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becoming a popular and everyday part of the Internet culture, thus establishing new
viewing patterns and novel forms of social behavior.
This section reviews different collections of user-contributed media like YouTube
[ 1 ] (see Sect. 2.2.1 ), Flickr [ 2 ](seeSect. 2.2.2 ), and Wikipedia [ 3 ] (see Sect. 2.2.3 )by
discussing their main features. We discuss these media collections as representatives
of the rapid growth witnessed in the Internet-based multimedia domain. In addition,
the interest of Web users in these Web services has significantly grown, attracting an
increasing attention from the scientific community.
2.2.1 YouTube
YouTube [ 1 ] is one of the largest and most successful online services allowing users
to upload, share, and watch video material freely and easily. Since its establishment
in early 2005, YouTube has become one of the fastest-growing Web sites and ranks
fourth in Alexa's [ 4 ] top popular site list, with 24 h of new video content uploaded
every minute, as of March 2010.
YouTube's primary features include the ability to upload and play back video
clips. Any user with a Web browser can view YouTube videos, but users are required
to create an account to publish their own content and interact with each other.
Registered users are assigned a profile page, named a “channel”, which serves as
an index to the user's uploaded material. Users may easily customize the looks of
their channel page by selecting an available graphical theme or by creating a new
one. They may optionally disclose their personal details, subscribe to other users'
videos, or “make friends” with them. Comments can be posted by registered users
on another user's profile page or on a specific video's page. Viewers can addition-
ally rate videos or join groups that focus on particular interests. YouTube thus
shows its strong community nature: it is a social networking site, with the added
feature of hosting video content [ 5 ].
Videos can be uploaded in most existing container formats and are automatically
converted into the Adobe Flash Video format (FLV). The Adobe Flash Player
browser plug-in, required to play the FLV format, is one of the most common pieces
of software installed on personal computers.
Currently, video clips uploaded by standard users are limited to 10 min in length
and a file size of 2 GB. In YouTube's early days, it was possible for users to upload
longer videos, but a time restriction was introduced in March 2006, when it was
found that the majority of videos exceeding this length were unauthorized uploads
of copyrighted materials. Since November 2008, YouTube has been making an
ongoing effort to improve picture quality. Videos were thus made available in HD
format and currently use the H.264/MPEG-4 AVC codec, with stereo AAC audio.
YouTube assigns each video a distinct 11-digit identifier, composed of digits and
(uppercase and lowercase) letters. Associated with the videos are some metadata,
including the name of the uploader, the date of upload, and the number of views,
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