1. A method to monitor an audience of a media presentation, the method comprising: grouping successive image frames depicting a location of the media presentation to form a sequence of scenes, respective scenes including respective groups of the image frames; grouping matching scenes into respective scene clusters having respective sizes, the scene clusters being represented by respective key frames; assigning respective ranks to the key frames of the respective scene clusters, the respective ranks being determined based on the respective sizes of the scene clusters; and processing the key frames in accordance with the respective ranks to monitor the audience of the media presentation.