splines, polyfit and polyval, wrong coefficients?!?

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David 2014-7-9

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https://ww2.mathworks.cn/matlabcentral/answers/141069-splines-polyfit-and-polyval-wrong-coefficients

编辑： David 2014-7-9

Good morning all together,

I have a question concerning the spline function and it´s coefficients (MATLAB R2012b). I have created a function out of two clothoids which looks like this.

Now I want to evaluate this function piecewise by fitting a cubic spline. Afterwards I took a look at the coefficients and tried to find the piecewise polynomial function:

y = spline(xges,yges);
xev = linspace (0,1,10);
hold on
for i =1:length(y.coefs)
      br = y.breaks(i:i+1); 
      xxp(i,:) = linspace(br(1),br(2),10);
      cf = y.coefs(i,:);
      yp(i,:) = polyval(cf,xev);
%     ypp (i,:) = ppval(y,xxp(i,:));
     plot(xxp(i,:),yp(i,:),'.g') ;
end

But it doesn´t fit the function. The piecewise polynomial functions are wrong. So are also the coefficients of the cubic spline wrong? Or am I doing some mistake by evaluating the piecewise polynomial functions? It´s also a little bit weird that the spline looks perfect, when I´m not using the polyval, but the ppval function. (Blue line is with ppval, green lines with polyval)

I hope you can help me