Feature detectors are used to build a semantically rich index entry for the original multimedia object. They do this on two different levels:
In the general case blackbox detectors will derive low-level feature data, e.g., the color distribution of an image. But they can also be used for more complex tasks, like finding a face in an image. The function of whitebox detectors is to relate low-level features to concepts, e.g., an image is a portrait because its color distribution classifies it as a photo and it contains exactly one face."
- Blackbox detectors are implemented in a programming language to access the raw multimedia data and to derive the desired features from it. Example: the web_header detector sends a HTTP HEAD request to the specified HTTP server and extracts the content type from the response.
- Whitebox detectors consist of queries over the already collected feature values. Example: the page_type detector uses the content type to determine if an object is a page.