## 3 Architecture for Semi-automatic Image Tagging and Annotation Based on Visual Features  
Social media sharing sites, e.g., Flickr ([[http://www.flickr.com](http://www.flickr.com)]([http://www.flickr.com](http://www.flickr.com))), Zooomr ([[http://www.zooomr.com](http://www.zooomr.com)]([http://www.zooomr.com](http://www.zooomr.com))) or Smugmug ([[http://smugmug.com](http://smugmug.com)]([http://smugmug.com](http://smugmug.com))) to mention but a few, allow users to form communities which reflect their (photo-taking) interests. Many photos are annotated using keywords called _tags_. Tags are chosen by the user and not restricted by a taxonomy or vocabulary. The process of assigning tags to resources is often referred to as _tagging_. Some examples of common tags assigned to digital photos on the Flickr web site (sourced from Flickr's all time most popular tags) are provided in Figure 3. On the most basic level, tags can be considered as keywords, categories labels or markers. Some systems may introduce more structure into tags and even so called machine readable tags that encode specific information intended to be read by the system itself. Despite the fact that many platforms allow users to enter free text tags (including spaces and punctuation), the work described in this paper only considers clean, case-invariant single word tags (largely comparable with Flickr's clean tags). Most tagging systems allow multiple tags to be assigned to each object. In this work, the set of tags assigned to an object (_i.e._ a photo) is referred to as a _tag set_ (see image in Figure 4 and the assigned tags). The Flickr web service, for example, allows users to provide up to 75 tags for each photo. It is important to note that tag sets are not lists--they do not preserve order. The highly flexible nature of tagging systems allows tags to be used for many different purposes. When applied in a wider sense, tagging leads to emergent structures that serve a particular purpose [13]. For example, the tag _FlickrElite_, despite not providing any explicit meaning, is used on Flickr to indicate that a photo belongs to a group of elite photographers. In their examination of the social issues surrounding tagging, [PERSON] and [PERSON] imply that user groups collaboratively tend  
Figure 3: Flickr’s all-time most popular tags. In this visualization—called _tag cloud_—the font size of the tag visualizes roughly the frequency of a tag. to develop a tagging vocabulary to suit their individual needs and as a result, tags from such \"folksonomies\" [13] often hold little value for users outside this group [22]. Similar phenomena may also be observed in the tagging structures utilized on the Flickr web service. Despite these differences, tags may be classified into the following four broad classes:  
(a)Tags relating to direct subject matter of the photo, _i.e._ describing the objects visible in the photo. For example _old town_ or _architecture_. (b)Tags that provide technical information about the photo itself or the camera, such as whether a flash was used or the particular camera model. For example _nikon_ or _i500_. (c)Tags describing the circumstances surrounding the photo or the emotions invoked by it. For example _awesome_ or _speechless_. (d)Additional organizational tags, often pertaining to the perceived quality of the image or used to identify individual users or groups. For example _diamondclassphotographer_ or _flickrelite_. Photos are typically tagged using tags from all four categories, as demonstrated by the example shown in Figure 4, in which the photo is tagged with a total of 25 tags: 12 relate to the content of the image _(bee, flower, clouds, sky, fly, nature, fleur, fleurs, flowers, insect, insect_ and _animal_), one concerns the technicalities of the photo (_macro_), eight describe the circumstances and emotions (_buzz, mywinner, anawesomeshot, abigfav, abigfave, abphotoday, eyecatcher_ and _bravo_), and the remaining four have an organizational function (_project365, freedp, shieldofexcellence_ and _frhwofavs_).