Autostereograms: analysis and algorithms.

by Lau Shek Kwan Mark.Thesis (M.Phil.)--Chinese University of Hong Kong, 2001.Includes bibliographical references (leaves 85-86).Abstracts in English and Chinese.Chapter 1 --- Introduction --- p.1Chapter 1.1 --- Historical Background --- p.2Chapter 1.2 --- Introduction to Autostereograms --- p.5Chapter 1.2.1 --- Geometrical Model --- p.5Chapter 1.2.2 --- IS-separation --- p.6Chapter 1.2.3 --- The Hidden Surfaces --- p.7Chapter 1.2.4 --- False Target and Echo --- p.8Chapter 1.3 --- The Autostereogram Generation Algorithm --- p.10Chapter 1.4 --- Further Applications of Autostereograms --- p.15Chapter 1.5 --- Organization of Thesis --- p.17Chapter 2 --- Analysis of Autostereograms --- p.20Chapter 2.1 --- IS-separation --- p.21Chapter 2.2 --- Autostereogram Generations --- p.25Chapter 2.3 --- Surface Reconstructions --- p.26Chapter 2.4 --- Visual Distortions --- p.28Chapter 2.4.1 --- Problem Model For Vertical Distortions --- p.30Chapter 2.4.2 --- Change of Depth Field --- p.33Chapter 2.4.3 --- Non-linear Distortion --- p.35Chapter 2.4.4 --- Lateral Distortions --- p.38Chapter 2.5 --- Discrete Autostereograms --- p.40Chapter 2.5.1 --- Truncation Problem --- p.41Chapter 2.5.2 --- Computer Algorithms for Autostereograms --- p.42Chapter 3 --- Analysis of Echoes --- p.48Chapter 3.1 --- Causes of Echoes --- p.49Chapter 3.1.1 --- Insufficient Lengths of The Periods of Repeating Patterns --- p.51Chapter 3.1.2 --- Overlapping of Copying Steps --- p.51Chapter 3.2 --- Avoidance of Type 1 Echoes --- p.52Chapter 3.3 --- Avoidance of Type 2 Echoes --- p.55Chapter 3.4 --- Autostereogram Encoding Any Surface --- p.58Chapter 4 --- Autostereogram as A Cryptosystem --- p.65Chapter 4.1 --- Introduction to Cryptography --- p.66Chapter 4.1.1 --- Mathematical Structure of Cryptosystems --- p.67Chapter 4.1.2 --- A Classical Cryptosystem´ؤSubstitution Cipher --- p.68Chapter 4.2 --- Autostereogram as a Cryptosystem --- p.72Chapter 4.2.1 --- Autostereogram as a Variation of Substitution Cipher --- p.74Chapter 4.2.2 --- Practical Considerations --- p.76Chapter 5 --- Conclusion and Future Works --- p.79Chapter 5.1 --- Future Works --- p.80Chapter A --- Excessive Removal of Copying Steps --- p.81Chapter B --- Publications Resulted from the Study --- p.8

The Impact of 2-D and 3-D Grouping Cues on Depth From Binocular Disparity

Stereopsis is a powerful source of information about the relative depth of objects in the world. In isolation, humans can see depth from binocular disparity without any other depth cues. However, many different stimulus properties can dramatically influence the depth we perceive. For example, there is an abundance of research showing that the configuration of a stimulus can impact the percept of depth, in some cases diminishing the amount of depth experience. Much of the previous research has focused on discrimination thresholds; in one example, stereoacuity for a pair of vertical lines was shown to be markedly reduced when these lines were connected to form a rectangle apparently slanted in depth (eg: McKee, 1983). The contribution of Gestalt figural grouping to this phenomenon has not been studied. This dissertation addresses the role that perceptual grouping plays in the recovery of suprathreshold depth from disparity. First, I measured the impact of perceptual closure on depth magnitude. Observers estimated the separation in depth of a pair of vertical lines as the amount of perceptual closure was varied. In a series of experiments, I characterized the 2-D and 3-D properties that contribute to 3-D closure and the estimates of apparent depth. Estimates of perceived depth were highly correlated to the strength of subjective closure. Furthermore, I highlighted the perceptual consequences (both costs and benefits) of a new disparity-based grouping cue that interacts with perceived closure, which I call good stereoscopic continuation. This cue was shown to promote detection in a visual search task but reduces depth percepts compared to isolated features. Taken together, the results reported here show that specific 2-D and 3-D grouping constraints are required to promote recovery of a 3-D object. As a consequence, quantitative depth is reduced, but the object is rapidly detected in a visual search task. I propose that these phenomena are the result of object-based disparity smoothing operations that enhance object cohesion

The efficiency of visual transparency

This thesis examines the phenomenon of visual transparency in a novel application of the efficiency approach. Transparency provides a useful stimulus to probe the visual mechanisms that underlie the visual surface representation, introduced in Chapter One. Previous research has found that there is a cost in processing visual transparency defined purely by motion or stereo cues. This has been interpreted in terms of visual mechanisms constraining the recovery of transparency. However, the cost for transparency may reflect the increased complexity of the stimuli. To address this issue I computed the efficiency for motion and stereo defined transparency tasks by comparing human performance with that of the ideal observer. The efficiency approach has two key advantages over traditional psychophysical measures: 1) it provides a performance measure normalised relative to the available information, 2) it is an absolute measure and can be compared directly across diverse tasks. I provide a review of the efficiency approach in Chapter Two. In Chapter Three, I present a study of the efficiency for speed discrimination of transparent random dot stimuli and comparable non-transparent random dot stimuli, as a function of the speed ratio and the dot density of the stimuli. In Chapter Four, I present a study of the efficiency for depth discrimination of transparent and non-transparent random dot stereograms, across a range of disparity ratios and dot densities. In Chapter Five, I present an extension of the efficiency approach to the motor domain, for the smooth pursuit of high-density transparent and non-transparent random-dot stimuli. Finally, in Chapter Six I provide physiologically plausible accounts of the findings

Spatial and temporal integration of binocular disparity in the primate brain

Le système visuel du primate s'appuie sur les légères différences entre les deux projections rétiniennes pour percevoir la profondeur. Cependant, on ne sait pas exactement comment ces disparités binoculaires sont traitées et intégrées par le système nerveux. D'un côté, des enregistrements unitaires chez le macaque permettent d'avoir accès au codage neuronal de la disparité à un niveau local. De l'autre côté, la neuroimagerie fonctionnelle (IRMf) chez l'humain met en lumière les réseaux corticaux impliqués dans le traitement de la disparité à un niveau macroscopique mais chez une espèce différente. Dans le cadre de cette thèse, nous proposons d'utiliser la technique de l'IRMf chez le macaque pour permettre de faire le lien entre les enregistrements unitaires chez le macaque et les enregistrements IRMf chez l'humain. Cela, afin de pouvoir faire des comparaisons directes entre les deux espèces. Plus spécifiquement, nous nous sommes intéressés au traitement spatial et temporal des disparités binoculaires au niveau cortical mais aussi au niveau perceptif. En étudiant l'activité corticale en réponse au mouvement tridimensionnel (3D), nous avons pu montrer pour la première fois 1) qu'il existe un réseau dédié chez le macaque qui contient des aires allant au-delà du cluster MT et des aires environnantes et 2) qu'il y a des homologies avec le réseau trouvé chez l'humain en réponse à des stimuli similaires. Dans une deuxième étude, nous avons tenté d'établir un lien entre les biais perceptifs qui reflètent les régularités statistiques 3D ans l'environnement visuel et l'activité corticale. Nous nous sommes demandés si de tels biais existent et peuvent être reliés à des réponses spécifiques au niveau macroscopique. Nous avons trouvé de plus fortes activations pour le stimulus reflétant les statistiques naturelles chez un sujet, démontrant ainsi une possible influence des régularités spatiales sur l'activité corticale. Des analyses supplémentaires sont cependant nécessaires pour conclure de façon définitive. Néanmoins, nous avons pu confirmer de façon robuste l'existence d'un vaste réseau cortical répondant aux disparités corrélées chez le macaque. Pour finir, nous avons pu mesurer pour la première fois les points rétiniens correspondants au niveau du méridien vertical chez un sujet macaque qui réalisait une tâche comportementale (procédure à choix forcé). Nous avons pu comparer les résultats obtenus avec des données également collectées chez des participants humains avec le même protocole. Dans les différentes sections de discussion, nous montrons comment nos différents résultats ouvrent la voie à de nouvelles perspectives.The primate visual system strongly relies on the small differences between the two retinal projections to perceive depth. However, it is not fully understood how those binocular disparities are computed and integrated by the nervous system. On the one hand, single-unit recordings in macaque give access to neuronal encoding of disparity at a very local level. On the other hand, functional neuroimaging (fMRI) studies in human shed light on the cortical networks involved in disparity processing at a macroscopic level but with a different species. In this thesis, we propose to use an fMRI approach in macaque to bridge the gap between single-unit and fMRI recordings conducted in the non-human and human primate brain, respectively, by allowing direct comparisons between the two species. More specifically, we focused on the temporal and spatial processing of binocular disparities at the cortical but also at the perceptual level. Investigating cortical activity in response to motion-in-depth, we could show for the first time that 1) there is a dedicated network in macaque that comprises areas beyond the MT cluster and its surroundings and that 2) there are homologies with the human network involved in processing very similar stimuli. In a second study, we tried to establish a link between perceptual biases that reflect statistical regularities in the three-dimensional visual environment and cortical activity, by investigating whether such biases exist and can be related to specific responses at a macroscopic level. We found stronger activity for the stimulus reflecting natural statistics in one subject, demonstrating a potential influence of spatial regularities on the cortical activity. Further work is needed to firmly conclude about such a link. Nonetheless, we robustly confirmed the existence of a vast cortical network responding to correlated disparities in the macaque brain. Finally, we could measure for the first time retinal corresponding points on the vertical meridian of a macaque subject performing a behavioural task (forced-choice procedure) and compare it to the data we also collected in several human observers with the very same protocol. In the discussion sections, we showed how these findings open the door to varied perspectives

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task