1. A method implemented by one or more computing devices, the method comprising: obtaining an input frame of a user's face; fitting two-dimensional feature points to the user's face in the input frame; generating a batch of frames that includes the input frame; constructing an energy function that includes the two-dimensional feature points and other two-dimensional feature points, three-dimensional head shape parameters, three-dimensional head pose parameters, and three-dimensional facial expression parameters for individual frames of the batch of frames; solving the energy function to obtain a three-dimensional head shape parameter update; and determining a three-dimensional shape of the user's face using the three-dimensional head shape parameter update.