1. A method comprising: capturing a series of images with depth data of a head of a subject; obtaining a reference pose for the head of the subject from one image in the series of images and using the reference pose to define a reference coordinate frame for the series of images; determining a rotation matrix and a translation vector associated with a pose of the head in each image in the series of images relative to the reference coordinate frame using the depth data; extracting a face on the head from a background in the series of images using the depth data, wherein extracting the face from the background comprises: calculating a depth of the face in each image using the depth data; and segmenting out the face from the background using a threshold and the depth of the face; and tracking the face in the series of images after extracting the face from the background.