1. A method comprising: evaluating, at a mobile device, a first area of pixels according to a summed area operation to generate a first result associated with summed pixel intensity values, the first area of pixels corresponding to a scene at a first time; evaluating, at the mobile device, a second area of pixels to generate a second result, the second area of pixels corresponding to the scene at a second time subsequent to the first time; and determining that the second area of pixels corresponds to one of a background portion of the scene or a foreground portion of the scene based on comparing a threshold with a difference between the first result and the second result.