  We propose Hi4D, a method and dataset for the automatic analysis of
physically close human-human interaction under prolonged contact. Robustly
disentangling several in-contact subjects is a challenging task due to
occlusions and complex shapes. Hence, existing multi-view systems typically
fuse 3D surfaces of close subjects into a single, connected mesh. To address
this issue we leverage i) individually fitted neural implicit avatars; ii) an
alternating optimization scheme that refines pose and surface through periods
of close proximity; and iii) thus segment the fused raw scans into individual
instances. From these instances we compile Hi4D dataset of 4D textured scans of
20 subject pairs, 100 sequences, and a total of more than 11K frames. Hi4D
contains rich interaction-centric annotations in 2D and 3D alongside accurately
registered parametric body models. We define varied human pose and shape
estimation tasks on this dataset and provide results from state-of-the-art
methods on these benchmarks.
