The human visual system's representation of light sources and the objects they illuminate

The light sources in a scene can drastically affect the pattern of intensities falling on the retina. However, it is unclear how the visual system represents the light sources in a scene. One possibility is that a light source is treated as a scene component: an entity that exists within a scene and interacts with other scene components (object shape and object reflectance) to produce the retinal image. The aim of this thesis was to test two key predictions arising from a perceptual framework in which light sources and the objects they illuminate are considered to be scene components by the visual system. We begin examining the first prediction in Chapter 3, focusing on the role of a dynamic shape cue in the interaction between shape, reflectance, and lighting. In two psychophysics experiments, we show that the visual system can "explain away'" alternative interpretations of luminance gradients using the information provided by a dynamic shape cue (kinetic depth). In subsequent chapters, the research focus shifts to the second prediction, investigating whether multiple objects in a scene are integrated to estimate light source direction. In Chapter 4, participants were presented with scenes that contained 1, 9, and 25 objects and asked to judge whether the scenes were illuminated from the left or right, relative to their viewpoint. We found that increasing the number of objects in a scene worsened, if anything, discrimination sensitivity. To further understand this result, we conducted an equivalent noise experiment in Chapter 5 to examine the contributions of internal noise and integration to estimates of light source direction. Our results indicate that participants used only 1 or 2 objects to judge light source direction for scenes with 9 and 25 objects. Chapter 6 presents a shape discrimination experiment that required participants to make an implicit, rather than explicit, judgement of light source direction. Consistent with the results reported in Chapters 4 and 5, we find that shape discrimination sensitivity was comparable for scenes containing 1, 9, and 25 objects. Taken together, the findings presented here suggest that while object shape and reflectance may be represented as scene components, lighting seems to be associated with individual objects rather than having a scene-level representation

Spectral spatiality in the acousmatic listening context

Sounds are often experienced as being spatially higher or lower in congruence with their frequency ‘height’ (i.e. pitch register). The term ‘spectral spatiality’ refers to this impression of spatial height and vertical depth as evoked by the perceived occupancy of evolving sound-shapes (spectromorphologies) within the continuum of audible frequencies. Chapters One and Two draw upon a diverse body of literature to explore the cognitive and physiological processes involved in human spatial hearing in general, and spectral spatiality in particular. Thereafter the potential pertinence of a spectral space consciousness in the acousmatic listening experience is highlighted, particularly with regard to more abstract acousmatic contexts where sounds do not directly invoke familiar source identities. Chapters Three and Four further elaborate aspects of spectral space consciousness and propose a terminological framework for discussing musical contexts in terms of their spectral space design. Consequently, it is argued that in acousmatic music, spectral spatiality must be considered as an inseparable aspect of spatiality in general, although its pertinence only becomes directly highlighted in particular musical contexts. The recurring theme in this thesis is that, in acousmatic music, 'space' is not a parameter but a multifaceted quality that is inherent to all sounds. As well as providing an analytical framework for discussing spatiality in acousmatic music, this thesis highlights the compositional potentials offered by spectral spatiality, particularly in relation to the creation of perspectival image in multichannel works. For instance, the possibility of (re)distributing the spectral components of a sound around the listener (circumspectral image) is discussed in context, and a software tool is presented that enables an intuitive and experimental approach to the composition of circumspectral sounds for 6 and 8 channel loudspeaker configurations. This thesis is useful for both composers and analysts interested in aspects of spatiality in acousmatic music. It also offers some insight into spectral space consciousness in non-acousmatic music, and may therefore contribute towards a more general understanding of the nature of our spatial experience in music

Spectral spatiality in the acousmatic listening context

Sounds are often experienced as being spatially higher or lower in congruence with their frequency ‘height’ (i.e. pitch register). The term ‘spectral spatiality’ refers to this impression of spatial height and vertical depth as evoked by the perceived occupancy of evolving sound-shapes (spectromorphologies) within the continuum of audible frequencies. Chapters One and Two draw upon a diverse body of literature to explore the cognitive and physiological processes involved in human spatial hearing in general, and spectral spatiality in particular. Thereafter the potential pertinence of a spectral space consciousness in the acousmatic listening experience is highlighted, particularly with regard to more abstract acousmatic contexts where sounds do not directly invoke familiar source identities. Chapters Three and Four further elaborate aspects of spectral space consciousness and propose a terminological framework for discussing musical contexts in terms of their spectral space design. Consequently, it is argued that in acousmatic music, spectral spatiality must be considered as an inseparable aspect of spatiality in general, although its pertinence only becomes directly highlighted in particular musical contexts. The recurring theme in this thesis is that, in acousmatic music, 'space' is not a parameter but a multifaceted quality that is inherent to all sounds. As well as providing an analytical framework for discussing spatiality in acousmatic music, this thesis highlights the compositional potentials offered by spectral spatiality, particularly in relation to the creation of perspectival image in multichannel works. For instance, the possibility of (re)distributing the spectral components of a sound around the listener (circumspectral image) is discussed in context, and a software tool is presented that enables an intuitive and experimental approach to the composition of circumspectral sounds for 6 and 8 channel loudspeaker configurations. This thesis is useful for both composers and analysts interested in aspects of spatiality in acousmatic music. It also offers some insight into spectral space consciousness in non-acousmatic music, and may therefore contribute towards a more general understanding of the nature of our spatial experience in music.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Spectral spatiality in the acousmatic listening context

Sounds are often experienced as being spatially higher or lower in congruence with their frequency ‘height’ (i.e. pitch register). The term ‘spectral spatiality’ refers to this impression of spatial height and vertical depth as evoked by the perceived occupancy of evolving sound-shapes (spectromorphologies) within the continuum of audible frequencies. Chapters One and Two draw upon a diverse body of literature to explore the cognitive and physiological processes involved in human spatial hearing in general, and spectral spatiality in particular. Thereafter the potential pertinence of a spectral space consciousness in the acousmatic listening experience is highlighted, particularly with regard to more abstract acousmatic contexts where sounds do not directly invoke familiar source identities. Chapters Three and Four further elaborate aspects of spectral space consciousness and propose a terminological framework for discussing musical contexts in terms of their spectral space design. Consequently, it is argued that in acousmatic music, spectral spatiality must be considered as an inseparable aspect of spatiality in general, although its pertinence only becomes directly highlighted in particular musical contexts. The recurring theme in this thesis is that, in acousmatic music, 'space' is not a parameter but a multifaceted quality that is inherent to all sounds. As well as providing an analytical framework for discussing spatiality in acousmatic music, this thesis highlights the compositional potentials offered by spectral spatiality, particularly in relation to the creation of perspectival image in multichannel works. For instance, the possibility of (re)distributing the spectral components of a sound around the listener (circumspectral image) is discussed in context, and a software tool is presented that enables an intuitive and experimental approach to the composition of circumspectral sounds for 6 and 8 channel loudspeaker configurations. This thesis is useful for both composers and analysts interested in aspects of spatiality in acousmatic music. It also offers some insight into spectral space consciousness in non-acousmatic music, and may therefore contribute towards a more general understanding of the nature of our spatial experience in music.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The Computation of Surface Lightness in Simple and Complex Scenes

The present thesis examined how reflectance properties and the complexity of surface mesostructure (small-scale surface relief) influence perceived lightness in centresurround displays. Chapters 2 and 3 evaluated the role of surface relief, gloss, and interreflections on lightness constancy, which was examined across changes in background albedo and illumination level. For surfaces with visible mesostructure (“rocky” surfaces), lightness constancy across changes in background albedo was better for targets embedded in glossy versus matte surfaces. However, this improved lightness constancy for gloss was not observed when illumination varied. Control experiments compared the matte and glossy rocky surrounds to two control displays, which matched either pixel histograms or a phase-scrambled power spectrum. Lightness constancy was improved for rocky glossy displays over the histogram-matched displays, but not compared to phase-scrambled variants of these images with equated power spectrums. The results were similar for surfaces rendered with 1, 2, 3 and 4 interreflections. These results suggest that lightness perception in complex centre-surround displays can be explained by the distribution of contrast across space and scale, independently of explicit information about surface shading or specularity. The results for surfaces without surface relief (“homogeneous” surfaces) differed qualitatively to rocky surfaces, exhibiting abrupt steps in perceived lightness at points at which the targets transitioned from being increments to decrements. Chapter 4 examined whether homogeneous displays evoke more complex mid-level representations similar to conditions of transparency. Matching target lightness in a homogeneous display to that in a textured or rocky display required varying both lightness and transmittance of the test patch on the textured display to obtain the most satisfactory matches. However, transmittance was only varied to match the contrast of targets against homogeneous surrounds, and not to explicitly match the amount of transparency perceived in the displays. The results suggest perceived target-surround edge contrast differs between homogeneous and textured displays. Varying the mid-level property of transparency in textured displays provides a natural means for equating both target lightness and the unique appearance of the edge contrast in homogeneous displays

The Computation of Surface Lightness in Simple and Complex Scenes

The present thesis examined how reflectance properties and the complexity of surface mesostructure (small-scale surface relief) influence perceived lightness in centresurround displays. Chapters 2 and 3 evaluated the role of surface relief, gloss, and interreflections on lightness constancy, which was examined across changes in background albedo and illumination level. For surfaces with visible mesostructure (“rocky” surfaces), lightness constancy across changes in background albedo was better for targets embedded in glossy versus matte surfaces. However, this improved lightness constancy for gloss was not observed when illumination varied. Control experiments compared the matte and glossy rocky surrounds to two control displays, which matched either pixel histograms or a phase-scrambled power spectrum. Lightness constancy was improved for rocky glossy displays over the histogram-matched displays, but not compared to phase-scrambled variants of these images with equated power spectrums. The results were similar for surfaces rendered with 1, 2, 3 and 4 interreflections. These results suggest that lightness perception in complex centre-surround displays can be explained by the distribution of contrast across space and scale, independently of explicit information about surface shading or specularity. The results for surfaces without surface relief (“homogeneous” surfaces) differed qualitatively to rocky surfaces, exhibiting abrupt steps in perceived lightness at points at which the targets transitioned from being increments to decrements. Chapter 4 examined whether homogeneous displays evoke more complex mid-level representations similar to conditions of transparency. Matching target lightness in a homogeneous display to that in a textured or rocky display required varying both lightness and transmittance of the test patch on the textured display to obtain the most satisfactory matches. However, transmittance was only varied to match the contrast of targets against homogeneous surrounds, and not to explicitly match the amount of transparency perceived in the displays. The results suggest perceived target-surround edge contrast differs between homogeneous and textured displays. Varying the mid-level property of transparency in textured displays provides a natural means for equating both target lightness and the unique appearance of the edge contrast in homogeneous displays

Light environment - A. Visible light. B. Ultraviolet light

Visible and ultraviolet light environment as related to human performance and safety during space mission

Modelling the human perception of shape-from-shading

Shading conveys information on 3-D shape and the process of recovering this information is called shape-from-shading (SFS). This thesis divides the process of human SFS into two functional sub-units (luminance disambiguation and shape computation) and studies them individually. Based on results of a series of psychophysical experiments it is proposed that the interaction between first- and second-order channels plays an important role in disambiguating luminance. Based on this idea, two versions of a biologically plausible model are developed to explain the human performances observed here and elsewhere. An algorithm sharing the same idea is also developed as a solution to the problem of intrinsic image decomposition in the field of image processing. With regard to the shape computation unit, a link between luminance variations and estimated surface norms is identified by testing participants on simple gratings with several different luminance profiles. This methodology is unconventional but can be justified in the light of past studies of human SFS. Finally a computational algorithm for SFS containing two distinct operating modes is proposed. This algorithm is broadly consistent with the known psychophysics on human SFS

Proceedings experiencing light 2009 : international conference on the effects of light on welbeing

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Proceedings experiencing light 2009 : international conference on the effects of light on welbeing

no abstrac