  Due to the scarcity of annotated scene flow data, self-supervised scene flow
learning in point clouds has attracted increasing attention. In the
self-supervised manner, establishing correspondences between two point clouds
to approximate scene flow is an effective approach. Previous methods often
obtain correspondences by applying point-wise matching that only takes the
distance on 3D point coordinates into account, introducing two critical issues:
(1) it overlooks other discriminative measures, such as color and surface
normal, which often bring fruitful clues for accurate matching; and (2) it
often generates sub-par performance, as the matching is operated in an
unconstrained situation, where multiple points can be ended up with the same
corresponding point. To address the issues, we formulate this matching task as
an optimal transport problem. The output optimal assignment matrix can be
utilized to guide the generation of pseudo ground truth. In this optimal
transport, we design the transport cost by considering multiple descriptors and
encourage one-to-one matching by mass equality constraints. Also, constructing
a graph on the points, a random walk module is introduced to encourage the
local consistency of the pseudo labels. Comprehensive experiments on
FlyingThings3D and KITTI show that our method achieves state-of-the-art
performance among self-supervised learning methods. Our self-supervised method
even performs on par with some supervised learning approaches, although we do
not need any ground truth flow for training.
