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Title: Modelling colour appearance : applications in skin image perception
Author: Chauhan, T.
ISNI:       0000 0004 6059 7435
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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Humans are trichromatic, and yet their perception of colours is rich and complex. The research presented in this thesis explores the process of colour appearance of uniform patches and natural polychromatic stimuli. This is done through the measurement and analysis of the achromatic locus (Chapter 2), modelling of chromatic adaptation in a large dataset of unique hues settings (Chapter 3), and measurement of thresholds for uniform and polychromatic stimuli derived from simulated skin images (Chapter 4). Chapter 2 proposes a novel navigation scheme based on unique hues for traversing colour space. The results show that when colour adjustments are made using this novel scheme, the variability of achromatic settings made by observers is reduced compared to the classical method of making colour adjustments along the cardinal axes of the CIELUV colour space. This result holds across the tested luminance levels (5,20,50 cd/m^2) in each of the three tested ambient illumination conditions – dark, simulated daylight and cool white fluorescent lighting. The analysis also shows that the direction of maximum variance of the achromatic settings lies along the daylight locus. Chapter 3 evaluates models of chromatic adaptation by using unique hues settings measured under different ambient illumination conditions. It is shown that a simple diagonal model in cone excitation space is the most efficient in terms of the trade-off between accuracy and degrees of freedom. It is also found that diagonal and linear models show similar performances, reiterating their theoretical equivalence. Performances of these diagonalisable models are found to be worse for UR and UG unique hue planes compared to UY and UB planes. Chapter 4 presents a set of three experiments reporting estimations of perceptual thresholds for polychromatic and uniform stimuli in a 3-D chromaticity-luminance colour space. The first experiment reports thresholds for simulated skin images and uniform stimuli of the corresponding mean CIELAB colour. The second and third experiments investigate the effect of ambient illumination and the location of the stimuli in colour space. The thresholds for the polychromatic stimuli are found to be consistently higher than those for the uniform patches, for both the chromatic, and the luminance projections. The area of the chromaticity ellipses shows a gradual increase with distance from the illuminant chromaticity. The orientations of these ellipses for simulated skin are found to align with the vector joining the mean patch chromaticity and the illuminant chromaticity.
Supervisor: Wuerger, S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral