In one aspect, a method includes receiving a sequence of images of a subject, the sequence captured by an image acquisition device over a time period in which relative locations of the image acquisition device varies with respect to the subject. A first image and a second image are selected from the first sequence of images. The first and second images represent a first relative location and a second relative location, respectively, of the image acquisition device with respect to the subject. A three-dimensional representation of the first subject is generated using the first image and the second image as a stereo pair. It is determined, based on the first three dimensional representation, that the first subject is an alternative representation of a live person, and in response, access to a secure system is prevented.