Correction and control of ocular aberrations with adaptive optics : effects on human vision

El objetivo de esta tesis doctoral es la comprensión de la relación entre la óptica de la imagen que se proyecta en la retina (en particular, la degradación impuesta por las aberraciones ópticas de alto orden) y la calidad visual. Se ha llevado a cabo el desarrollo de un nuevo sistema de Óptica Adaptativa, que incluye un sensor de frente de onda tipo Hartmann-Shack y un espejo deformable, implementando vías de psicofísica para la medida de la función visual y percepción neuronal bajo control de las aberraciones oculares mediante el espejo deformable. En esta tesis, se presenta una serie de estudios psicofísicos desarrollados para investigar el efecto de la aberraciones sobre la visión humana y la adaptación neuronal a las aberraciones oculares y también para demostrar la correlación entre la codificación interna del emborronamiento y el emborronamiento impuesto por las aberraciones de alto orden en términos de magnitud y orientació

High-fidelity colour reproduction for high-dynamic-range imaging

The aim of this thesis is to develop a colour reproduction system for high-dynamic-range (HDR) imaging. Classical colour reproduction systems fail to reproduce HDR images because current characterisation methods and colour appearance models fail to cover the dynamic range of luminance present in HDR images. HDR tone-mapping algorithms have been developed to reproduce HDR images on low-dynamic-range media such as LCD displays. However, most of these models have only considered luminance compression from a photographic point of view and have not explicitly taken into account colour appearance. Motivated by the idea to bridge the gap between crossmedia colour reproduction and HDR imaging, this thesis investigates the fundamentals and the infrastructure of cross-media colour reproduction. It restructures cross-media colour reproduction with respect to HDR imaging, and develops a novel cross-media colour reproduction system for HDR imaging. First, our HDR characterisation method enables us to measure HDR radiance values to a high accuracy that rivals spectroradiometers. Second, our colour appearance model enables us to predict human colour perception under high luminance levels. We first built a high-luminance display in order to establish a controllable high-luminance viewing environment. We conducted a psychophysical experiment on this display device to measure perceptual colour attributes. A novel numerical model for colour appearance was derived from our experimental data, which covers the full working range of the human visual system. Our appearance model predicts colour and luminance attributes under high luminance levels. In particular, our model predicts perceived lightness and colourfulness to a significantly higher accuracy than other appearance models. Finally, a complete colour reproduction pipeline is proposed using our novel HDR characterisation and colour appearance models. Results indicate that our reproduction system outperforms other reproduction methods with statistical significance. Our colour reproduction system provides high-fidelity colour reproduction for HDR imaging, and successfully bridges the gap between cross-media colour reproduction and HDR imaging

Engineering data compendium. Human perception and performance. User's guide

The concept underlying the Engineering Data Compendium was the product of a research and development program (Integrated Perceptual Information for Designers project) aimed at facilitating the application of basic research findings in human performance to the design and military crew systems. The principal objective was to develop a workable strategy for: (1) identifying and distilling information of potential value to system design from the existing research literature, and (2) presenting this technical information in a way that would aid its accessibility, interpretability, and applicability by systems designers. The present four volumes of the Engineering Data Compendium represent the first implementation of this strategy. This is the first volume, the User's Guide, containing a description of the program and instructions for its use

Rendering non-pictorial (Scientific) high dynamic range images

In recent years, the graphics community is seeing an increasing demand for the capture and usage of high-dynamic-range (HDR) images. Since the production of HDR imagery is not solely the domain of the visualization of real life or computer generated scenes, novel techniques are also required for imagery captured from non-visual sources such as remote sensing, medical imaging, astronomical imaging, etc. This research proposes to integrate the techniques used for the display of high-dynamic-range pictorial imagery for the practical visualization of non-pictorial (scientific) imagery for data mining and interpretation. Nine algorithms were utilized to overcome the problem associated with rendering the high-dynamic-range image data to low-dynamic-range display devices, and the results were evaluated using a psychophysical experiment. Two paired-comparison experiments and a target detection experiment were performed. Paired-comparison results indicate that the Zone System performs the best on average and the Local Color Correction method performs the worst. The results show that the performance of different encoding schemes depend on the type of data being visualized. The correlation between the preference and scientific usefulness judgments (R2 = 0.31) demonstrates that observers tend to use different criteria when judging the scientific usefulness versus image preference. The experiment was conducted using observers with expertise (Radiologists) for the Medical image to further elucidate the success of HDR rendering on these data. The results indicated that both Radiologists and Non-radiologists tend to use similar criteria regardless of their experience and expertise when judging the usefulness of rendered images. A target detection experiment was conducted to measure the detectability of an embedded noise target in the Medical image to demonstrate the effect of the tone mapping operators on target detection. The result of the target detection experiment illustrated that the detectability of targets the image is greatly influenced by the rendering algorithm due to the inherent differences in tone mapping among the algorithms

Computational mechanisms for colour and lightness constancy

Attributes of colour images have been found which allow colour and lightness constancy to be computed without prior knowledge of the illumination, even in complex scenes with three -dimensional objects and multiple light sources of different colours. The ratio of surface reflectance colour can be immediately determined between any two image points, however distant. It is possible to determine the number of spectrally independent light sources, and to isolate the effect of each. Reflectance edges across which the illumination remains constant can be correctly identified.In a scene illuminated by multiple distant point sources of distinguishalbe colours, the spatial angle between the sources and their brightness ratios can be computed from the image alone. If there are three or more sources then reflectance constancy is immediately possible without use of additional knowledge.The results are an extension of Edwin Land's Retinex algorithm. They account for previously unexplained data such as Gilchrist's veiling luminances and his single- colour rooms.The validity of the algorithms has been demonstrated by implementing them in a series of computer programs. The computational methods do not follow the edge or region finding paradigms of previous vision mechanisms. Although the new reflectance constancy cues occur in all normal scenes, it is likely that human vision makes use of only some of them.In a colour image all the pixels of a single surface colour lie in a single structure in flux space. The dimension of the structure equals the number of illumination colours. The reflectance ratio between two regions is determined by the transformation between their structures. Parallel tracing of edge pairs in their respective structures identifies an edge of constant illumination, and gives the lightness ratio of each such edge. Enhanced noise reduction techniques for colour pictures follow from the natural constraints on the flux structures

Modelling chromatic processes in human vision

This investigation programme has concentrated on a number of topics concerning visual function in humans, with particular emphasis on the processing of chromatic information. The programme included an investigation of colour constancy which was carried out using a computer - controlled colour display. A new dynamic colour matching method was developed and the experimental conditions arranged so as to test the constancy of colour appearance. The technigue makes possible the definition of an index of colour constancy which simplifies the interpretation of experimental results. Various spatial, temporal and chromatic parameters of the stimulus configuration were investigated and the results show the importance of the stimulus boundary and near surround in determining the magnitude of the constancy effects observed. Pupillary function was investigated in normal and amblyopic observers. The results suggest that the pupil light reflex is essentially of normal amplitude in amblyopic eyes, although a latency anomaly does exist. Pupillary responses to achromatic, sinusoidal grating stimuli were anomalous in the affected eyes of many amblyopic subjects. Interestingly, certain response parameters were found to be anomalous in the normal eyes of amblyopic observers. Pupillary responses in normal observers were measured following stimulation with isoluminant, red - green chromatic gratings. Responses were found to vary systematically with grating spatial frequency, and broadly reflect the way in which detection thresholds for the same stimuli vary with spatial frequency. Response latencies were approximately 80 ms longer than for equivalent responses to achromatic gratings. No responses were observed when the same experiments were carried out with a protanope. Chromatic discrimination thresholds were measured using a spatio - temporal luminance masking technique. The technique is implemented on a carefully calibrated colour monitor and obviates the requirement of stimulus isoluminance. Preliminary experiments employing psychophysical and pupillometric procedures illustrate the use of the technique in the subjective and objective assessment of colour vision. The rapid changes in visual sensitivity which occur following an intense flash of light were investigated by measuring increment thresholds for a small, brief test stimulus. Results are presented which show the dependence of the sensitivity changes on various spatial, temporal and chromatic parameters. An attempt is made to relate the experimental findings to the properties of retinal neurones

Parametric studies on pattern recognition mechanisms in human vision

Imperial Users onl

Context-dependent lightness affects perceived contrast

Perceived contrast of a grating varies with its background (or mean) luminance: of the two gratings with the same photometric contrast the one on higher luminance background appears to have higher contrast. Does perceived contrast also vary with context-dependent background lightness even when the luminance remains constant? We investigated this question using a stimulus in which two equiluminant patches ("context squares", CSs) appear different in lightness. First we measured the lightness effect in a behavioral experiment. After ensuring that it was present for all participants, we conducted perceived contrast experiments, where participants judged the contrast of rectified incremental and decremental square-wave gratings superimposed on the CSs. For the incremental gratings participants' settings were significantly different for the two CSs. Specifically, perceived contrast was higher when the gratings were placed on the context square that was perceived lighter. In a follow-up experiment we measured perceived contrast of rectified gratings on isolated patches that differed in luminance. The pattern of results of the two experiments was consistent, demonstrating that possibly shared mechanisms underpin the effects of background luminance and context-dependent lightness on perceived contrast. © 2016 Elsevier Ltd

Visual assessment of object color chroma and colorfulness

A series of visual experiments were designed to determine whether naive observers typically evaluate chroma or colorfulness when judging color appearance. A total of 7 observers were asked to determine a color appearance match between Munsell samples under the same illuminant (C) at different levels of illuminance. Color appearance matches were determined for 12 Munsell samples, under five reference and matching scene illuminance conditions, for four experimental techniques. The four experimental techniques were haploscopic, simultaneous inspection, successive inspection, and short-term memory matching. Results suggested that a chroma match was most important when observers were evaluating the color appearance of two scenes at different levels of illuminance. Results were also compared to predictions of two color appearance models. While similar trends were apparent between the experimental results and the two model\u27s predictions, only the Hunt model\u27s chroma term satisfactorily predicted experimental observations

Filling-in the Forms: Surface and Boundary Interactions in Visual Cortex

Defense Advanced Research Projects Agency and the Office of Naval Research (NOOOI4-95-l-0409); Office of Naval Research (NOOO14-95-1-0657)