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

Modelling of colour appearance of textured colours and smartphones using CIECAM02

The international colour committee recommended a colour appearance model, CIECAM02 in 2002, to help to predict colours under various viewing conditions from a colour appearance point of view, which has the accuracy of an averaged observer. In this research, an attempt is made to extend this model to predict colours on mobile telephones, which is not covered in the model. Despite the limited size and capacity of a mobile telephone, the urge to apply it to meet quotidian needs has never been unencumbered due to its appealing appearance, versatility, and readiness, such as viewing/taking pictures and shopping online. While a smartphone can act as a mini-computer, it does not always offer the same functionality as a desktop computer. For example, the RGB values on a smartphone normally cannot be modified nor can white balance be checked. As a result, performing online shopping using a mobile telephone can be difficult, especially when buying colour sensitive items. Therefore, this research takes an initiative to investigate the variations of colours for a number of smartphones while making an effort to predict their colour appearance using CIECAM02, benefiting both telephone users and makers. This thesis studies the Apple iPhone 5, LG Nexus 4, Samsung, and Huawei models, and compares their performance with a CRT colour monitor that has been calibrated using the D65 standard, to be consistent with the normal way of viewing online colours. As expected, all the telephones tested present more colourful images than a CRT. Work was also undertaken to investigate colours with a degree of texture. It was found that, on CRT monitors, a colour with a texture appears to be darker but more colourful to a human observer. Linear modifications have been proposed and implemented to the CIECAM02 model to accommodate these textured colours

High Dynamic Range (HDR) Display Perception

Displays have undergone a huge development in the last several decades. From cathode-ray tube (CRT), liquid crystal display (LCD), to organic light-emitting diode (OLED), even Q-OLED, the new configurations of the display bring more and more functions into industry and daily life. In the recent several years, high dynamic range (HDR) displays become popular. HDR displays usually refer to that the black level of the display is darker and the peak being brighter compared with the standard dynamic range (SDR) display. Traditionally, the peak luminance level can be used as the white in characterization and calibration. However, for HDR displays, the peak luminance is higher than the traditional diffuse white level. Exploration of the perceptual diffuse white in HDR image when presented in displays is proposed, which can be beneficial to the characterizing and the optimizing the usage of the HDR display. Moreover, in addition to the ``diffuse white , 3D color gamut volume can be calculated in some specific color appearance models. Calculation and modeling of the 3D color gamut volume can be very useful for display design and better characterizing display color reproduction capability. Furthermore, the perceptional color gamut volume can be measured through psychophysical experiments. Comparison between the perceptional color gamut volume and the theoretical 3D gamut volume calculations will reveal some insights for optimizing the usage of HDR displays. Another advantage of the HDR display is its darker black compared with the SDR display. Compared with the real black object, what level of black is `perfect\u27 enough in displays? Experiments were proposed and conducted to evaluate that if the HDR display is capable of showing ``perfect black for different types of background images/patterns. A glare-based model was proposed to predict the visual ``perfect black. Additionally, the dynamic range of human vision system is very large. However, the simultaneous dynamic range of human vision system is much smaller and is important for the fine tuning usage of HDR displays. The simultaneous dynamic range was measured directly for different stimulus sizes. Also, it was found that the simultaneous dynamic range was peak luminance level dependent. A mathematical model was proposed based on the experimental data to predict the simultaneous dynamic range. Also the spatial frequency effect of the target pattern on the simultaneous dynamic range was measured and modeled. The four different assessments about HDR displays perception would provide experimental data and models for a better understanding of HDR perception and tuning of the HDR display

Modelling of chromatic contrast for retrieval of wallpaper images

Colour remains one of the key factors in presenting an object and consequently has been widely applied in retrieval of images based on their visual contents. However, a colour appearance changes with the change of viewing surroundings, the phenomenon that has not been paid attention yet while performing colour-based image retrieval. To comprehend this effect, in this paper, a chromatic contrast model, CAMcc, is developed for the application of retrieval of colour intensive images, cementing the gap that most of existing colour models lack to fill by taking simultaneous colour contrast into account. Subsequently, the model is applied to the retrieval task on a collection of museum wallpapers of colour-rich images. In comparison with current popular colour models including CIECAM02, HSI, and RGB, with respect to both foreground and background colours, CAMcc appears to outperform the others with retrieved results being closer to query images. In addition, CAMcc focuses more on foreground colours, especially by maintaining the balance between both foreground and background colours, while the rest of existing models take on dominant colours that are perceived the most, usually background tones. Significantly, the contribution of the investigation lies in not only the improvement of the accuracy of colour-based image retrieval, but also the development of colour contrast model that warrants an important place in colour and computer vision theory, leading to deciphering the insight of this age-old topic of chromatic contrast in colour science

Impact of colour adjustment on flavour stability of pale lager beers with a range of distinct colouring agents

The impact of colour adjustment on the flavour stability of a portfolio of locallybrewed pale lager beers with a range of colouring agents such as specialty malts, roasted barley, colouring beer and artificial caramel colorant was investigated. All brewing control parameters and beer specifications were defined and monitored under a rigorous regime in order to avoid processing factors that might interfere with or modify the two parameters under investigation. The colour appearance parameters of the beer samples at distinct ages (fresh, forced aged and 12 month-aged) were psychophysically assessed by means of sensory viewing method (magnitude estimation) by an expert panel of the Colour Imaging Group at the Department of Colour Science, University of Leeds. Likewise, the aforementioned samples were physically measured by tele-spectroradiometry and digital imaging system at two different environments. Significant differences between the beer samples at sameand distinct ageing conditions were detected in terms of lightness, colourfulness, hue angle, opacity and clarity, although all of the samples were colour-adjusted to the same colour units according to conventional procedures (EBC colour units). In addition, good agreement between the sensory viewing (magnitude estimation) method and telespectroradiometry was observed. In contrast, some discrepancies between the aforementioned methodologies and the digital imaging technology were detected. Flavour stability was assessed by the detection and quantification of fifteen flavour-active beer ageing compounds (10 aldehydes and 5 non-aldehydes compounds) by GC-MS using headspace-solid phase microextraction (HS-SPME) with on-fibre PFBOA [O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine] derivatisation for aldehydes compounds and on-fibre DVB-CAR-PDMS [divinylbenzene-carboxenpolydimethylsiloxane 50/30_m] derivatisation for non-aldehyde compounds. The results were correlated with the determination of the endogenous anti-oxidative potential (EAP) of the beer samples by electron spin resonance (ESR) using N-tert-butyl-α-(4- pyridyl)nitrone N’-oxide (POBN) and the sensory assessments provided by the I.C.B.D. sensory panel. Additionally, the quantification of organic radicals of the specialty malts, the roasted barley (whole intact kernel and milling fraction measurement) and the artificial caramel colorant were conducted by ESR. Based on the results of this holistic approach, a colouring agent was selected for improving the flavour stability of pale lagers according to its physicochemical-, sensorial and psychophysical effects as colour appearance

Measuring Perceptual Color Differences of Smartphone Photographs

Measuring perceptual color differences (CDs) is of great importance in modern smartphone photography. Despite the long history, most CD measures have been constrained by psychophysical data of homogeneous color patches or a limited number of simplistic natural photographic images. It is thus questionable whether existing CD measures generalize in the age of smartphone photography characterized by greater content complexities and learning-based image signal processors. In this paper, we put together so far the largest image dataset for perceptual CD assessment, in which the photographic images are 1) captured by six flagship smartphones, 2) altered by Photoshop, 3) post-processed by built-in filters of the smartphones, and 4) reproduced with incorrect color profiles. We then conduct a large-scale psychophysical experiment to gather perceptual CDs of 30,000 image pairs in a carefully controlled laboratory environment. Based on the newly established dataset, we make one of the first attempts to construct an end-to-end learnable CD formula based on a lightweight neural network, as a generalization of several previous metrics. Extensive experiments demonstrate that the optimized formula outperforms 33 existing CD measures by a large margin, offers reasonable local CD maps without the use of dense supervision, generalizes well to homogeneous color patch data, and empirically behaves as a proper metric in the mathematical sense. Our dataset and code are publicly available at https://github.com/hellooks/CDNet.Comment: 10 figures, 8 tables, 14 page

Novel methods for the determination of colour and translucency in selected alcoholic beverages

At present, the various beverage industries have a significant challenge to effectively monitor the colour and translucency of their products, whether this is by visual assessments or by physical measurements. The visual assessment of translucent beverages is hampered in that the different industry sectors often have their own words/scales for translucency assessments, with specificity across a narrow range of products making them not easily transferred to other sectors of the beverage industry. Several industrial sensory evaluation methods were compared and correlated in this study. Similarly, there is a lack of accurate instrumental measurements either of the colour or the translucency of translucent beverage products in samples that are both coloured and translucent. A new multiple path-length cell together with a digital imaging system have been designed. The cell was built to deliver a specified analytical path length distribution, the overall light scattering performance of the liquid body would behave to the expected analytical path. This cell was applied at-line to determine the colour and translucency critical control points (CCPs) throughout beer production. In this study, descriptive language and psychophysical models to assess colour and translucency were investigated using twelve so-called pseudo-beers (solutions of colorants and scatterers), six commercial beers and fifteen red wine samples. The colour appearance attributes of these samples were assessed by trained experts, who scaled lightness and hue compositions with less variation than their scaling of colourfulness. From the observers‘ performance, it was indicated that beers presented in glasses of different volumes and geometries had different visual colour appearance and that the perceived colour appearance was affected by different levels of scatterers. For the investigation of translucency, observers demonstrated that they could correlate different words or methodologies used for translucency evaluations. But they performed more consistently on the term ―transparency‖, and thus, scaling ―transparency‖ can reasonably be used as a term by which the translucency of a iii beverage might be judged. The magnitudes of colour/translucency changes between any two of the adjacent samples taken during the production of pilot-brewed beer indicated critical control points (CCPs) on colour/translucency monitoring throughout the brewing process. Five CCPs were established. Here, the novel cell and the digital imaging system were set as a off-line monitoring instrument to make determinations at each of the CCPs. The findings clearly showed that the colour/translucency results of most samples followed similar trends. Commercial final products were also tested with the new system, and the results were consistent with conventional methodology results and human observations. In comparison with conventional instruments and methods used for beer colour/translucency analyses, this novel system was demonstrated to be more sensitive, allowing for the simultaneous monitoring of colour of both in-process streams and final products. Some more improvements of the new system are still needed on translucency correlating with conventional methods, e.g. with EBC turbidity