Dr. Alex Ioannides
Alex's PhD work pursued a single line of enquiry: lightness, brightness, and visual illusions. In particular, Alex's thesis focuses on White's effect, simultaneous brightness contrast, and theories that can account for both phenomenon. In the first part of his thesis (Chapters 1-3), the problem-space is defined before a review of lightness and brightness theories from both low- and high-level vision. In the second part (Chapter 4), the only two low-level V1 models of brightness, capable of accounting for both White's effect and simultaneous brightness contrast, are shown to be reliant on the amplification of low spatial frequency information, to accurately reconstruct images and account for the illusory brightness apparent in both effects. It is argued that this low spatial frequency information is not available to V1. Hence, it is concluded that these models are not biologically plausible. In the third part (Chapter 5), the issue of recovering low spatial frequency information without explicitly sampling it is considered. The problem is formally defined in the Scale- Space framework and solved analytically. That is, an algorithm for recovering local mean luminance (and low spatial frequencies), from the information implicit in contrast coding cells typically found in V1, is constructed, and is referred to as the Implicit Luminance Coding (ILC) model. It is argued that the ILC model is not biologically-plausible. Subsequently, Alex proposed a new algorithm, based on a numerical approximation to the analytical solution. The biologically plausible ILC algorithm is developed into a complete low-level model of brightness, which makes use of the information present in multiple scale channels. The model is shown to be capable of accounting for both White's effect and simultaneous brightness contrast, by means of an interplay between two independent assimilation and contrast mechanisms. The final part (Chapter 6), is concerned with the application of the model to visual phenomenon synonymous with lightness and brightness, including all known variants of White's effect and simultaneous brightness contrast, and some effects that are traditionally accounted for by appealing to mechanisms from high-level vision. The biologically-plausible ILC model is shown to be in good accordance with experimental data. Alex currently works as a Modelling Advisor at KPMG Advisory.
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