Recursive filtering for splines on hexagonal lattices

Hex-splines are a novel family of bivariate splines which are well suited to handle hexagonally sampled data. Similar to classical 1D B-splines, the spline coefficients need to be computed by a prefilter. Unfortunately, the elegant implementation of this prefilter by causal and anti-causal recursive filtering is not applicable for the (non-separable) hex-splines. Therefore, in this paper we introduce a novel approach from the viewpoint of approximation theory. We propose three different recursive filters and optimize their parameters such that a desired order of approximation is obtained. The results for third and fourth order hex-splines are discussed. Although the proposed solutions provide only quasi-interpolation, they tend to be very close to the interpolation prefilter.