AuthorsP. Ni
TitleVisual Perception of Scalable Video Streaming: Applied to Mobile Scenarios
AfilliationCommunication Systems, Communication Systems
Publication TypePhD Thesis
Year of Publication2016
PublisherUniversity of Oslo (UiO)
Other NumbersISSN 1501 - 7710

Modern video coding techniques provide multidimensional adaptation options for adaptive video streaming over networks. For instance, a video server can adjust the frame-rate, frame-size or signal-to-noise ratio (SNR) of the video being requested to cope with the available bandwidth. However, these adaptation operations give rise to distinct visual artefacts, so it follows that they are not perceived in the same manner. Subjective evaluation shows that we can no longer assume a monotonic rate-distortion function for scalable video. In fact, the perceived video quality that is expressed as the overall viewing experience of the video being delivered, is mutually and interactively affected by many factors ranging from configuration parameters to source material. Performing subjective evaluation is a tedious task, and to facilitate conducting field experiments of quality assessment, we introduce a practical and economic method, denoted by Randomized Paired Comparison. The performance of this method has been examined by experimental practice and simulations. To help formulating optimal adaptation strategies for streaming services, a sequence of field studies have been conducted to evaluate the perceived video quality, with the focus mainly on mobile streaming scenarios. These studies reveal that dynamic bit-rate variations may bring about the so-called flicker effects, which have negative influence on the perceived quality. Furthermore, the perceptual impacts can be controlled by the intensity of bit-rate changes (amplitude) and the number of bit-rate changes per seconds (frequency). The amplitude of bit-rate fluctuation is the most dominant factor. Thus, the greater amplitude an adaptation scheme has, the lower perceived quality will be brought about. Meanwhile, the frequency factor affects visual quality significantly when the bit-rate changes occurs in the spatial domain. To ensure stability of the perceived video quality, the spatial components (pixel values and frame size) of a video should be kept unchanged for a period more than 2 seconds. Moreover, we have explored the acceptance thresholds of quality degradations in different scaling dimensions, and the general preference order of scaling dimension has been suggested. We made also some preliminary analyses of the effect of content type in all of these studies.