An object classifier using a set of object definitions arranged in an object definition hierarchy including at least a first group of coarse-level object definitions and a second group of finer-level object definitions. The object classifier is arranged to configure a first object classification cycle and a second, subsequent, object classification cycle by selectively executing a first subset of object definitions from the categorization data in the first object classification cycle; and selectively executing a second, different, subset in the second object classification cycle.