source: sasmodels/sasmodels/models/cylinder.c @ 2a0b2b1

#define INVALID(v) (v.radius<0 || v.length<0)

static double
form_volume(double radius, double length)
{
    return M_PI*radius*radius*length;
}

static double
fq(double qab, double qc, double radius, double length)
{
    return sas_2J1x_x(qab*radius) * sas_sinx_x(qc*0.5*length);
}

static double
orient_avg_1D(double q, double radius, double length)
{
    // translate a point in [-1,1] to a point in [0, pi/2]
    const double zm = M_PI_4;
    const double zb = M_PI_4;

    double total = 0.0;
    for (int i=0; i<76 ;i++) {
        const double theta = Gauss76Z[i]*zm + zb;
        double sin_theta, cos_theta; // slots to hold sincos function output
        // theta (theta,phi) the projection of the cylinder on the detector plane
        SINCOS(theta , sin_theta, cos_theta);
        const double form = fq(q*sin_theta, q*cos_theta, radius, length);
        total += Gauss76Wt[i] * form * form * sin_theta;
    }
    // translate dx in [-1,1] to dx in [lower,upper]
    return total*zm;
}

static double
Iq(double q,
    double sld,
    double solvent_sld,
    double radius,
    double length)
{
    const double s = (sld - solvent_sld) * form_volume(radius, length);
    return 1.0e-4 * s * s * orient_avg_1D(q, radius, length);
}

static double
Iqxy(double qx, double qy,
    double sld,
    double solvent_sld,
    double radius,
    double length,
    double theta,
    double phi)
{
    double q, sin_alpha, cos_alpha;
    ORIENT_SYMMETRIC(qx, qy, theta, phi, q, sin_alpha, cos_alpha);
    const double qab = q*sin_alpha;
    const double qc = q*cos_alpha;
    const double s = (sld-solvent_sld) * form_volume(radius, length);
    const double form = fq(qab, qc, radius, length);
    return 1.0e-4 * square(s * form);
}