Traditional 3D morphable face models (3DMMs) provide fine-grained control
over expression but cannot easily capture geometric and appearance details.
Neural volumetric representations approach photorealism but are hard to animate
and do not generalize well to unseen expressions. To tackle this problem, we
propose IMavatar (Implicit Morphable avatar), a novel method for learning
implicit head avatars from monocular videos. Inspired by the fine-grained
control mechanisms afforded by conventional 3DMMs, we represent the expression-
and pose- related deformations via learned blendshapes and skinning fields.
These attributes are pose-independent and can be used to morph the canonical
geometry and texture fields given novel expression and pose parameters. We
employ ray marching and iterative root-finding to locate the canonical surface
intersection for each pixel. A key contribution is our novel analytical
gradient formulation that enables end-to-end training of IMavatars from videos.
We show quantitatively and qualitatively that our method improves geometry and
covers a more complete expression space compared to state-of-the-art methods.

提出了一种名为 IMavatar 的新方法，可以从单目视频中生成隐式头像。通过学习混合形状和蒙皮场来表示表情和姿势相关的变形，使得这些属性可以用于完成新表情和姿态下的几何和纹理场的变形，并通过光线行进和迭代根查找，为每个像素定位规范表面交点，从而实现了头像的生成，并通过端到端的训练，得出了比现有技术更完整的表情空间和几何形状。