1. A method comprising: accessing multiple images including a face presented by a user; determining, based on the multiple images, whether the face included in the multiple images is a 3-dimensional structure or a flat surface by: accessing a first image and a second image of the multiple images; generating a warped image by warping the first image using a homography matrix; determining an image difference between the warped image and the second image; and determining that the face included in the multiple images is a 3-dimensional structure if the difference between the warped image and the second image meets a threshold value, and otherwise determining that the face included in the multiple images is a flat surface; indicating that the user can be authenticated if the face included in the multiple images is a 3-dimensional structure; and indicating that the user cannot be authenticated if the face included in the multiple images is a flat surface.