  Recent works in implicit representations, such as Neural Radiance Fields
(NeRF), have advanced the generation of realistic and animatable head avatars
from video sequences. These implicit methods are still confronted by visual
artifacts and jitters, since the lack of explicit geometric constraints poses a
fundamental challenge in accurately modeling complex facial deformations. In
this paper, we introduce Dynamic Tetrahedra (DynTet), a novel hybrid
representation that encodes explicit dynamic meshes by neural networks to
ensure geometric consistency across various motions and viewpoints. DynTet is
parameterized by the coordinate-based networks which learn signed distance,
deformation, and material texture, anchoring the training data into a
predefined tetrahedra grid. Leveraging Marching Tetrahedra, DynTet efficiently
decodes textured meshes with a consistent topology, enabling fast rendering
through a differentiable rasterizer and supervision via a pixel loss. To
enhance training efficiency, we incorporate classical 3D Morphable Models to
facilitate geometry learning and define a canonical space for simplifying
texture learning. These advantages are readily achievable owing to the
effective geometric representation employed in DynTet. Compared with prior
works, DynTet demonstrates significant improvements in fidelity, lip
synchronization, and real-time performance according to various metrics. Beyond
producing stable and visually appealing synthesis videos, our method also
outputs the dynamic meshes which is promising to enable many emerging
applications.
