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Title: Colour constancy for glossy objects under complex lighting environments
Author: Morimoto, Takuma
ISNI:       0000 0004 8507 8607
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Colour constancy refers to our visual ability to identify the colour of objects under different illuminations. As a result of research over decades, our understanding of the mechanisms underlying this ability has advanced considerably. However, a significant limitation of past studies has been that they have mostly employed oversimplified experimental stimuli that only partially reflect the properties of surfaces and illuminations in the natural environment. Stimuli were mostly matte, two-dimensional, and uniformly illuminated by single illumination. In the real world, in contrast, we know that objects sometimes include gloss (perceptual attribute of specular reflection), and their shape is usually three-dimensional rather than flat. Moreover, an object placed in the natural environment receives illumination from every direction as light is reflected from other objects in the scene, and the spectra of those lights might vary from one direction to another. In this thesis, I report five research projects. The first three are all experiment based, and they were designed to find mechanisms that underpin human colour constancy for three-dimensional glossy objects that are placed under complex lighting environments. We conducted first an experiment where observers were asked to judge surface and illuminant colour changes for glossy objects under point light sources. The second and third experiments explored our ability to identify surface colours of glossy objects under environmental illumination that introduces a complex spatial pattern of specular highlights on the surfaces. All experiments used computer-rendered objects that take account of the complex interaction between material and illumination in the real world. Collectively these experiments suggest that observers are able to extract surface colour, even with complex stimuli in which simple heuristics are unlikely to be reliable. Instead, the results suggest that observers may build up complex internal models from which they can segregate colours of surface and illumination. The fourth chapter is based on field work that was conducted in collaboration with Prof. Sergio Nascimento at University of Minho, Portugal. We report newly collected hyperspectral environmental illuminations recorded in eight outdoor scenes and four indoor scenes. The aim was to characterise the directional spectral variation that occurs in natural environments and its application to more precise simulation of objects using computer graphics. Subsequent fifth chapter was motivated by the striking variation of illuminant spectra in natural environments. We revisited a classical problem in human colour vision, called illuminant metamerism where two distinct surfaces appear the same under a specific illumination. We showed theoretically that this metamerism is less of an issue than previously thought thanks to the presence of directional variation of illumination.
Supervisor: Smithson, Hannah E. Sponsor: Fundação para a Ciência e a Tecnologia ; Experimental Psychology Society ; Japan Student Services Organization ; Great Britain Sasakawa Foundation ; Kikawada Foundation ; Wellcome Trust ; Aso Group
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Human Colour Vision ; Vision Science ; Experimental Psychology