KEYWORDS: Photovoltaics, Signal processing, Electronic imaging, Switches, 3D vision, Medical research, Analytical research, Biomedical engineering, Psychology, Human vision and color perception
There are at least two broad classes of three-dimensional (3D) stimuli that tend to be perceived in illusory reverse depth: hollow masks and “reverspectives”, the latter having been invented by Patrick Hughes in 1964. Because of the depth inversion, these stimuli appear to move when observers move in front of them. The illusion is diminished significantly when a hollow mask is inverted, as compared to an upright mask; the same trend is observed with inverted reverspectives, as compared to upright reverspectives, but the inversion effect is weaker than that in faces. The inversion effect can be attributed to top-down influences in perception, and the results point to a stronger role of such influences for the perception of faces than scenes.
When the moon is half, one would expect that a line starting from the moon’s center and being perpendicular to the “shadow diameter” would, if extended, go through the center of the light source, namely, the sun. It turns out that, when the sun is visible, this extended line appears to aim significantly above the sun, which is the essence of the “half-moon illusion”. The explanation advanced here is that this is not an optical illusion; instead, it can be explained by the relative sizes and distances of the earth, moon, and sun, and it hinges on the fact that the sunrays are nearly parallel with respect to the earth-moon system. It turns out that the ancients knew and used this near-parallelism of the sunrays. Eratosthenes, for example, used a simple but ingenious scheme to obtain a good estimate of the earth’s circumference. An interesting question is: How did the ancients arrive at the conclusion that the sunrays are nearly parallel? This was probably a corollary, based on the immense size of the sun and its huge distance from the earth, as estimated by, among others, Aristarchus of Samos by a brilliantly simple method.
We present a psycho-visual and analytical framework for automatic measurement of motion activity in view sequences. We construct a test-set of video segments by carefully selecting video segments form the MPEG-7 video test set. We construct a ground truth, based on subjective test with naive subjects. We find that the subjects agree reasonably on the motion activity of video segments, which makes the ground truth reliable. We present a set of automatically extractable, known and novel, descriptors of motion activity based on different hypotheses about subjective perception of motion activity. We show that all the descriptors perform well against the ground truth. We find that the MPEG-7 motion activity descriptor, based on variance of motion vector magnitudes, is one of the best in overall performance over the test set.
'Reverspectives' is one of many titles given to a remarkable class of Patrick Hughes's paintings that share two basic properties: (1) They are painted on a set of intersecting planar surfaces, rather than a single flat surface. (2) The perspective produced by the painting elicits the percept of a depth configuration that is the reverse of the actual depth configuration, as dictated by the construction of the multi- planar combination of surfaces (hence the name reverspective). As a result of these two properties, reverspectives appear to turn vividly as viewers walk past the paintings. This strange motion, which can rarely be observed in flat-canvas paintings unless they elicit strong perspective depth, can be explained by a neural process that matches up kinesthetic information about the viewer's own motion to changes in the retinal images. In this paper I study the role played by the perspective painting, by comparing percepts elicited by conventional Hughes reverspectives to those elicited by the same multi-planar surfaces that are left blank (unpainted). I examine differences in the two precepts in terms of the critical distance, i.e. that viewing distance for which the depth percept switches from veridical to reverse. This paradigm is one example in which the adoption of a depth schema determines the interpretation of motion signals.
We describe psychophysical experiments conducted to study PicHunter, a content-based image retrieval (CBIR) system. Experiment 1 studies the importance of using (a) semantic information, (2) memory of earlier input and (3) relative, rather than absolute, judgements of image similarity. The target testing paradigm is used in which a user must search for an image identical to a target. We find that the best performance comes from a version of PicHunter that uses only semantic cues, with memory and relative similarity judgements. Second best is use of both pictorial and semantic cues, with memory and relative similarity judgements. Most reports of CBIR systems provide only qualitative measures of performance based on how similar retrieved images are to a target. Experiment 2 puts PicHunter into this context with a more rigorous test. We first establish a baseline for our database by measuring the time required to find an image that is similar to a target when the images are presented in random order. Although PicHunter's performance is measurably better than this, the test is weak because even random presentation of images yields reasonably short search times. This casts doubt on the strength of results given in other reports where no baseline is established.
This study reports on experiments conducted with human observers to investigate the properties of linear and non- linear, perceptual grouping mechanisms by using reverse- polarity sparse random-dot patterns. The stimuli were generated by spatially superimposing a sparse set of randomly distributed square elements onto a copy of the original set that was expanded or rotated about the center of the screen. In the control experiment both the original and transformed sets contained elements of identical luminance contrast with the background. The main experiments involved a reverse- contrast random-dot pattern, in which the transformed set consisted of elements of equal contrast magnitude but opposite polarity to that of the original set. At least two competing global percepts are possible: 'forward grouping' in which perceived grouping agrees with the physical transformation; or 'reverse grouping' in a direction orthogonal to that of the 'forward grouping.' The two-alternative forced-choice (2AFC) task was to report the direction of the global grouping. For the control experiment, the observers reported forward grouping both at the fovea and eccentricities of up to 4 degrees; as expected, no reverse grouping was observed. With the reverse-polarity stimulus, reverse grouping was observed at high eccentricities and low contrasts, but forward grouping dominated under foveal viewing. In another experiment, the influence of chromatic mechanisms was studied by using opposite-contrast red elements on a yellow background. In this experiment reverse grouping was not observed, which indicates that color mechanisms veto reverse grouping. Reverse grouping can be hypothesized to be the result of processing by linear oriented spatial mechanisms, in analogy with reverse-phi motion. Forward grouping, on the other hand, can be explained by non-linear preprocessing (such s squaring or full-wave rectification).
We report on psychovisual experiments designed to obtain subjective-based thresholds for a novel conditional-replenishment image-sequence coder. This coder attempts to avoid the replenishment of textured blocks for which no subjective change has occurred from the previous to the current frame. Typically, such blocks give rise to a large difference signal with respect to the corresponding block in the previous image, and hence are coded (replenished) in commonly used coders. We designed and conducted extensive visual experiments to study the response of the human visual system to stimuli that are relevant to the coding algorithm. Three major classes of experiments were conducted with numerous parametric variations for each, in which the observers were asked to discriminate target elements with properties that differed from those of the background: (1) Uniform targets on uniform background of different intensity. (2) Textured targets of varying standard deviation on a uniform background of the same average intensity. (3) Textured targets on a textured background with the same standard deviation, but different average intensity. We report on the results of these experiments and on the improvement in the performance of the coder, as a result of implementing these results in the encoding algorithm.
KEYWORDS: Visualization, Visual process modeling, Silicon, Human vision and color perception, Colorimetry, Electronic imaging, Silver, Motion models, Information operations, 3D image processing
We report on several experiments that we designed to study the relative strength of visual attributes in the perception of
texture. Our stimuli are composed of microelements that are arranged regularly in two-dimensional space. Each
microelement can be characterized by the conjunction of several attributes (shape, size, color, etc.). Although the spatial
regularity results in a "homogeneous" texture, we can manipulate the arrangement of selected attributes so as to break this
homogeneity by forming textural patterns, whose discriminability can be tested experimentally.
The stimuli allow flexibility in choosing the ways in which different attributes can be matched spatially to generate
patterns. We have used these stimuli to study the roles of color, orientation, luminance and polarity in forming textures.
Preliminary results indicate an inherent similarity between the mechanisms subserving texture perception and those
mediating the perception of motion; the latter were studied with a similar type of stimuli. Finally, we report on a method
for isolating the role of specific texture mechanisms by comparing the results of carefully selected experiments.
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