Pengpeng Ni successfully defended her PhD

On the 18th of January 2016, Pengpeng Ni successfully defended her PhD thesis “Visual Perception of Scalable Video Streaming: Applied to Mobile Scenarios". The defense took place at 12:15, “Storstua”, Simula Ressearch Laboratory, Martin Linges vei 17, Fornebu.

Quality of Experience in the context of video streaming services rests mainly in the perceived video quality, which is a typical subjective measure. We can no longer assume a monotonic relationship between video bitrate and user perceived quality. The quality evaluation of streaming video relies highly on subjective quality assessment.  In this thesis, we present a subjective quality evaluation method for conducting audiovisual quality assessment studies in the field. Field studies are known to be expensive and difficult to administer. Our method is designed with realistic assumptions of the time and effort that an assessor will have to spend. With the use of this method, an experimenter can easily obtain stable results with accuracy close to traditional experiment designs at a much lower cost. We demonstrate the efficiency and practicality of this method by simulations.

This thesis presents also a sequence of user studies that evaluates the effects of multidimensional video adaptation techniques on human quality perception. These studies reveal that frequent quality variations may create additional visual artefacts denoted flicker effects, and it is not worthwhile making quality changes unless the negative impact of flicker on visual quality is eliminated. We identify the main influential factors on the visibility of flicker effects and determine the threshold quantities of these factors for acceptable visual quality of video. These findings can help improving video adaptation strategy or bit-rate controllers deployed in video streaming solutions, such as Scalable video streaming, Dynamic Adaptive Streaming over HTTP, and Microsoft Smooth Streaming. In all of our studies, the quality assessments were made on different types of video content, some preliminary analyses of content effects on human quality perception are provided.

The thesis is written within the field of Distributed multimedia systems. The work has been conducted at Simula Research Laboratory and University of Oslo.

Prior to the defense, at 10:15, Pengpeng Ni presented her trial lecture “Crowdsourcing user studies related to digital media”.

The adjudication committee

  • Professor Klara Nahrstedt, Computer Science, University of Illinois Professor Klara Nahrstedt, Computer Science, University of Illinois
  • Assistant Professor Hendrik Knoche, Department of Architecture, Design and Media, Ålborg University
  • Professor Frank Eliassen, University of Oslo, Department of informatics

Chair of the disputation

  • Professor, Knut Liestøl, Department of Informatics, University of Oslo


  • Professor Carsten Griwodz, Simula Research Laboratory
  • Professor Pål Halvorsen, Simula Research Laboratory

Read more

Announcement of the PhD defense at the University of Oslo's web pages (in Norwegian).

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