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010_Introduction/Introduction.tex
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\graphicspath{{010_Introduction}}
\glsreset{hevc}
\chapter{Introduction \VERSION{1}}
\chapter{Introduction \VERSION{2}}
\label{Chapter:Intro}
\section{Context and motivation}
\glsreset{h264}
Over the recent years new advances in video compression and networking technologies have been enabling the development of many multimedia applications over the internet, such as video conferencing, video streaming, video publishing, among others.
The increasing diversity of services, the demands for high quality multimedia and the growing popularity of \ac{uhd} formats (\emph{e.g.}, $4k\times2k$ or $8k\times4k$ resolution and beyond) are pushing technologies forward and opening new research directions.
Over the recent years new advances in video compression and networking technologies have enabled the development of many multimedia applications over the internet, such as video conferencing, video streaming, video publishing, among others.
The increasing diversity of services, the demands for high quality multimedia and the growing popularity of \ac{uhd} formats (\emph{e.g.}, $4k$ or $8k$ resolution and beyond) are pushing technologies forward and opening new research directions.
Moreover, the emerging new video formats with increased resolutions have created stronger needs for higher coding efficiency, beyond the previous standards, i.e., the MPEG-4/\ac{h264}~\cite{H264AVCStandard2003,H264OverviewJul2003}.
Such need has been driven not only by higher resolutions but also by stereoscopic and multi-view video formats, which increase the number of viewpoints that need to be captured, coded and delivered simultaneously.
Such need has been driven not only by higher resolutions but also by stereoscopic and multi-view video formats, which increase the number of viewpoints that need to be encoded and delivered simultaneously.
Moreover, the ever increasing traffic generated by mobile multimedia applications \cite{CiscoMobile1621_Feb2017} and different video services with ever increasing popularity, e.g., Netflix, impose new challenges to the existing networks.
To cope with such demanding requirements a new coding standard was developed by the ITU-T \ac{vceg} and the \ac{iso}/\ac{iec} \ac{mpeg} standardisation organisations, referred to as H.265 - \ac{hevc}~\cite{H265StandardFeb2018,HEVCOverviewDez2012}.
\glsunset{h264}
The \ac{hevc} is currently the main video coding framework to extend current services towards higher resolution formats delivered with better compression efficiency.
Similar to its predecessor, \ac{hevc} is a block-based hybrid video codec using predictive transform and entropy coding.
The coding flexibility and efficiency of \ac{hevc}, results from the use of new coding tools and block structures, which enables new block partitions\cite{HEVCBlock2012}, improved prediction modes \cite{IntraCodingHEVC2012,AMPCodingHEVCJun2012} and new high-level features~\cite{HEVCHighLevelReferenceDez2012}, such as explicit reference picture management and new parameter sets.
However, it is known that higher coding efficiency leads to lower robustness to transmission errors and data loss.
Therefore, the new coding tools in \ac{hevc} bring not only high compression efficiency~\cite{HEVCComparisonDec2012}, but also some other disadvantages, such as increased complexity~\cite{PerformanceComplexityDez2012} and inherent reduction in error robustness.
Similar to its predecessor, \ac{hevc} is a block-based hybrid video codec using predictive, transform and entropy coding.
The coding flexibility and efficiency of \ac{hevc} results from the use of new coding tools and block structures, which enables new block partitions\cite{HEVCBlock2012}, improved prediction modes \cite{IntraCodingHEVC2012,AMPCodingHEVCJun2012} and new high-level features~\cite{HEVCHighLevelReferenceDez2012}, such as explicit reference picture management and new parameter sets.
However, it is known that higher coding efficiency leads to lower robustness in the presence of transmission errors and data loss.
Therefore, the \ac{hevc} new coding tools bring not only high compression efficiency~\cite{HEVCComparisonDec2012}, but also some other disadvantages, such as increased complexity~\cite{PerformanceComplexityDez2012} and inherent reduction of error robustness.
%\cite{StudyHEVCErrorsDez2012}
Earlier studies about the error robustness characteristics of \ac{hevc} have shown that in general the use of high efficiency coding tools results in poor error resilience performance, in comparison with previous standards.
Earlier studies about the error robustness characteristics of \ac{hevc} have shown that, in general, the use of high efficiency coding tools results in poor error resilience performance, in comparison with previous standards.
Thus, when \ac{hevc} streams incur in transmission losses, this leads to significant degradation of both objective and subjective quality~\cite{QoEHEVCMay2014,SubjectiveEvalSep2013}.
This is mostly due to the strong decoding dependencies imposed by the highly complex prediction modes of \ac{hevc}, which assume \mbox{error-free} transmission~\cite{ROPE2007}.
A possible approach to encode robust video streams is to increase the error resilience of \ac{hevc} by including \mbox{error-aware} mechanisms in the coding process. As a result, the effect of error propagation due to prediction mismatch at the decoding side, will be limited.
A possible approach to encode robust video streams is to increase the error resilience of \ac{hevc} by including \mbox{error-aware} mechanisms in the coding process. As a result, the effect of error propagation, due to prediction mismatch at the decoding side, will be limited.
The widespread use of \ac{hevc} in broadcast and multimedia delivery services, is be based on transport technologies such as \ac{rtp}~\cite{HEVCStreamMay2012}, \mbox{MPEG-2 TS}~\cite{HEVCLayerIntegrationDez2012}, or more recent standards as MPEG-DASH~\cite{HEVCHTTPStreamAug2016}.
The widespread use of \ac{hevc} in broadcast and multimedia delivery services is based on transport technologies such as \ac{rtp}~\cite{HEVCStreamMay2012}, \mbox{MPEG-2 TS}~\cite{HEVCLayerIntegrationDez2012}, or more recent standards as MPEG-DASH~\cite{HEVCHTTPStreamAug2016}.
Recent developments also enable combinations of different technologies, e.g, \mbox{HTTP-based} streaming and \mbox{MPEG-2 TS}, to increase the flexibility of over-the-top (OTT) video services\cite{Broadcast2017}. For content delivery over heterogeneous networks, the \ac{mmt}~\cite{ATSCMMT2016} is another possible technology to be used with the \ac{hevc} standard. Besides the transport technologies,
the overall error robustness of video transmission may also benefit from the increasing flexibility of multimedia systems to cope with heterogeneous networks, by using auxiliary streams to aid the video decoder to increase the quality of recovered lost slices~\cite{ErrorResilienceHeterogenousNetworks2016,HeterogeneousStreaming2016}. Moreover, error resilience techniques may also take advantage of the multi-stream based approach, where each stream allows different levels of robustness to errors~\mbox{\cite{HEVCHTTPStreamAug2016,PerfMediaTransport2017}}.
the overall error robustness of video transmission may also benefit from the increasing flexibility of multimedia systems to cope with heterogeneous networks, by using auxiliary streams that allow the video decoder to increase the quality of the recovered lost slices~\cite{ErrorResilienceHeterogenousNetworks2016,HeterogeneousStreaming2016}. Moreover, error resilience techniques may also take advantage of the multi-stream based approach, where each stream allows different levels of robustness to errors~\mbox{\cite{HEVCHTTPStreamAug2016,PerfMediaTransport2017}}.
%Despite the different characteristics of the existing services and applications, the users' \ac{qoe} consistently depends on how the compressed streams are adapted to the diverse networking technologies, as well as on how the transmission errors and/or packet losses are handled at the application level ~\cite{HEVCStreamMay2012,HEVCLayerIntegrationDez2012,StudyHEVCErrorsDez2012}. As mentioned before, the \ac{hevc} standard is expected to replace the \ac{h264} in future video services.
@@ -51,17 +51,19 @@ the overall error robustness of video transmission may also benefit from the inc
%follows.
\begin{figure}[tb]
\centering
\input{./010_Introduction/RobustVideoSystem.tikz}
\caption{Generalised diagram of a robust video transmission system.}
\label{FIG1:RobustVideoTransmission}
\end{figure}
In summary, increasing the error robustness of the highly compressed video streams is a relevant research problem whose solutions will be useful to increase the adaptability for transmission diverse network conditions and to achieve robust communications in order to provide acceptable levels of \ac{qoe} to the end users at all times.
In general, increased error robustness in video transmission can be jointly achieved at three different stages: (i) using robust video encoding, by selecting robust coding modes, (ii) after encoding, by adding robust features to the compressed streams as side-information and (iii) using robust video decoding techniques, e.g., efficient \ac{ec}. This is exemplified in Figure~\ref{FIG1:RobustVideoTransmission} where at the encoder different layer are used to increase the error robustness, which results in a video stream composed by a \ac{vcl} and \ac{sei} layers. Finally, at the decoder two processing layers are used to achieve robust video decoding.
The \ac{hevc} reference codec is continuously being improved and extended development by multiple proponents from around the world, also addressing the problem of error resilience and robust delivered transmission
Although the \ac{ec} methods have been developed during the past years for \ac{h264}~\cite{2DRecursiveRefinement2010} and recently for \ac{hevc}~\cite{HEVCMCBlockAug2013}, a combined and optimised approach between encoder and decoder is yet to be developed, which would improve the users’ \ac{qoe} by exploiting shared information between the encoder and the decoder sides.
In summary, increasing the error robustness of the highly compressed video streams is a relevant research problem, whose solutions will be useful to increase the adaptability for transmission over diverse network conditions and to achieve robust communications, thus providing acceptable levels of \ac{qoe} to the end users at all times.
In general, increased error robustness in video transmission can be jointly achieved at three different stages: (i) using robust video encoding, by selecting robust coding modes, (ii) after encoding, by adding robust features to the compressed streams, as side-information, and (iii) using robust video decoding techniques, e.g., efficient \ac{ec}. This is exemplified in Figure~\ref{FIG1:RobustVideoTransmission} where at the encoder different layers are used to increase the error robustness, which results in a video stream composed by a \ac{vcl} and \ac{sei} layers. Finally, at the decoder two processing layers are used to achieve robust video decoding.
The \ac{hevc} reference codec is continuously being improved and extended by multiple proponents from around the world, which also address the problem of error resilience and robust delivered transmission.
Although different \ac{ec} methods have been developed during the past years for \ac{h264}~\cite{2DRecursiveRefinement2010} and recently for \ac{hevc}~\cite{HEVCMCBlockAug2013}, a combined and optimised approach involving the encoder and decoder is yet to be developed, which would improve the users’ \ac{qoe}, by exploiting shared information between the encoder and the decoder sides.
\begin{figure}[tb]
\centering
\input{./010_Introduction/RobustVideoSystem.tikz}
\caption{Generalised diagram of a robust video transmission system.}
\label{FIG1:RobustVideoTransmission}
\end{figure}
\section{Main objectives and contributions}