Photo Search Engines

The phrase 'Image Search Engine' is most often used of Web-based services that collect and index images from other sites on the Internet. Image searching is sometimes offered by general search engines, like Google or Altavista, but there are also specialised image search engines - services devoted to indexing images or multimedia. In addition, there are meta- search engines, which pass on search requests to more than one search engine and then bring back the results.

Sometimes 'Image Search Engine' is also used to refer to collection-based search engines - services that index a single or small number of image collections. Large digital libraries or commercial stock photo collections, like Corbis, typically offer such search engine-like facilities.

All of the categories of image search engines mentioned above are text-based - their indexes are created from words associated with the images. There have also been attempts to create content-based search engines, which 'index' visual characteristics of an image, such as its shape and colour. However, these tend to be experimental and are often limited to single image collections.

Image search engines are based on existing search engine technology, but they use additional strategies to identify, categorise and rank images.

A search engine's indexing of images is done automatically, rather than using human indexers, so it must find ways to guess at the image's content. It might take into account its filename or any accompanying 'ALT' picture tags (these are coded into the HTML page). It might look for clues from the image's context - for example, the words or phrases that are close to the image, or the 'META' tags found at the top of the HTML coding. The characteristics of the Web site and its server will also often be taken into account.

Analysis of an image's text and context can be used to exclude images as well as include them - for example, an image engine will usually consider an image's context and associated words when it is blocking out adult material.

Currently, the main English-language-based search engines offering image searches are AllTheWeb, AltaVista, Google and Lycos. They are all US-based. Yahoo (strictly speaking a directory rather than a search engine) offers a picture search, but this is drawn from the Corbis collection (reviewed in section 3, below). HotBot and MSN Search do not offer true image search features, they simply enable users to limit their search results to 'only pages with images'.

Conclusion

Image search engines attempt to give access to the wide range of images available on the Internet.

For those used to viewing well-indexed collections of quality images, the results of the large automated image search engines will probably disappoint. The poor quality of their offerings is not surprising, since they reflect the randomness and unevenness of the Web. The frequent irrelevancy of their results is also explicable, since the automated engines are guessing at their images' visual subject content using indirect textual clues.

Anything, then, that enables the user to have more control over their image searching is helpful. The ability to filter a search - to include and exclude items - is important in any Web searching, but particularly so when searching for images. Many users will wish to exclude adult imagery from their search results, but it can also be very useful to limit by file type, file size, or colour - and the ability to use Boolean logic or phrases will greatly improve the relevancy of the results. An image search engine also needs to return a reasonable number of results, since in any given search a fair proportion of the images found are likely to be irrelevant or of insufficient quality.

Collection-based image search engines include images selected for quality and indexed by hand. The images they contain are seldom found within the results of general Web search engines. Collection-based engines, then, will usually offer much better results than their search engine counterparts. The commercial and copyright issues will also be much clearer - although many users seem to prefer to look to image search engines for images they can 'freely' re-use, as if easy access and absent copyright notices lets them off the moral and legal hook. True copyright-free images are rare on the Web, and many image search engines do operate on commercial imperatives, even subtlety skewing their results towards commercial ends.