It was good here, but Google Code is shutting down

From 24 August 2015

nn is hosted at GitHub

nn is a C code for Natural Neighbours interpolation of 2D scattered data. It provides a C library and a command line utility nnbathy.

Algorithmically, it was initially loosely based on the Dave Watson's description of nngridr; code-wise it is an independent development. You may see a comparison of performance of a (rather old) version of nn with nngridr in

Quanfu Fan, Alon Efrat, Vladlen Koltun, Shankar Krishnan, and Suresh Venkatasubramanian. Hardware-assisted Natural Neighbor Interpolation. In Proc. 7th Workshop on Algorithm Engineering and Experiments (ALENEX), 2005. pdf

nn is coded for robustness (to handle degenerate data) and scalability (to handle millions of data points), subject to using double precision calculations. For the underlying Delaunay triangulation it calls exact arithmetic code from triangle.

Checkout nn by running "svn checkout https://nn-c.googlecode.com/svn/nn"

You may also be interested in cubic spline approximation code csa.

README