## 1 Introduction
The same keyframes, for instance the first frame of a video, are always used although [15] has shown that these keyframes are not suited to present the results of each video query. Video retrieval is usually possible through (a) direct navigation to videos using external links; (b) internal links such as personalized recommendations and related videos that are shown in the portal website; and (c) text-based searches. According to [16], internal links of option (b) account for about 60% of all views in YouTube. The presented recommendations are personalized for each user and they reflect her recent activities on the site. Related video links are shown for videos that stem from the same user of a selected video that are similarly tagged or share other similarities like internal user recommendations or comments. However, it has been shown that there is an extremely weak social connectivity between users of a video portal [17, 18] and that views from user recommendations mainly account to direct links that are contained in Facebook postings, emails, or somewhere else. Such external links provide a way of advertising the videos, and thus they are especially important for recently uploaded videos with 10% to 30% of all views in the first 4 days of a YouTube video [19]. In the same study, the overall views from external links only accounted for about 7% views of all YouTube videos while a much higher impact of external links has been measured in YouKo. However, the current number of YouTube visits that consists of a single page view (bounce rate) and its constant increase over the last 20 months (up to 33% measured in November 2011 by Alexa [7]) indicate a higher number of views from external links today and supports their importance. The last class of video retrieval in online portals stems from search queries and goal-oriented browsing around a certain topic [16]. For this, it follows that only a small partition of all video views originates from a search oriented retrieval process, although no exact numbers are available. Most video portals only allow text-based video searches while visual queries and their combination with text queries [20] are mainly integrated in research prototypes [21, 22, 23, 24] that stem from computer vision research. Video portal _users_ follow the same intentions as users of traditional broadcast television. On the one hand, they visit video portals with the simple, unarticulated want to be entertained by content that they find interesting. On the other hand, they are interested in specific information and content. According to [25] users can be classified in (a) browsers with no clear end goal that perform a series of unrelated searches within a session and jump across multiple topics; (b) surfers that have a moderate clarity of an end goal. Their actions may be somewhat exploratory in the beginning but increase the clarity in subsequent searches; and (c) searchers that are very clear about what they are searching for in the system and only need short sessions to retrieve an end result. Users further adapt to the provided technologies and thus there exists a relationship between the most used query types (about videos as a whole that can be answered with bibliographic metadata) and the availability of results [9]. In other words, this means that people tend to ask queries where they expect reasonable answers. Queries about the visual content usually involved actors and their actions while questions about objects and the video structure have only been raised by people that were involved with multimedia work. The reasons why users want to share video content are manifold. One group of users produces content by themselves and simply like to share this content with other people. Another group wants to share or promote videos that they find interesting. The generation of tags and comments for video content is motivated by the similar goals, as shown in an investigation of user-generated YouTube tags [26].