Fitting with Convolution of Two Functions


In this tutorial, we will show you how to define a convolution of two functions, and perform a fit of the data with non-evenly spaced X using this fitting function.

If your data is a convolution of Gauss and Exponential functions, you can simply use built-in fitting function GaussMod in Peak Functions category to directly fit your data.

Minimum Origin Version Required: Origin 9.0 SR0

What you will learn

This tutorial will show you how to:

  • Sample a function.
  • Calculate a convolution of two functions.
  • Define constants in the fitting function.
  • Pad zeroes before the convolution.
  • Interpolate the convolution result for non-evenly spaced X.
  • Use a parameter to balance the speed and the precision.
  • Use Y Error Bar as weight.

Example and Steps

Import Data

  1. Open a new workbook.
  2. Click the Import Single ASCII button Import Single ASCII.png to bring up the Open dialog. Browse to \Samples\Curve Fitting folder and select the file ConvData.dat. Note that column A is not evenly spaced. We can use LabTalk diff function to verify it.
  3. Right click on column C, and select Set As: Y Error from the short-cut menu. Highlight column B and C, and select Plot: Symbol: Scatter from the Origin menu. The graph should look like:
Conv Data G1.png

Define Fitting Function

The fitting function is a convolution of two functions. It can be described as follows:


where f(x)=\frac{s}{\pi}\cdot\frac{\tau_Lx_0^2(x_L^2-x_0^2)}{(x-x_{c1})\tau_L((x-x_{c1})^2-x_L^2)^2+((x-x_{c1})^2-x_0^2)^2},


And x_0, x_L, \tau_L, s, y_0, b_1 and b_2 are fitting parameters. w_1, x_{c1}, w_2, x_{c2} and A_2 are constants in the fitting function.

The fitting function can be defined using the Fitting Function Builder tool.

  1. Select Tools: Fitting Function Builder from Origin menu.
  2. In the Fitting Function Builder dialog's Goal page, click Next.
  3. In the Name and Type page, select User Defined from Select or create a Category drop-down list, type convfunc in the Function Name field, and select Origin C in Function Type group. And click Next.
  4. In the Variables and Parameters page, type x0,xL,tL,s,y0,b1,b2 in the Parameters field, w1,xc1,w2,xc2,A2 in the Constants field. Click Next.
  5. In the Origin C Fitting Function page, set initial parameters as follows:
    x0 = 3.1
    xL = 6.3
    tL = 0.4
    s = 0.14
    y0 = 1.95e-3
    b1 = 2.28e-5
    b2 = 0.2

    Click Constants tab, set constants as follows:

    w1 = 1.98005
    xc1 = -0.30372
    w2 = 5.76967
    xc2 = 3.57111
    A2 = 9.47765e-2

    Click the User-Defined Fitting Functions-2.png button on the right of the Function Body edit box and define the fitting function in Code Builder as follows:

    Include header files,

    #include <ONLSF.H>
    #include <fft_utils.h>

    Define the function body

      NLFitContext *pCtxt = Project.GetNLFitContext();
      if ( pCtxt )
        // Vector for the output in each iteration.
        static vector vX, vY;
        static int nSize;
        BOOL bIsNewParamValues = pCtxt->IsNewParamValues();
        // If parameters were updated, we will recalculate the convolution result.
        if ( bIsNewParamValues )
          //Sampling Interval 
          double dx = 0.05;
          vX.Data(-16.0, 16.0, dx);
          nSize = vX.GetSize();
          vector vF, vG, vTerm1, vTerm2, vDenominator, vBase, vAddBase;
          double Numerator = tL * x0^2 * (xL^2 - x0^2);
          vTerm1 = ( (vX - xc1) * tL * ( (vX - xc1)^2 - xL^2 ) )^2;
          vTerm2 = ( (vX - xc1)^2 - x0^2 )^2;
          vDenominator = vTerm1 + vTerm2;
          //Function f(x)
          vF = (s/pi) * Numerator / vDenominator;
          //Function g(x)
          vG =  1/(w1*sqrt(pi/2))*exp(-2*vX^2/w1^2);
          //Pad zeroes at the end of f and g before convolution
          vector vA(2*nSize-1), vB(2*nSize-1);
          vA.SetSubVector( vF );
          vB.SetSubVector( vG );
          //Perform circular convolution
          int iRet = fft_fft_convolution(2*nSize-1, vA, vB);
          //Truncate the beginning and the end
          vA.GetSubVector( vY, floor(nSize/2), nSize + floor(nSize/2)-1 );
          vBase = (b1*vX + y0);
          vAddBase =  b2 * A2/(w2*sqrt(pi/2))*exp( -2*(vX-xc2)^2/w2^2 );
          //Fitted Y
          vY = dx*vY + vBase + vAddBase;
        //Interpolate y from x for the fitting data on the convolution result.
        ocmath_interpolate( &x, &y, 1, vX, vY, nSize );

    Click Compile button to compile the function body. And click the Return to Dialog button.

    Click Evaluate button, and it shows y=0.02165 at x =1. And this indicates the defined fitting function is correct. Click Next.

  6. Click Next. In the Bounds and General Linear Constraints page, set the following bounds:
    0 < x0 < 7
    0 < xL < 10
    0 < tL < 1
    0 <= s <= 5
    0 < b2 <= 3

    Click Finish.

Note: In order to monitor the the fitted parameters, NLFitContext class was introduced in defining fitting function to achieve some key information within the fitter

Fit the Curve

  1. Select Analysis: Fitting: Nonlinear Curve Fit from the Origin menu. In the NLFit dialog, select Settings: Function Selection, in the page select User Defined from the Category drop-down list and convfunc function from the Function drop-down list. Note that Y Error Bar is shown in the active graph, so column C is used as Y weight, and Instrumental weighting method is chosen by default.
  2. Click the Fit button to fit the curve.

Fitting Results

The fitted curve should look like:

Conv Data G2.png

Fitted Parameters are shown as follows:

Parameter Value Standard Error
x0 3.1424 0.07318
xL 6.1297 0.1193
tL 0.42795 0.02972
s 0.14796 0.00423
y0 0.00216 1.76145E-4
b1 4.90363E-5 1.61195E-5
b2 0.07913 0.02855

Note that you can set a smaller value for dx in the fitting function body, the result may be more accurate, but at the same time it may take a longer time for fitting.