Progress in multimedia technologies has resulted in a plethora of services and devices that capture, compress, transmit and display audiovisual stimuli. Humans -- the ultimate receivers of such stimuli -- now have access to visual entertainment at their homes, their workplaces as well as on mobile devices. With increasing visual signals being received by human observers, in the face of degradations that occur to due the capture, compression and transmission processes, an important aspect of the quality of experience of such stimuli is the \emph{perceived visual quality}. This dissertation focuses on algorithm development for assessing such visual quality of natural images, without need for the `pristine' reference image, i.e., we develop computational models for no-reference image quality assessment (NR IQA). Our NR IQA model stems from the theory that natural images have certain statistical properties that are violated in the presence of degradations, and quantifying such deviations from \emph{naturalness} leads to a blind estimate of quality. The proposed modular and easily extensible framework is distortion-agnostic, in that it does not need to have knowledge of the distortion afflicting the image (contrary to most present-day NR IQA algorithms) and is not only capable of quality assessment with high correlation with human perception, but also is capable of identifying the distortion afflicting the image. This additional distortion-identification, coupled with blind quality assessment leads to a framework that allows for blind general-purpose image repair, which is the second major contribution of this dissertation. The blind general-purpose image repair framework, and its exemplar algorithm described here stem from a revolutionary perspective on image repair, where the framework does not simply attempt to ameliorate the distortion in the image, but to ameliorate the distortion, so that visual quality at the output is maximized. Lastly, this dissertation describes a large-scale human subjective study that was conducted at UT to assess human behavior and opinion on visual quality of videos when viewed on mobile devices. The study lead to a database of 200 distorted videos, which incorporates previously studied distortions such as compression and wireless packet-loss, and also dynamically varying distortions that change as a function of time, such as frame-freezes and temporally varying compression rates. This study -- the first of its kind -- involved over 50 human subjects and resulted in 5,300 summary subjective scores and time-sampled subjective traces of quality for multiple displays. The last part of this dissertation analyzes human behavior and opinion on time-varying video quality, opening up an extremely interesting and relevant field for future research in the area of quality assessment and human behavior. / text

With the wide spread of wireless technology, many devices, such as tablets, smartphones, notebooks, fixed and portable navigation systems are becoming immensely convenient to access and are making a significant contribution to our daily life. Compared to conventional PC, portable devices are used at different times of the day and in various conditions and environments, both indoors and outdoors. As a consequence, devices need to be adaptive and adaptable to a wide range of use contexts and conditions. It is often argued that contextual factors, such as the ambient illuminance in relation to characteristics of the display (e.g., surface treatment, size, screen reflectance, and display luminance) may have a strong influence on the use of such devices and corresponding user experiences. Yet, the current understanding of these influence factors is still rather limited. Therefore, in this thesis work, emphasis is given particularly to the impact of lighting on readability, visual performance and affective state. As a first step, a preparatory investigation focusing on the objective measure- ment of display luminance and contrast, display gloss, and luminance in different scenes, took place. For this first step, a large number of tablet and laptop dis- plays were evaluated. This preparatory investigation served as a complement to and as an input for step two, namely a subjective study with human test subjects which was organized in a controlled lab environment. In the subjective test (N=18), different types of displays, luminance settings and lighting conditions were included to investigate the impact on display read- ability. The results of subjective tests indicate that for matte, visual acuity and reading speed is both impaired by the increase of ambient light luminance. However, they are compensated effectively by the increase of display luminance when it is below 500-600 cd/ m2. Compared with matte display, the glossy dis- play shows to be more robust in relation to the ambient light, though it indeed has a negative effect on visual acuity and reading speed. However, for subjective feelings, there is no significant difference between the two displays. Both of them are influenced by the ambient light only. In the post-experiment evaluation of the preference of displays, the majority (61%) of the test subjects indicated a preference for the glossy display. / I och med spridningen av tradlos teknologi, blir enheter sasom surfplattor, smarta mobiler och GPS:er alltmer anvandbara och blir darmed en allt viktigare del av var vardag. Jamfort med en vanlig dator, anvands dessa barbara enheter i manga olika sammanhang i olika miljoer, bade inomhus och utomhus. Darfor maste de kunna anpassa sig till olika miljoer och forutsattningar. Det pastas ofta att omgivande faktorer sasom bakgrundsbelysning och hur dessa samverkar med skarmens egenskaper (yta, storlek, reflektans och ljusstyrka) kan han en viktig inverkan pa anvandningen av sadana enheter och motsvarande anvan- darupplevelse. Trots det ar var forstaelse av dessa faktorer fortfarande ratt begransad. Darfor fokuserar detta examensarbete specifikt pa hur lasbarhet, lasupplevelse och humor paverkas av ljusforhallandena. Som ett forsta steg, gjordes en forberedande studie med objektiva matningar av displayers ljusstyrka, kontrast och matthet i olika omgivningar. I detta forsta steg, utvarderades ett stort antal olika lasplattor och laptop-skarmar. Denna in- ledande undersokning bildade ett komplement och underlag till det andra steget, vilket var en subjektiv anvandbarhetsstudie med ett antal forsokspersoner, or- ganiserad i en reglerad labbmiljo. I anvandartesten (N=18), undersoktes lasbarheten for olika typer av dis- player under olika ljusstyrkeinstallningar och omgivande ljusforhallanden. Re- sultaten av anvandartesten indikerar att for matta displayer, paverkas skarpa och lashastighet negativt da den omgivande ljusstyrka okar. Detta kan dock effektivt kompenseras genom att oka displayens ljusstyrka, da den ar under 500-600 cd/m2. I jamforelse med matta displayer, visar sig blanka displayer vara mer robusta mot variationer i bakgrundsbelysningen. Daremot, for sub- jektiva kanslan, ar det ingen signifikant skillnad mellan de tva displaytyperna. I bada fallen ar det bara omgivande belysningen som paverkar. I en utvardering efter avslutat experiment, foredrog en majoritet (61%) av forsokspersonerna den blanka displayen.