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Thomas Forgione
2019-10-16 17:01:56 +02:00
parent 8b9303b1a5
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src/state-of-the-art/video.tex
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@@ -4,24 +4,22 @@
\subsection{DASH\@: the standard for video streaming\label{sote:dash}}
\copied{}
Dynamic Adaptive Streaming over HTTP (DASH), or MPEG-DASH \citep{dash-std,dash-std-2}, is now a widely deployed
standard for streaming adaptive video content on the Web \citep{dash-std-full}, made to be simple and scalable.
\fresh{}
DASH is based on a clever way of preparing and structuring a video in order to allow a great adaptability of the streaming without requiring any server side computation.
\subsubsection{DASH structure}
All the content structure is described in a Media Presentation Description (MPD) file, written in the XML format.
This file has 4 layers: the periods, the adaptation sets, the representations and the segments.
A MPD has a tree-structure, meaning that it has many periods, each period can have many adaptation sets, each adaptation set can have many representation, and each representation can have many segments.
A MPD has a tree-structure, meaning that it has multiple periods, each period can have multiple adaptation sets, each adaptation set can have multiple representation, and each representation can have multiple segments.
\paragraph{Periods.}
Periods are used to delimit content depending on time.
It can be used to delimit chapters, or to add advertisements that occur at the beginning, during or at the end of a video.
\paragraph{Adaptation sets.}
Adaptation sets are used to delimit content depending on format.
Adaptation sets are used to delimit content according to the format.
Each adaptation set has a mime-type, and all the representations and segments that it contains share this mime-type.
In videos, most of the time, each period has at least one adaptation set containing the images, and one adaptation set containing the sound.
It may also have an adaptation set for subtitles.
@@ -33,18 +31,18 @@ This allows a user to choose its representation and change it during the video,
\paragraph{Segments.}
Until this level in the MPD, content has been divided but it is still far from being sufficiently divided to be streamed efficiently.
In fact, a representation of the images of a chapter of a movie is still a long video, and keeping such a big file is not possible since heavy files prevent the dynamic of streaming: if the user requests to change the level of resolution of a video, the system would either have to wait until the file is totally downloaded, or cancel the request, making all the progress done unusable.
In fact, a representation of the images of a chapter of a movie is still a long video, and keeping such a big file is not possible since heavy files prevent streaming adaptability: if the user requests to change the level of resolution of a video, the system would either have to wait until the file is totally downloaded, or cancel the request, making all the progress done unusable.
Segments are used to prevent this issue.
They typically encode files that contain approximately one second of video, and give the software a great ability to dynamically adapt to the system.
If a user wants to seek somewhere else in the video, only one second of data can be lost, and only one second of data has to be downloaded for the playback to resume.
They typically encode files that contain approximately one second of video, and give the software a greater ability to dynamically adapt to the system.
If a user wants to seek somewhere else in the video, only one second of data can be lost, and only one second of data needs to be downloaded for the playback to resume.
\subsubsection{Content preparation and server}
Encoding a video in DASH format consists in partitioning the content into periods, adaptation sets, representations and segments as explained above, and generating a Media Presentation Description file (MPD) that describes this organisation.
Once the data is prepared, it can simply be hosted on a static HTTP server that does no computation other than serving files when it receives requests.
All the intelligence and the decision making is moved to the client side.
This is one of the strengths of DASH\@: no powerful server is required, and since static HTTP server are studied since the beginning of the internet, they are stable and efficient and all DASH clients can benefit from it.
This is one of the DASH strengths: no powerful server is required, and since static HTTP server are stable and efficient, all DASH clients can benefit from it.
\subsubsection{Client side computation}