Image sizes have increased exponentially in recent years. The resulting high-resolution images are often viewed via remote image browsing. Zooming and panning are desirable features in this context, which result in disparate spatial regions of an image being displayed at a variety of ( spatial) resolutions. When an image is displayed at a reduced resolution, the quantization step sizes needed for visually lossless quality generally increase. This paper investigates the quantization step sizes needed for visually lossless display as a function of resolution, and proposes a method that effectively incorporates the resulting ( multiple) quantization step sizes into a single JPEG 2000 codestream. This codestream is JPEG 2000 Part 1 compliant and allows for visually lossless decoding at all resolutions natively supported by the wavelet transform as well as arbitrary intermediate resolutions, using only a fraction of the full-resolution codestream. When images are browsed remotely using the JPEG 2000 Interactive Protocol ( JPIP), the required bandwidth is significantly reduced, as demonstrated by extensive experimental results.

Image quality measures are used to optimize image processing algorithms and evaluate their performances. The only reliable way to assess image quality is subjective evaluation by human observers, where the mean value of their scores is used as the quality measure. This is known as mean opinion score (MOS). In addition to this measure there are various objective (quantitative) measures. Most widely used quantitative measures are: mean squared error (MSE), peak signal to noise ratio (PSNR) and signal to noise ratio (SNR). Since these simple measures do not always produce results that are in agreement with subjective evaluation, many other quality measures have been proposed. They are mostly various modifications of MSE, which try to take into account some properties of human visual system (HVS) such as nonlinear character of brightness perception, contrast sensitivity function (CSF) and texture masking. In these approaches quality measure is computed as MSE of input image intensities or frequency domain coefficients obtained after some transform (DFT, DCT etc.), weighted by some coefficients which account for the mentioned properties of HVS. These measures have some advantages over MSE, but their ability to predict image quality is still limited. A different approach is presented here. Quality measure proposed here uses simple model of HVS, which has one user-defined parameter, whose value depends on the reference image. This quality measure is based on the average value of locally computed correlation coefficients. This takes into account structural similarity between original and distorted images, which cannot be measured by MSE or any kind of weighted MSE. The proposed measure also differentiates between random and signal dependant distortion, because these two have different effect on human observer. This is achieved by computing the average correlation coefficient between reference image and error image. Performance of the proposed quality measure is illustrated by examples involving images with different types of degradation.

Title from PDF of title page. / Document formatted into pages; contains 82 pages. / Thesis (M.S.E.E.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: Image quality measures are used to optimize image processing algorithms and evaluate their performances. The only reliable way to assess image quality is subjective evaluation by human observers, where the mean value of their scores is used as the quality measure. This is known as mean opinion score (MOS). In addition to this measure there are various objective (quantitative) measures. Most widely used quantitative measures are: mean squared error (MSE), peak signal to noise ratio (PSNR) and signal to noise ratio (SNR). Since these simple measures do not always produce results that are in agreement with subjective evaluation, many other quality measures have been proposed. They are mostly various modifications of MSE, which try to take into account some properties of human visual system (HVS) such as nonlinear character of brightness perception, contrast sensitivity function (CSF) and texture masking. / ABSTRACT: In these approaches quality measure is computed as MSE of input image intensities or frequency domain coefficients obtained after some transform (DFT, DCT etc.), weighted by some coefficients which account for the mentioned properties of HVS. These measures have some advantages over MSE, but their ability to predict image quality is still limited. A different approach is presented here. Quality measure proposed here uses simple model of HVS, which has one user-defined parameter, whose value depends on the reference image. This quality measure is based on the average value of locally computed correlation coefficients. This takes into account structural similarity between original and distorted images, which cannot be measured by MSE or any kind of weighted MSE. The proposed measure also differentiates between random and signal dependant distortion, because these two have different effect on human observer. / ABSTRACT: This is achieved by computing the average correlation coefficient between reference image and error image. Performance of the proposed quality measure is illustrated by examples involving images with different types of degradation. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

<p>This study is a thesis for the Master of Science degree in Media Technology and Engineering at the Department of Science and Technology, Linkoping University. It was accomplished from November 2002 to May 2003. </p><p>Objective image quality measures play an important role in various image processing applications. In this paper quality measures applied on halftoned images are aimed to be in focus. Digital halftoning is the process of generating a pattern of binary pixels that create the illusion of a continuous- tone image. Algorithms built on this technique produce results of very different quality and characteristics. To evaluate and improve their performance, it is important to have robust and reliable image quality measures. This literature survey is to give a general description in digital halftoning and halftone image quality methods.</p>

Made available in DSpace on 2015-05-14T13:16:34Z (GMT). No. of bitstreams: 1 arquivototal.PDF: 1526331 bytes, checksum: 38e40aec6b111371aebd1a5cba081a3c (MD5) Previous issue date: 2009-04-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Epilepsy is a brain dysfunction characterized by the repeated tendency of the brain to cause epileptic crisis and by the neurobiological, cognitive, psychological and social consequences of this condition. In these terms, the aim of this work is to determine the visual sensory threshold of children and adolescents who have and who do not have epilepsy, analyzing possible alterations in the form related to this dysfunction. The sensory threshold is defined as the smallest intensity of incentive a human being is capable of detecting. The contrast sensitivity function (CSF) is the inverse of the threshold curve of contrast (1/CSF). This way, the CSF is a tool that allows us to label the answer of SV to visual patterns in low, medium and high levels of contrast and evaluate possible sensorial alterations associated with the visual processing. The main idea is that the elevation or reduction of the CSF can happen deem to sensorial, cognitive and behavioral alterations. Twenty volunteers participated on this study, from both genders, between 7 and 17 years of age, 10 of whom nonepileptic and 10 epileptic (tonic-clonic crisis). All of them presented either normal or corrected visual accuracy and were distributed in groups that were equivalent in age. The resolutions were taken from the psychophysical forced-choice method, using the static sine wave gratings with spatial frequencies of 0.25, 2.0 and 8.0 cpd (cycles per degree of visual angle). During each experimental session, there were stimulus pares presented randomly (test stimulus and neutral), and the task of the participants was to choose always the test stimulus with one of the frequencies above. The criterion used was the one of three consecutive correct answers to low the contrast in one unity and just one mistake to increase the contrast in the same unity (20%). After each session, a sheet of results with the experimental situation was released and the six values of contrast were calculated by the reversions. The values of contrast obtained to each frequency were grouped in spreadsheets separated by condition (with or without epilepsy) and the big average was used as an estimative of the sensory threshold or of the sensibility to the contrast due to each spatial frequency tested. The analysis of variance (ANOVA) to repeated measures showed significant differences between the groups [F(1,238) = 11.80); p<0.01]. The analysis obtained with the help of the test posthoc Tukey HSD revealed a relevant dissimilarity only in the 0.25cpg (p<0.01)]. In other words, the results indicated an expressive alteration in the visual perception children and adolescents with epilepsy have. / A epilepsia é um distúrbio cerebral caracterizado pela predisposição persistente do cérebro para gerar crises epilépticas e pelas conseqüências neurobiológicas, cognitivas, psicológicas e sociais desta condição. Algumas pesquisas demonstram que distúrbios visuais são bastante freqüentes em pessoas com epilepsia. Nestes termos, o objetivo deste trabalho foi determinar o limiar sensório visual de crianças e adolescentes com e sem epilepsia, verificando possíveis alterações na percepção visual da forma relacionadas a este transtorno. O limiar sensório é definido como a menor intensidade de um estímulo que um ser humano é capaz de detectar. A função de sensibilidade ao contraste (FSC) é o inverso da curva de limiar de contraste (1/FSC). Assim, a FSC é uma ferramenta que permite caracterizar a resposta do SV para padrões visuais em níveis baixos, médios e altos de contraste e avaliar possíveis alterações sensoriais relacionadas ao processamento visual. A idéia principal é que a elevação ou redução da FSC pode está relacionada a alterações sensoriais, cognitivas e comportamentais. Participaram deste estudo, 20 voluntários de ambos os sexos com idades entre 7 e 17 anos, sendo 10 sem epilepsia e 10 com epilepsia (crises tônicoclônicas). Todos apresentavam acuidade visual normal ou corrigida e foram distribuídos pelos grupos atendendo ao critério de equivalência de idade. As medidas foram realizadas com o método psicofísico da escolha forçada, utilizando grade senoidal vertical estática com freqüências espaciais de 0,25; 2,0 e 8,0 cpg (ciclos por grau de ângulo visual). Durante cada sessão experimental, foram apresentados aleatoriamente pares de estímulos (estímulo de teste e neutro), e a tarefa dos participantes foi escolher sempre o estímulo de teste com uma das freqüências acima. O critério adotado foi o de três acertos consecutivos para diminuir o contraste em uma unidade, e apenas um erro para aumentar o contraste na mesma unidade (20%). Após cada sessão, o programa produzia uma folha de resultados com a situação experimental e os seis valores de contraste conseguidos pelas reversões. Os valores de contraste obtidos para cada freqüência foram agrupados em planilhas por condição (com e sem epilepsia) e a grande média foi utilizada como estimativa do limiar sensório ou da sensibilidade ao contraste em função de cada freqüência espacial testada. A análise de variância (ANOVA) para medidas repetidas mostrou diferenças significantes entre os dois grupos [F(1, 238) =11,80); p < 0,01]. Já a análise com o teste post-hoc Tukey HSD revelou diferença significante apenas na faixa de freqüências de 0,25 cpg (p < 0,01). Em termos gerais, os resultados demonstraram alteração significante na percepção visual da forma de crianças e adolescentes com epilepsia.

This study is a thesis for the Master of Science degree in Media Technology and Engineering at the Department of Science and Technology, Linkoping University. It was accomplished from November 2002 to May 2003. Objective image quality measures play an important role in various image processing applications. In this paper quality measures applied on halftoned images are aimed to be in focus. Digital halftoning is the process of generating a pattern of binary pixels that create the illusion of a continuous- tone image. Algorithms built on this technique produce results of very different quality and characteristics. To evaluate and improve their performance, it is important to have robust and reliable image quality measures. This literature survey is to give a general description in digital halftoning and halftone image quality methods.

La courbe de sensibilité au contraste – la façon selon laquelle la sensibilité diffère selon les fréquences spatiale – a été mesurée pour la première fois en 1956 (Schade, 1956). Elle diffère d’individu en individu et, quoiqu’elle ait été observée pour la première fois il y a plus de 60 ans, certains facteurs ayant un impact sur ces différences individuelles sont mal compris. La figure de Campbell-Robson est une grille sinusoïdale dont la fréquence spatiale varie sur l’axe des x et le contraste varie sur l’axe des y, de sorte que l’observateur perçoit une courbe directement sur la figure. Si cette figure contenait de l’information sur la courbe de sensibilité au contraste d’un individu, elle aurait pu être utilisée pour développer une méthode rapide permettant de mesurer la courbe de sensibilité au contraste. Or, les résultats de l’article 1 montrent qu’il n’existe que peu d’information à propos de la courbe de sensibilité au contraste dans la figure de Campbell-Robson. La maturation de la sensibilité au contraste n’est pas bien comprise. Puisque les études antérieures ont utilisé, entre autres, des méthodes et des tâches différentes, les résultats rapportés par ces études sont contradictoires. Nous nous sommes penchés sur la question dans l’article 2 en utilisant une méthode objective pour mesurer la sensibilité au contraste pour une grande étendue de fréquences spatiales (0.5 à 30 cycles par degré) et une grande étendue d’âges (4 à 27 ans). Au lieu d’utiliser l’âge comme variable catégorielle en séparant les participants en groupes, nous avons utilisé une méthode de régression locale (LOESS) pour utiliser l’âge comme variable continue et ainsi obtenir plus de précision sur l’âge de maturation. Les résultats montrent que la sensibilité devient semblable à celle d’un adulte autour de 12 ans pour les fréquences spatiales basses et hautes, et autour de 17 ans pour les fréquences spatiales moyennes. Après l’âge, la culture dans laquelle une personne grandit est un autre facteur pouvant avoir un impact sur la sensibilité au contraste. Dans l’article 3, nous avons vérifié l’effet de la culture sur la courbe de sensibilité au contraste en la mesurant de deux manières différentes chez des participant.es chinois.es et canadien.nes. Les résultats montrent que la courbe de sensibilité au contraste ne semble pas être différente chez les deux cultures. Compte tenu de différences interculturelles dans les fixations oculaires sur les visages, nous avons vérifié s’il existe des différences dans les fréquences spatiales contenues dans la représentation interne des visages chez les deux cultures. Nos résultats montrent que les participant.es chinois.es utilisent de plus basses fréquences spatiales et les participant.es canadien.nes utilisent de plus hautes fréquences spatiales pour reconnaître les visages. En somme, les résultats présentés dans cette thèse permettent de mieux comprendre les différences individuelles dans la sensibilité au contraste. / The contrast sensitivity function – the curve defining the way in which sensitivity differs according to spatial frequencies – was first measured in 1956 (Schade, 1956). The contrast sensitivity function differs from person to person and, although it was observed over 60 years ago, some factors which have an impact on these individual differences are not well understood. The Campbell-Robson figure is a sinusoidal grating on which the spatial frequency varies on the x axis and contrast varies on the y axis, resulting in a perceived curve, between the grating and the perceived gray area of the grating, directly on the figure. If this figure contained information on the contrast sensitivity function, it would have been useful to develop a quick method to measure it. However, the results of article 1 show that there is little information on the contrast sensitivity function contained in the Campbell-Robson chart. The way in which contrast sensitivity matures is not well understood. Because anterior studies used, among other things, different methods and tasks, the results they report vary greatly and are often contradictory. We have studied the question in article 2, using an objective method to measure contrast sensitivity for a large array of spatial frequencies (0.5 to 30 cycles per degree) and a large span of ages (4 to 27 years). Instead of using age as a categorial variable by separating the participants in age bins, we used a local regression technique (LOESS) in order to use age as a continuous variable and obtain a more precise estimate of the maturation age of contrast sensitivity. Results show that sensitivity becomes similar to an adult’s around 12 years old for low and high spatial frequencies, and around 17 years old for mid-range spatial frequencies. Other than age, the culture in which a person grows up is another factor that could have an impact on contrast sensitivity. In article 3, we verified the effect of culture on the contrast sensitivity function by measuring it using two different methods in Chinese and Canadian participants. Results don’t show that the contrast sensitivity function differs between the two cultures. Because of intercultural differences in ocular fixations on faces, we further verified if there are differences in the spatial frequencies contained in the internal representations of faces in the two cultures. Our results show that Chinese participants use lower spatial frequencies and Canadian participants use higher spatial frequencies when identifying faces. In sum, the results presented in this thesis help better understand individual differences in contrast sensitivity.