Product Of Exponentials (POE) Splines on Lie-Groups: Limitations, Extensions, and Application to SO(3) and SE(3)

Abstract

Existing methods for constructing splines and Bezier curves on a Lie group G involve repeated products of exponentials deduced from local geodesics, w.r.t. a Riemannian metric, or rely on general polynomials. Moreover, each of these local curves is supposed to start at the identity of $G$. Both assumptions may not reflect the actual curve to be interpolated. This paper pursues a different approach to construct splines on $G$. Local curves are expressed as solutions of the Poisson equation on G. Therewith, the local interpolations satisfies the boundary conditions while respecting the geometry of $G$. A $k$th-order approximation of the solutions gives rise to a $k$th-order product of exponential (POE) spline. Algorithms for constructing 3rd- and 4th-order splines are derived from closed form expressions for the approximate solutions. Additionally, spline algorithms are introduced that allow prescribing a vector field the curve must follow at the interpolation points. It is shown that the established algorithms, where $k$th-order POE-splines are constructed by concatenating local curves starting at the identity, cannot exactly reconstruct a $k$th-order motion. To tackle this issue, the formulations are extended by allowing for local curves between arbitrary points, rather than curves emanating from the identity. This gives rise to a global $k$th-order spline with arbitrary initial conditions. Several examples are presented, in particular the shape reconstruction of slender rods modeled as geometrically non-linear Cosserat rods.