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fix parametrization of UniformInShell function #379

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fix parametrization of UniformInShell function #379

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fb4312c
fixed parametrization of UniformInShell function
heberlr Jul 24, 2025
6fe2f7b
UniformInShell function simplified using UniformOnUnitSphere
heberlr Jul 24, 2025
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29 changes: 10 additions & 19 deletions core/PhysiCell_rules.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2321,28 +2321,19 @@ std::vector<double> UniformInAnnulus( double r1, double r2 )
return {x,y,0.0};
}

std::vector<double> UniformInShell( double r1, double r2 )
std::vector<double> UniformInShell(double r1, double r2)
{
static double two_pi = 6.283185307179586;
double r1_3 = r1 * r1 * r1;
double r2_3 = r2 * r2 * r2;

double xi = UniformRandom();
double r = pow(r1_3 + xi * (r2_3 - r1_3), 1.0 / 3.0);

double T = UniformRandom();
double sqrt_T = sqrt(T);
double sqrt_one_minus_T = 1.0;
sqrt_one_minus_T -= T;
sqrt_one_minus_T = sqrt( sqrt_one_minus_T );
std::vector<double> dir = UniformOnUnitSphere();
for (int i = 0; i < 3; ++i)
dir[i] *= r;

double param1 = pow( UniformRandom() , 0.33333333333333333333333333333333333333 ); // xi^(1/3),
// param1 *= (r2-r1);
// param1 += r1;
double param2 = param1; // xi^(1/3)
param2 *= 2.0; // 2 * xi^(1/3)
param2 *= sqrt_T; // 2 * xi(1) * T^(1/2)
param2 *= sqrt_one_minus_T; // 2 * xi(1) * T^(1/2) * (1-T)^(1/2)

double theta = UniformRandom(); // U(0,1)
theta *= two_pi; // U(0,2*pi)

return { param2*sin(theta) , param2*cos(theta), param1*(1-2*T) };
return dir;
}

void setup_cell_rules( void )
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