Loading...
6 results
Search Results
Now showing 1 - 6 of 6
- Audiovisual Quality of Live Music Streaming over Mobile Networks using MPEG-DASHPublication . Rodrigues, Rafael; Pocta, Peter; Melvin, Hugh; Bernardo, Marco V.; Pereira, Manuela; Pinheiro, Antonio M. G.The MPEG-DASH protocol has been rapidly adopted by most major network content providers and enables clients to make informed decisions in the context of HTTP streaming, based on network and device conditions using the available media representations. A review of the literature on adaptive streaming over mobile shows that most emphasis has been on adapting the video quality whereas this work examines the trade-off between video and audio quality. In particular, subjective tests were undertaken for live music streaming over emulated mobile networks with MPEG-DASH. A group of audio/video sequences was designed to emulate varying bandwidth arising from network congestion, with varying trade-off between audio and video bit rates. Absolute Category Rating was used to evaluate the relative impact of both audio and video quality in the overall Quality of Experience (QoE). One key finding from the statistical analysis of Mean Opinion Scores (MOS) results using Analysis of Variance indicates that providing reduced audio quality has a much lower impact on QoE than reducing video quality at similar total bandwidth situations. This paper also describes an objective model for audiovisual quality estimation that combines the outcomes from audio and video metrics into a joint parametric model. The correlation between predicted and subjective MOS was computed using several outcomes (Pearson and Spearman correlation coefficients, Root Mean Square Error (RMSE) and epsilon-insensitive RMSE). The obtained results indicate that the proposed approach is a viable solution for objective audiovisual quality assessment in the context of live music streaming over mobile network.
- Quality perception and chromatic changes in digital imagesPublication . Bernardo, Marco André Vieira Andrade; Sousa, Maria Manuela Areias da Costa Pereira de; Fiadeiro, Paulo TorrãoThe work presented in this thesis intends to provide significant contributions to the research and development on multimedia quality assessment. It is particularly focused on the evaluation of image quality perception and models, and also on the study of consumer sensitivity to induced chromatic changes in digital images. The main motivation is to evaluate the perception of quality, providing the means to improve the consumer immersion capability of new trends in imaging technology, a more realistic perception of the contents, and consequently to reach new levels of quality of experience. New representation models of visual information are being developed to offer visual fidelity closer to the human perceptual limits. Such representation models, like hyperspectral and multiexposure technologies, lead to new image formats that recently had acquired large attraction from scientific, professional, and artistic communities. The first research topic in this thesis reports a study on the induced chromatic changes effects on perceived colors in images and subsequent quality of experience influence on consumers. The results had demonstrated that subjects reveal larger perception to a loss of color naturalness than to the error magnitude itself. Moreover, the eye tracking data revealed a high correlation with the perceived image quality. The second research topic is devoted to the quality evaluation of the JPEG XT performance on high dynamic range images coding. This study, made in cooperation with other research groups inside Qualinet, benefit from the experience and knowledge acquired on the first research topic. Furthermore, allowed to apply the acquired knowledge on quality in standardization activities. The JPEG XT profiles were verified and the application of quality metrics for quality evaluation was studied. It was concluded that metrics specifically developed for high dynamic range images revealed to be reliable predictors. Moreover, good performance can also be archived with objective metrics developed for low dynamic range domain, when used on perceptual domains. This thesis is concluded with a final comment on the achieved results and possible future research and development directions.
- Quality comparison of the HEVC and VP9 encoders performancePublication . Fernandes, Pedro; Bernardo, Marco; Pinheiro, Antonio M. G.; Fiadeiro, Paulo; Pereira, ManuelaThis paper reports a comparison between two recent video codecs, namely the HEVC and the VP9, using High Definition Video Sequences encoded with different bit rates. A subjective test for the evaluation of the provided Quality of Experience is reported. The video sequences were shown to a panel of subjects on a High Definition LED display and the subjective tests were performed using a Single Stimulus Methodology. The results shown that the HEVC encoder provides a better visual quality on low bit rates than the VP9. Similar performance was obtained for visually lossless conditions, although the HEVC requires lower bit rates to reach that level. Moreover, the correlation of the subjective evaluation and three tested objective metrics (PSNR, SSIM, and FSIM) revealed a good representation of the subjective results, particularly the SSIM and the FSIM metrics.
- MedCor: caracterização, modelação e implementação de transformações cromáticas em imagens digitais para observadores com cromatopsia visualPublication . Bernardo, Marco André Vieira Andrade; Sousa, Maria Manuela Areias da Costa Pereira de; Fiadeiro, Paulo TorrãoOs indivíduos de portadores de cromatopsia visual possuem certas limitações, quer na visualização, quer na discriminação de algumas cores no mundo real. Estas limitações reflectem-se também na percepção, por estes indivíduos, de cores emitidas por qualquer tipo de ambiente gráfico. Esta incapacidade visual poderá, de certa forma, ser prejudicial, na medida em que, o portador deste tipo de limitação poderá não obter uma acessibilidade à informação disponibilizada, nas mais variadas plataformas informáticas, tal como pode um observador com visão normal das cores. Existe, assim, desta forma, uma lacuna no que diz respeito a um tipo de conhecimento ou de tecnologia que procure contornar estes aspectos fundamentais, que poderão, portanto, contribuir para um melhoramento da qualidade de vida de um indivíduo portador deste tipo de anomalia. Por esta razão, e devido ao facto do estudo destas anomalias a nível informático ser ínfimo, propõem-se, então, uma metodologia de alteração colorimétrica em imagens digitais, assente na realização de um estudo intensivo e rigoroso através de experiências, com observadores, em cenário tridimensional, de forma a calcular o erro, ou desvio cromático em portadores de cromato psi a visual. Estas experiências que foram efectuadas em cada tipo de cor primária (RGB), sob a influência de dois tipos de iluminantes, de forma a tentar quantificar e modelar a variação de percepção colorimétrica em indivíduos portadores de cromatopsia visual.
- Assessment of speckle denoising filters for digital holography using subjective and objective evaluation modelsPublication . Fonseca, Elsa; Fiadeiro, Paulo; Bernardo, Marco V.; Pinheiro, Antonio M. G.; Pereira, ManuelaDigital holography is an emerging imaging technique for displaying and sensing three dimensional objects. The perceived image quality of a hologram is frequently corrupted by speckle noise due to coherent illumination. Although several speckle noise reduction methods have been developed so far, there are scarce quality assessment studies to address their performance and they typically focus solely on objective metrics. However, these metrics do not reflect the visual quality perceived by a human observer. In this work, the performance of four speckle reduction algorithms, namely the nonlocal means, the Lee, the Frost and the block matching 3D filters, with varying parameterizations, were subjectively evaluated. The results were ranked with respect to the perceived image quality to obtain the mean opinion scores using pairwise comparison. The correlation between the subjective results and twenty different no-reference objective quality metrics was evaluated. The experiment indicates that block matching 3D and Lee are the preferred filters, depending on hologram characteristics. The best performing objective metrics were identified for each filter.
- Holographic representation: Hologram plane vs. object planePublication . Bernardo, Marco V.; Fernandes, Pedro; Arrifano, Ângelo Miguel; Antonini, Marc; Fonseca, Elsa; Fiadeiro, Paulo; Pinheiro, Antonio M. G.; Pereira, ManuelaDigital holography allows the recording, storage and subsequent reconstruction of both amplitude and phase of the light field scattered by an object. This is accomplished by recording interference patterns that preserve the properties of the original object field essential for 3D visualization, the so-called holograms. Digital holography refers to the acquisition of holograms with a digital sensor, typically a CCD or a CMOS camera, and to the reconstruction of the 3D object field using numerical methods. In the current work, the different representations of digital holographic information in the hologram and in the object planes are studied. The coding performance of the different complex field representations, notably Amplitude-Phase and Real-Imaginary, in both the hologram plane and the object plane, is assessed using both computer generated and experimental holograms. The HEVC intra main coding profile is used for the compression of the different representations in both planes, either for experimental holograms or computer generated holograms. The HEVC intra compression in the object plane outperforms encoding in the hologram plane. Furthermore, encoding computer generated holograms in the object plane has a larger benefit than the same encoding over the experimental holograms. This difference was expected, since experimental holograms are affected by a larger negative influence of speckle noise, resulting in a loss of compression efficiency. This work emphasizes the possibility of holographic coding on the object plane, instead of the common encoding in the hologram plane approach. Moreover, this possibility allows direct visualization of the Object Plane Amplitude in a regular 2D display without any transformation methods. The complementary phase information can easily be used to render 3D features such as depth map, multi-view or even holographic interference patterns for further 3D visualization depending on the display technology.