  Building animatable 3D models is challenging due to the need for 3D scans,
laborious registration, and manual rigging, which are difficult to scale to
arbitrary categories. Recently, differentiable rendering provides a pathway to
obtain high-quality 3D models from monocular videos, but these are limited to
rigid categories or single instances. We present RAC that builds category 3D
models from monocular videos while disentangling variations over instances and
motion over time. Three key ideas are introduced to solve this problem: (1)
specializing a skeleton to instances via optimization, (2) a method for latent
space regularization that encourages shared structure across a category while
maintaining instance details, and (3) using 3D background models to disentangle
objects from the background. We show that 3D models of humans, cats, and dogs
can be learned from 50-100 internet videos.
