%'tps_eval_dxy': calculate the derivatives of thin plate spline (tps) interpolation at a set of points (limited to the 2D case)
%------------------------------------------------------------------------
% function [DMX,DMY] = tps_eval_dxy(dsites,ctrs)
%------------------------------------------------------------------------
% OUTPUT:
% DMX: Mx(N+3) matrix representing the contributions to the X
% derivatives at the M sites from unit sources located at each of the N
% centers, + 3 columns representing the contribution of the linear gradient part.
% DMY: idem for Y derivatives
%
% INPUT:
% dsites: M x s matrix of interpolation site coordinates (s=space dimension=2 here)
% ctrs: N x s matrix of centre coordinates (initial data)
%
% related functions:
% tps_coeff, tps_eval
function [DMX,DMY] = tps_eval_dxy(dsites,ctrs)
%% matrix declarations
[M,s] = size(dsites); [N,s] = size(ctrs);
Dsites=zeros(M,N);
DM = zeros(M,N);
% DMXY = zeros(M,N+1+s);
%% Accumulate sum of squares of coordinate differences
% The ndgrid command produces two MxN matrices:
% Dsites, consisting of N identical columns (each containing
% the d-th coordinate of the M interpolation sites)
% Ctrs, consisting of M identical rows (each containing
% the d-th coordinate of the N centers)
[Dsites,Ctrs] = ndgrid(dsites(:,1),ctrs(:,1));%d coordinates of interpolation points (Dsites) and initial points (Ctrs)
DX=Dsites-Ctrs;% set of x wise distances between sites and centres
[Dsites,Ctrs] = ndgrid(dsites(:,2),ctrs(:,2));%d coordinates of interpolation points (Dsites) and initial points (Ctrs)
DY=Dsites-Ctrs;% set of y wise distances between sites and centres
DM = DX.*DX + DY.*DY;% add d component squared
%% calculate matrix of tps derivatives
DM(DM~=0) = log(DM(DM~=0))+1; %=2 log(r)+1 derivative of the tps r^2 log(r)
DMX=[DX.*DM zeros(M,1) ones(M,1) zeros(M,1)];% effect of mean gradient
DMY=[DY.*DM zeros(M,1) zeros(M,1) ones(M,1)];% effect of mean gradient