Non-rigid face tracking was first popularized in the early to mid-1990s with the advent of Active Shape Models (ASM) by Cootes and Taylor. Since then, a tremendous amount of research has been dedicated to solving the difficult problem of generic face tracking with many improvements over the original method that ASM proposed. The first milestone was the extension of ASM to Active Appearance Models (AAM) in 2001, also by Cootes and Taylor. This approach was later formalized though the principled treatment of image warps by Baker and colleges in the mid-2000s. Another strand of work along these lines was the 3D morphable model (3DMM) by Blanz and Vetter, which like AAM, not only modeled image textures as opposed to profiles along object boundaries as in ASM, but took it one step further by representing the models with a highly dense 3D data learned from laser scans of faces. From the mid- to late 2000s, the focus of research on face tracking shifted away from how the face was parameterized...