the inconsistency.

Given the ubiquity of image manipulation, it is important to consider what our results reveal about how people process visual information. The current findings add to our theoretical understanding of how the visual system processes information.

Although people seemingly experience a detailed and coherent picture of the world, the striking finding that people are slow to detect even large changes that occur during a real or simulated eye blink suggests that this is not the case e. Why, then, do people have the impression of observing a richly detailed and coherent world? From the standpoint of coherence theory, this impression is the result of a visual system that generates a sparse and incomplete representation of the scene whereby most parts are represented only at a preattentive level Rensink, , This incomplete representation is achieved via a low-level subsystem that involves an automatic and continual processing of the visual scene to generate simple visual elements without the awareness of the observer.

According to coherence theory, a limited-capacity attentional subsystem can form a subset of visual elements into a coherent and detailed object representation—this is the basis of conscious perception. Owing to a finite attentional capacity, these detailed, conscious representations are only created for the objects needed for the task at hand.

The attentional subsystem is guided via a combination of low-level factors e. Together, these subsystems can provide the impression that perceptions are stable and highly detailed, even though a complete representation of the scene is never constructed. Furthermore, previous research has shown that when people take an effortful approach to attend to the details of a scene, aspects such as shadows and reflections rarely receive attention Ehinger et al. Indeed, this insensitivity to shadow and reflection information can account for our finding that, even when cued to the target object and its shadow, people struggled to make an accurate subjective judgement about whether or not these aspects of the scene were consistent or inconsistent.

Specifically, if people discard information about shadows and reflections at an early stage of visual processing, then it follows that they will not have an opportunity to learn how these aspects should appear. That is, under the current experimental conditions, people were more likely to detect inconsistent shadows and reflections when they were positioned further from the correct position. As such, our results fit with the notion of a perceptual threshold for detecting lighting inconsistencies based on cues within the image plane Lopez-Moreno et al.

It seems possible, then, that there is a discernible point at which the inconsistent shadows are sufficiently different from the consistent position to be detected preattentively. But for shadow inconsistencies that do not pass this perceptual threshold, a more effortful strategy is required for people to detect the inconsistency. Indeed, our results from the three reflection experiments revealed a similar possibility—that people might have a perceptual threshold for noticing inconsistencies in the reflections in a scene.

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As such, even if forgers leave behind inconsistencies in the shadows or reflections in an image, our results suggest that people are unlikely to capitalize on such tell-tale signs to help to detect forgeries in the real world. Essentially, our results suggest that when image manipulations create quite large inconsistencies in the shadows or reflections of the scene, people might be able to detect the image as a fake. Yet it is important to note that subjects in our study were explicitly asked to examine the images and determine if the image had been manipulated.

Moreover, they were even cued as to exactly which shadow or reflection might have been changed. Despite this, and the fact that they took a relatively long time examining the images the shortest mean response time per image in an experiment was In more casual and limited viewing conditions, the tolerances for accepting faked images as real with shadow and reflection inconsistencies is likely to be much higher than those reported here.

These types of analyses form the basis of some of the digital image forensic computer programs that can help to verify the authenticity of images e. Yet our results suggest that people are reasonably insensitive to inconsistencies in shadows and reflections.