CLIPPER+: A Fast Maximal Clique Algorithm for Robust Global Registration
Kaveh Fathian Tyler Summers
We present CLIPPER+, an algorithm for finding maximal cliques in unweighted
graphs for outlier-robust global registration. The registration problem can be
formulated as a graph and solved by finding its maximum clique. This
formulation leads to extreme robustness to outliers; however, finding the
maximum clique is an NP-hard problem, and therefore approximation is required
in practice for large-size problems. The performance of an approximation
algorithm is evaluated by its computational complexity (the lower the runtime,
the better) and solution accuracy (how close the solution is to the maximum
clique). Accordingly, the main contribution of CLIPPER+ is outperforming the
state-of-the-art in accuracy while maintaining a relatively low runtime.
CLIPPER+ builds on prior work (CLIPPER [1] and PMC [2]) and prunes the graph by
removing vertices that have a small core number and cannot be a part of the
maximum clique. This will result in a smaller graph, on which the maximum
clique can be estimated considerably faster. We evaluate the performance of
CLIPPER+ on standard graph benchmarks, as well as synthetic and real-world
point cloud registration problems. These evaluations demonstrate that CLIPPER+
has the highest accuracy and can register point clouds in scenarios where over
$99\%$ of associations are outliers. Our code and evaluation benchmarks are
released at https://github.com/ariarobotics/clipperp.