source: sasview/sansmodels/src/sans/models/c_extensions/fuzzysphere.c @ 18695bf

/**
 * Scattering model for a fuzzy sphere
 */

#include <math.h>
#include "libSphere.h"
#include "fuzzysphere.h"
#include <stdio.h>
#include <stdlib.h>

// scattering from a uniform sphere w/ fuzzy surface
// Modified from FuzzySpheres in libigor/libSphere.c without polydispersion: JHC
double fuzzysphere_kernel(double dp[], double q){
        double pi,x,xr;
        double radius,sldSph,sldSolv,scale,bkg,delrho,fuzziness,f2,bes,vol,f;           //my local names

        pi = 4.0*atan(1.0);
        x= q;
        scale = dp[0];
        radius = dp[1];
        fuzziness = dp[2];
        sldSph = dp[3];
        sldSolv = dp[4];
        bkg = dp[5];
        delrho=sldSph-sldSolv;

        xr = x*radius;
        //handle xr==0 separately
        if(xr == 0.0){
                bes = 1.0;
        }else{
                bes = 3.0*(sin(xr)-xr*cos(xr))/(xr*xr*xr);
        }
        vol = 4.0*pi/3.0*radius*radius*radius;
        f = vol*bes*delrho;             // [=] A
        f *= exp(-0.5*fuzziness*fuzziness*x*x);
        // normalize to single particle volume, convert to 1/cm
        f2 = f * f / vol * 1.0e8;               // [=] 1/cm

        f2 *= scale;
        f2 += bkg;

    return(f2); //scale, and add in the background
}


/**
 * Function to evaluate 1D scattering function
 * @param pars: parameters of the fuzzysphere
 * @param q: q-value
 * @return: function value
 */
double fuzzysphere_analytical_1D(FuzzySphereParameters *pars, double q) {
        double dp[6];

        dp[0] = pars->scale;
        dp[1] = pars->radius;
        dp[2] = pars->fuzziness;
        dp[3] = pars->sldSph;
        dp[4] = pars->sldSolv;
        dp[5] = pars->background;

        return fuzzysphere_kernel(dp, q);
}

/**
 * Function to evaluate 2D scattering function
 * @param pars: parameters of the fuzzysphere
 * @param q: q-value
 * @return: function value
 */
double fuzzysphere_analytical_2D(FuzzySphereParameters *pars, double q, double phi) {
        return fuzzysphere_analytical_1D(pars,q);
}

double sphere_analytical_2DXY(FuzzySphereParameters *pars, double qx, double qy){
        return fuzzysphere_analytical_1D(pars,sqrt(qx*qx+qy*qy));
}