# SasView/sasmodels

Fetching contributors…
Cannot retrieve contributors at this time
58 lines (54 sloc) 1.9 KB
 /** * Spherical Bessel function 3*j1(x)/x * * Used for low q to avoid cancellation error. * Note that the values differ from sasview ~ 5e-12 rather than 5e-14, but * in this case it is likely cancellation errors in the original expression * using double precision that are the source. */ double sas_3j1x_x(double q); // The choice of the number of terms in the series and the cutoff value for // switching between series and direct calculation depends on the numeric // precision. // // Point where direct calculation reaches machine precision: // // single machine precision eps 3e-8 at qr=1.1 ** // double machine precision eps 4e-16 at qr=1.1 // // Point where Taylor series reaches machine precision (eps), where taylor // series matches direct calculation (cross) and the error at that point: // // prec n eps cross error // single 3 0.28 0.4 6.2e-7 // single 4 0.68 0.7 2.3e-7 // single 5 1.18 1.2 7.5e-8 // double 3 0.01 0.03 2.3e-13 // double 4 0.06 0.1 3.1e-14 // double 5 0.16 0.2 5.0e-15 // // ** Note: relative error on single precision starts increase on the direct // method at qr=1.1, rising from 3e-8 to 5e-5 by qr=1e3. This should be // safe for the sans range, with objects of 100 nm supported to a q of 0.1 // while maintaining 5 digits of precision. For usans/sesans, the objects // are larger but the q is smaller, so again it should be fine. // // See explore/sph_j1c.py for code to explore these ranges. // Use 4th order series #if FLOAT_SIZE>4 #define SPH_J1C_CUTOFF 0.1 #else #define SPH_J1C_CUTOFF 0.7 #endif double sas_3j1x_x(double q) { // 2017-05-18 PAK - support negative q if (fabs(q) < SPH_J1C_CUTOFF) { const double q2 = q*q; return (1.0 + q2*(-3./30. + q2*(3./840. + q2*(-3./45360.))));// + q2*(3./3991680.))))); } else { double sin_q, cos_q; SINCOS(q, sin_q, cos_q); return 3.0*(sin_q/q - cos_q)/(q*q); } }