Computational and psychophysical investigations of perceptual transparency.

The work presented in this thesis has investigated, both computationally and psychophysically, the chromatic conditions for the phenomenon of perceptual transparency. A series of physical measurements and numerical simulations have been carried out to show that the cone-excitation ratios between two opaque surfaces and the cone-excitation ratios between the same surfaces covered by a transparent filter are almost statistically invariant (see also Westland & Ripamonti, 2000) for many physically transparent systems. The relationship between physical transparency and perceptual transparency has been explored and strong psychophysical evidence has been presented to show that, when the invariance holds, the filter is perceived as transparent. Furthermore, the degree of invariance seems to be a good measure of the degree of perceived transparency. The more invariant the cone-excitation ratios are, the more the filter is perceived to be transparent. This shows that perceptual transparency is not a categorical perception but rather a continuously graded perception: the more the cone-excitation ratios approximate invariance the more the filtered area is perceived to be transparent. A set of further psychophysical experiments has been carried out to ascertain whether the invariance is required for all three classes of cones or whether some cone classes are more important than others. The data seem to support the notion that the S-cone class responses contribute relatively little to transparency perception. The role of image complexity on transparency perception was also explored. Specifically it has been shown that the strength of the transparency percept increases with the number of patches in a Mondrian display partially covered by a transparent filter. Finally, it is acknowledged that the chromatic conditions for transparency perception that are defined by the invariance model may well be alternatively expressed by other models or transparency or at other levels of visual processing. However, the invariance model makes explicit an intriguing link between transparency perception and colour constancy.