An N-object detector comprises an N-object decision structure incorporating decision sub-structures of N object detectors. Some decision sub-structures have multiple different versions composed of the same classifiers with the classifiers rearranged. Said multiple versions associated with an object detector are arranged in the N-object decision structure so that the order in which the classifiers are evaluated is dependent upon the results of the evaluation of a classifier of another object detector. Each version of the same decision sub-structure produces the same logical behaviour as the other versions. Such an N-object decision structure is generated by generating multiple candidate N-object decision structures and analysing the expected computational cost of these candidates to select one of them.