Two Cameras and a Screen: How to Calibrate Mobile Devices?

We propose a new approach to estimate the geometric extrinsic calibration of all the elements of a smart phone or tablet (such as the screen, the front and the back cameras) by using a planar mirror. By moving a smart phone in front of a single static planar mirror, it is possible to establish correspondences between the images and a pattern displayed on the screen, and therefore estimate the geometric relationship between the non-overlapping cameras with respect to the screen location. The newly proposed setup (static mirror, moving smart phone) enables to both improve the state-of-the-art by working in the minimal case of two images, and improve the accuracy when more images are available. We analyze the minimal case for different calibration scenarios and evaluate the proposed approach on several data. We also show an application of this geometric calibration for specular surface reconstruction, by observing the reflection of a known pattern displayed on the screen.