Shape analysis of planar PH curves with the Gauss–Legendre control polygons

•We propose a topological criterion to select the best PH curves with a planar Gauss-Legendre control polygon.•We classify planar PH curves of degree 2n+1 into 2n subclasses by defining the types of PH curves.•By exploiting the types, we present a lower bound of the topological index of a PH curve.•We prove the uniqueness of the best PH curve with a given Gauss-Legendre control polygon.•We also show the existence of the best PH curves for cubic and quintic PH curves. Kim and Moon (2017) have recently proposed rectifying control polygons as an alternative to Bézier control polygons and a way of controlling planar PH curves by the rectifying control polygons. While a Bézier control polygon determines a unique polynomial curve, a rectifying control polygon gives a multitude of PH curves. This multiplicity of PH curves naturally raises the selection problem of the “best” PH curves, which is the main topic of this paper. To resolve the problem, we first classify PH curves of degree 2n+1 into 2n subclasses by defining the types of PH curves, and propose the absolute hodograph winding number as a topological index to characterize the topological behavior of PH curves in shape. We present a lower bound of the topological index of a PH curve which is given solely by its type, and prove the uniqueness of the best PH curve by exploiting it. The existence theorems are also proved for cubic and quintic PH curves. Finally, we propose a selection rule of the best PH curve only based on its type.