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lammps/lib/voro++/examples/extra/cut_region.cc
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// Cell cutting region example code
//
// Author : Chris H. Rycroft (LBL / UC Berkeley)
// Email : chr@alum.mit.edu
// Date : August 30th 2011
#include "voro++.hh"
using namespace voro;
const double pi=3.1415926535897932384626433832795;
// This constant sets the tolerance in the bisection search algorithm
const double tolwidth=1e-7;
// This constant determines the density of points to test
const double theta_step=pi/200;
int main() {
double x,y,z,r,rmin,rmax;
double theta,phi,phi_step;
voronoicell v;
FILE *fp=safe_fopen("cell_cut_region.gnu","w");
// Initialize the Voronoi cell to be an octahedron and make a single
// plane cut to add some variation
v.init_octahedron(1);
v.plane(0.4,0.3,1,0.1);
// Output the cell in gnuplot format
v.draw_gnuplot(0,0,0,"cell.gnu");
// Now test over direction vectors from the center of the sphere. For
// each vector, carry out a search to find the maximum distance along
// that vector that a plane will intersect with cell, and save it to
// the output file.
for(theta=theta_step*0.5;theta<pi;theta+=theta_step) {
phi_step=2*pi/(int(2*pi*sin(theta)/theta_step)+1);
for(phi=phi_step*0.5;phi<2*pi;phi+=phi_step) {
// Calculate a direction to look along
x=sin(theta)*cos(phi);
y=sin(theta)*sin(phi);
z=cos(theta);
// Now carry out a bisection search. Here, we initialize
// a minimum and a maximum guess for the distance
// along this vector. Keep multiplying rmax by two until
// the plane no longer makes a cut.
rmin=0;rmax=1;
while (v.plane_intersects(x,y,z,rmax)) rmax*=2;
// We now know that the distance is somewhere between
// rmin and rmax. Test the point halfway in-between
// these two. If it intersects, then move rmin to this
// point; otherwise, move rmax there. At each stage the
// bounding interval is divided by two. Exit when the
// width of the interval is smaller than the tolerance.
while (rmax-rmin>tolwidth) {
r=(rmax+rmin)*0.5;
if (v.plane_intersects(x,y,z,r)) rmin=r;
else rmax=r;
}
// Output this point to file
r=(rmax+rmin)*0.5;
x*=r;y*=r;z*=r;
fprintf(fp,"%g %g %g\n",x,y,z);
}
}
fclose(fp);
}
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