natcub.m

来自「改进等高线拟合代码」· M 代码 · 共 35 行

% NATCUB   : Determination of values for the first derivatives        [f]
%
% [B] = NATCUB (X,Y)  
%
% Returns in B the estimates for the first derivatives for a
% spline fit through the points given in X and Y with a continuous
% second derivative.


function [b]=natcub(x,y)


n= length(x);
if n <3,
  error('Number of points < 3.');
  return
end

gamma(1)=0.5;
for i = 2:n-1
   gamma(i) = 1/(4-gamma(i-1));
end
gamma(n) = 1/(2-gamma(n-1));

delta(1)=3*(y(2)-y(1))*gamma(1);
for i = 2:n-1
   delta(i) = (3*(y(i+1)-y(i-1))-delta(i-1))*gamma(i);
end
delta(n) = (3*(y(n)-y(n-1))-delta(n-1))*gamma(n);

b(n)=delta(n);
for i = n-1:-1:1
   b(i) = delta(i)-gamma(i)*b(i+1);
end