1. A computer-implemented method comprising: receiving a plurality of images that display visible physical features of people in the images; generating a data structure having representations of the visible physical features and associations that link the representations based on similarities in appearance between the visible physical features of the people in the images; and outputting a first classification value for a first representation of a first visible physical feature of a person in an image that indicates a classification for the first visible physical feature of the person in the image, into one or a plurality of available classification groups, based on one or more other corresponding classification values of one or more other representations of one or more other corresponding visible physical features that are linked to the first representation.