In this thesis, I investigated how human vision processes colour and luminance information to enable perception of our environment. I first tested how colour can alter the perception of depth from shading. A luminance variation can be interpreted as either variation of reflectance (patterning) or variation of shape. The process of shape-from-shading interprets luminance variation as changes in the shape of the object (e.g. the shading on an object might elicit the perception of curvature). The addition of colour variation is known to modify this shape-from-shading processing. In the experiments presented here I tested how luminance driven percepts can be modified by colour. My first series of experiments confirmed that depth is modulated by colour. I explored a larger number of participants than previously tested. Contrary to previous studies, a wide repertoire of behaviour was found; participants experienced variously more depth, or less depth, or no difference. I hypothesised that the colour modulation effect might be due to a low-level contrast modulation of luminance by colour, rather than a higher-level depth effect. In a second series of experiments, I therefore tested how the perceived contrast of a luminance target can be affected by the presence of an orthogonal mask. I found that colour had a range of effects on the perception of luminance, again dependant on the participants. Luminance also had a similar wide range of effects on the perceived contrast of luminance targets. This showed that, at supra-threshold levels, a luminance target's contrast can be modulated by a component of another orientation (colour or luminance defined). The effects of luminance and colour were not following a particular rule. In a third series of experiments, I explored this interaction at detection levels of contrast. I showed cross-interaction between luminance target and mask but no effects of a colour mask.

Flicker defined form (FDF) is a temporally-dependent illusion created by the counterphase flicker of randomly positioned element dots, that preferentially stimulates the magnocellular system. Previous studies have found improvement with peripheral presentation, a resistance to blur and a dependence on high temporal frequencies. (Quaid & Flanagan, 2005a; Quaid & Flanagan, 2005b). Although it is seemingly very different from most luminance defined, static stimuli, it is still unknown in what ways it differs. The current study aimed to determine how FDF varies or is similar to static, luminance defined stimuli. Current results showed that FDF could be matched to particular spatial frequencies, and improved with increasing background structure and area. Shapes could be discriminated from each other and recognized. These results suggest that although FDF is dependent on motion pathways for temporal dynamic perception, it could also benefit from the input of form perception pathways, depending on the cues present in the stimulus (e.g. background structure, area). Results also showed that FDF does not benefit from Gestalt rules of contour closure, unlike some static stimuli, although related studies have shown that FDF could still be detected in spite of blur. These studies suggest that FDF appears to rely on motion perception pathways, areas such as MT, but is easier to perceive at times due to overlap in function with shape perception pathways, areas such as IT. As such FDF shares many characteristics with other motion-defined-form stimuli, but uniquely shares aspects of form vision.

Flicker defined form (FDF) is a temporally-dependent illusion created by the counterphase flicker of randomly positioned element dots, that preferentially stimulates the magnocellular system. Previous studies have found improvement with peripheral presentation, a resistance to blur and a dependence on high temporal frequencies. (Quaid & Flanagan, 2005a; Quaid & Flanagan, 2005b). Although it is seemingly very different from most luminance defined, static stimuli, it is still unknown in what ways it differs. The current study aimed to determine how FDF varies or is similar to static, luminance defined stimuli. Current results showed that FDF could be matched to particular spatial frequencies, and improved with increasing background structure and area. Shapes could be discriminated from each other and recognized. These results suggest that although FDF is dependent on motion pathways for temporal dynamic perception, it could also benefit from the input of form perception pathways, depending on the cues present in the stimulus (e.g. background structure, area). Results also showed that FDF does not benefit from Gestalt rules of contour closure, unlike some static stimuli, although related studies have shown that FDF could still be detected in spite of blur. These studies suggest that FDF appears to rely on motion perception pathways, areas such as MT, but is easier to perceive at times due to overlap in function with shape perception pathways, areas such as IT. As such FDF shares many characteristics with other motion-defined-form stimuli, but uniquely shares aspects of form vision.

Three-dimensional (3D) object perception is critical for comprehending and interacting with the world. It develops during infancy and continues through adulthood. One powerful cue used for object perception is uniform coherent motion. The present paper first briefly reviews the current literature concerning object perception using random-dot stimuli and structure-from-motion (SFM) displays. To extend our knowledge in this area, two new studies were conducted to further our understanding of how infants process 3D shape in SFM stimuli. Study 1 examined infants of two age groups (3-5 month-olds and 8-9 month-olds) in a familiarization phase and a test phase. In the familiarization phase, infants were exposed to one of two SFM shapes (cube or cylinder) and in the test phase infants viewed both SFM shapes side-by-side. Extraction of shape was measured through novelty preferences. Results of Study 1 suggest that both age groups successfully extracted 3D shape. Study 2 served as a replication and extension, with the added control for the variable rotational axis. When this variable was controlled for, 3-5 month-olds failed to show a novelty preference during the test phase. These results suggest not only that infants were attending to both the global shape presented in the SFM stimuli as well as the detailed component of the rotational axis of the stimuli, but also that adding the extra change in the component of rotational axis to SFM stimuli makes the task of extracting shape more difficult for infants. These findings contribute to the infant literature by furthering the understanding of infant shape perception.

The human visual system has the remarkable capacity to transform spatio-temporal patterns of light into structured units of perception. Much research has focused on how the visual system integrates information around the perimeter of closed contours to form the perception of shape. This dissertation extends previous work by investigating how the perception of curvature along closed-contour shapes is affected by the presence of additional shapes that appear close to the target shape in space and/or time. Chapter 2 examined the ability of shape mechanisms at representing low frequency curvature in the presence of a higher frequency component along contours in multi-shape displays. We found that additions of high amplitude, high frequency curvature along a contour path can modulate the strength of interaction observed between shapes, and thus attenuates the contribution of low frequency components in interactions between neighbouring contours. Chapter 3 examined what curvature features are of importance in modulating phase dependent interactions between shapes. Results revealed that phase-dependent masking does not depend on curvature frequency, but is related to sensitivity for phase shifts in isolated contours, and is affected by both positive and negative curvature extrema. Computational simulations aimed at modelling the population responses evoked in intermediate shape processing areas (i.e., V4) suggest sensitivity to shifts in phase of shapes is not well captured by such a population code, and therefore alternative explanations are required. Chapter 4 examined how sensitivity to curvature deformations along the contour of a closed shape changes as a function of polar angle, angular frequency, and spatial uncertainty. Results show that human observers are, at first approximation, uniformly sensitivity to curvature deformations across all polar angles tested, and this result holds despite changes in angular frequency and spatial uncertainty. Chapter 5 examined whether the strength of spatial masking between shapes is affected by the presentation of a temporal mask. Our results demonstrate that a temporal mask affected spatial masking only when it preceded the target-mask stimulus by 130-180ms. Furthermore, the effects of a temporal mask on spatial masking are approximately additive, suggesting that separate components contribute to spatial and temporal interactions between shapes. / Thesis / Doctor of Philosophy (PhD)

The human visual system gathers information about three-dimensional object shape from a wide range of sources. How effectively we can use these sources, and how they are combined to form a consistent and accurate percept of the 3D world is the focus of much research. In complex scenes inter-reflections of light between surfaces (mutual illumination) can occur, creating chromatic illumination gradients. These gradients provide a source of information about 3D object shape, but little research has been conducted into the capabilities of the visual system to use such information. The experiments described here were conducted with the aim of understanding the influence of chromatic gradients from mutual illumination on 3D shape perception. Psychophysical experiments are described that were designed to investigate: If the human visual system takes account of mutual illumination when estimating 3D object shape, and how this might occur; How colour shading cues are integrated with other shape cues; The relative influence on 3D shape perception of achromatic (luminance) shading and chromatic shading from mutual illumination. In addition, one chapter explores a selection of mathematical models of cue integration and their applicability in this case. The results of the experiments suggest that the human visual system is able to quickly assess and take account of colour mutual illuminations when estimating 3D object shape, and use chromatic gradients as an independent and effective cue. Finally, mathematical modelling reveals that the chromatic gradient cue is likely integrated with other shape cues in a way that is close to statistically optimal.

The human visual system gathers information about three-dimensional object shape from a wide range of sources. How effectively we can use these sources, and how they are combined to form a consistent and accurate percept of the 3D world is the focus of much research. In complex scenes inter-reflections of light between surfaces (mutual illumination) can occur, creating chromatic illumination gradients. These gradients provide a source of information about 3D object shape, but little research has been conducted into the capabilities of the visual system to use such information. The experiments described here were conducted with the aim of understanding the influence of chromatic gradients from mutual illumination on 3D shape perception. Psychophysical experiments are described that were designed to investigate: If the human visual system takes account of mutual illumination when estimating 3D object shape, and how this might occur; How colour shading cues are integrated with other shape cues; The relative influence on 3D shape perception of achromatic (luminance) shading and chromatic shading from mutual illumination. In addition, one chapter explores a selection of mathematical models of cue integration and their applicability in this case. The results of the experiments suggest that the human visual system is able to quickly assess and take account of colour mutual illuminations when estimating 3D object shape, and use chromatic gradients as an independent and effective cue. Finally, mathematical modelling reveals that the chromatic gradient cue is likely integrated with other shape cues in a way that is close to statistically optimal.

Digital watermarking is the art of embedding secret messages in multimedia contents in order to protect their intellectual property. While the watermarking of image, audio and video is reaching maturity, the watermarking of 3D virtual objects is still a technology in its infancy. In this thesis, we focus on two main issues. The first one is the perception of the distortions caused by the watermarking process or by attacks on the surface of a 3D model. The second one concerns the development of techniques able to retrieve a watermark without the availability of the original data and after common manipulations and attacks. Since imperceptibility is a strong requirement, assessing the visual perception of the distortions that a 3D model undergoes in the watermarking pipeline is a key issue. In this thesis, we propose an image-based metric that relies on the comparison of 2D views with a Mutual Information criterion. A psychovisual experiment has validated the results of this metric for the most common watermarking attacks. The other issue this thesis deals with is the blind and robust watermarking of 3D shapes. In this context, three different watermarking schemes are proposed. These schemes differ by the classes of 3D watermarking attacks they are able to resist to. The first scheme is based on the extension of spectral decomposition to 3D models. This approach leads to robustness against imperceptible geometric deformations. The weakness of this technique is mainly related to resampling or cropping attacks. The second scheme extends the first to resampling by making use of the automatic multiscale detection of robust umbilical points. The third scheme then addresses the cropping attack by detecting robust prong feature points to locally embed a watermark in the spatial domain.

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La perception de la forme visuelle est le principal médiateur de la reconnaissance d’objets. S’il y a consensus sur le fait que la détection des contours et l’analyse de fréquences spatiales sont les fondements de la vision primaire, la hiérarchie visuelle et les étapes subséquentes du traitement de l’information impliquées dans la reconnaissance d’objets sont quant à elles encore méconnues. Les données empiriques disponibles et pertinentes concernant la nature des traits primitifs qu’utilise véritablement le système visuel humain sont rares et aucune ne semble être entièrement concluante. Dans le but de palier à ce manque de données empiriques, la présente étude vise la découverte des régions de l’image utilisées par des participants humains lors d’une tâche de reconnaissance d’objets. La technique des bulles a permis de révéler les zones diagnostiques permettant de discriminer entre les huit cibles de l’étude. Les zones ayant un effet facilitateur et celles ayant un effet inhibiteur sur les performances humaines et celles d’un observateur idéal furent identifiées. Les participants n’ont pas employé la totalité de l’information disponible dans l’image, mais seulement une infime partie, ce sont principalement les segments de contours présentant une discontinuité (i.e. convexités, concavités, intersections) qui furent sélectionnés par ces derniers afin de reconnaitre les cibles. L’identification des objets semble reposer sur des ensembles de caractéristiques distinctives de l’objet qui lui permettent d’être différencié des autres. Les informations les plus simples et utiles ont préséance et lorsqu’elles suffisent à mener à bien la tâche, le système visuel ne semble pas appliquer de traitement plus complexe, par exemple, l’encodage de caractéristiques plus complexes ou encore de conjonctions d’attributs simples. Cela appuie la notion voulant que le contexte influence la sélection des caractéristiques sous-tendant la reconnaissance d’objets et suggère que le type d’attributs varie en fonction de leur utilité dans un contexte donné. / The main mediator of visual object recognition is shape perception. While there is a consensus that contour detection and spatial frequency analysis are the foundations of early vision, the visual hierarchy and the nature of information processing in the subsequent stages involved in object recognition, remain widely unknown. Available and relevant empirical data concerning the nature of the primitive features used by the human visual system to recognize objects are scarce and none seems to be entirely conclusive. To overcome this lack of empirical data, this study aims to determine which regions of the images are used by humans when performing an object recognition task. The Bubbles technique has revealed the diagnostic areas used by 12 adults an ideal observer, to discriminate between eight target objects. stimulus areas with a facilitatory or inhibitory effect on performance were identified. Humans only used a small subset of the information available to recognize the targets which consisted mostly in discontinuous contour segments (i.e. convexities, concavities, intersections). Object recognition seems to rest upon contrasting sets of features which allow objects to be discriminated from one another. The simplest and most useful information seems to take precedence and it suffices to the task, the visual system does not engage in further processing involving for instance more complex features or the encoding of conjunctions of simple features. This implies that context influences the selection of features underlying human object recognition and suggests that attribute types can vary according to their utility in a given context.