The amount of audio visual information available in digital format has grown exponently in recent years. Gigabytes of new images, audio and video clips are generated and stored every day, building up a huge, distributed, and mostly unstructured repository of multimedia information, much of which can be accessed through the Internet.Digitalization, compression, and archival of multimedia information has become popular, inexpensive and straightforward, and there is a broad range of available hardware and software to support these tasks. Subsequent retrieval of the stored information, however, might require considerable additional work in order to be effective and efficient