1. A method comprising: at a mobile device with a camera and a display: capturing an image of a user with the camera; analyzing the captured image of the user; determining, from the analysis of the captured image of the user, whether positions of the user's eyes are determinable; in accordance with a determination that the positions of the user's eyes are determinable, determining a first viewpoint of the user relative to the display based on the positions of the user's eyes; in accordance with a determination that the positions of the user's eyes are not determinable, determining the first viewpoint of the user relative to the display using an alternate signature target; rendering a first image of a constructed scene on the display based on the first viewpoint relative to the display; after determining the first viewpoint of the user relative to the display, determining a second viewpoint of the user relative to the display; and rendering a different image of the constructed scene on the display based on the second viewpoint of the user relative to the display.