how to get the coeffients out of a sfit object after creating it on some data using 'cubicinterp'.

Hi, i want to fit some 3d data, and generate a mathematical description of the surface using the curve fitting toolbox. I used "cubicinterp" as the interpolation method and get the following output:

>> test = fit([X_cart,Y_cart],Z_cart,'cubicinterp','Exclude', nanz) Warning: Duplicate x-y data points detected: using average of the z values. > In curvefit.attention.Warning/throw (line 30) In curvefit.model.SurfaceInterpolantFactory/doCoalesceDuplicatePoints (line 206) In curvefit.model.SurfaceInterpolantFactory/create (line 74) In fit>iSurfaceInterpolation (line 895) In fit>iFit (line 489) In fit (line 108)

     Piecewise cubic interpolant:
       test(x,y) = piecewise cubic surface computed from p
     Coefficients:
       p = coefficient structure

I have no idea how to extract the coefficients or any kind of mathematical description out of the sfit object "test" which is generated.

Any ideas?

Thanks for any help.

Simon

Answer 1

[Mukul Rao]

Hi,

Here is an example that demonstrates how to extract fit coefficients from the "sfit" object:

coeffvalues

[Simon Freutel]

Hi,

thanks, but I guess this only works for cfit objects. Not for sfit?!

Answer 2

[haochen he]

I think we do not have access to the model parameters:

[fitresult, gof] = fit( [xData, yData], zData, 'cubicinterp', 'Normalize', 'on' );

names = coeffnames(fitresult); % result is: names = {'p'}
% if you try other properties like 'fitresult.others', an error will goes like:
% For the current fit, you can access these properties: p

% then we try this:
names = coeffnames(fitresult.p);
% result will be an error, if we see the fitresult.p, we can only find the property is Type='Cubic'

[Antonio Leanza]

names = fitresult.p.coefs works

[Antonio Machado Jorge]

It doesn't work for me... I'm on 2023b

>> fitresult.p

ans = 

  GriddataInterpolant with properties:

    Type: 'Cubic'
   
    
>> names = fitresult.p.coefs
Unrecognized method, property, or field
'coefs' for class
'curvefit.model.GriddataInterpolant'.

Error in indexing (line 26)
    out = subsref(out, subs);