How to use boost::uniform_on_sphere?

Question

I am trying to pick a random point on a unit sphere, and found that boost provides the distribution that does exactly this. But when I try to use it, all generated values are nan. I don't know what I am doing wrong, could you please enlighten me? This small code depicts what I am trying to do:

#include <iostream>
#include <fstream>
#include <vector>
#include <boost/random/mersenne_twister.hpp>
#include <boost/random/uniform_on_sphere.hpp>

int main()
{
  boost::mt19937 gen;
  boost::uniform_on_sphere<float> dist(3);

  { // Seed from system random device
    std::ifstream rand_device("/dev/urandom");
    uint32_t seed;
    rand_device >> seed;

    gen.seed(seed);
  }

  while(1) {
    // Generate a point on a unit sphere
    std::vector<float> res = dist(gen);

    // Print the coordinates
    for(int i = 0; i < res.size(); ++i) {
      std::cout << res[i] << ' ';
    }
    std::cout << '\n';
  }
}

The output is:

nan nan nan 
nan nan nan 
nan nan nan 
nan nan nan

ad infinitum...

Code works fine for me (using boost 1.48 and of course without the infinite loop). I think its a bug in your version of boost. — Jesse Good, Apr 18 '12 at 04:00

ely · Accepted Answer · 2012-04-18T04:48:56.150

I'm assuming you are using Boost 1.49. The method below works in that case.

This is where Boost is a little complicated. The object that is uniform_on_sphere is just one component of what you need in order to draw points, it is the distribution part.. You also need to create a boost::variate_generator along the lines of the following working .cpp file below.

Note, this has my own includes for using the system time to seed, etc. You should be able to modify this to suit you.

// Standards, w/ctime to seed based on system time.
#include <iostream>
#include <fstream>
#include <vector>
#include <ctime>

// Boost. Requires variate_generator to actually draw the values.
#include <boost/random/mersenne_twister.hpp>
#include <boost/random/uniform_on_sphere.hpp>
#include <boost/random/variate_generator.hpp>


int main(){

    typedef boost::random::mt19937 gen_type;

    // You can seed this your way as well, but this is my quick and dirty way.
    gen_type rand_gen;
    rand_gen.seed(static_cast<unsigned int>(std::time(0)));

    // Create the distribution object.
    boost::uniform_on_sphere<float> unif_sphere(3);

    // This is what will actually supply drawn values.
    boost::variate_generator<gen_type&, boost::uniform_on_sphere<float> >    random_on_sphere(rand_gen, unif_sphere);

    // Now you can draw a vector of drawn coordinates as such:
    std::vector<float> random_sphere_point = random_on_sphere();

    // Print out the drawn sphere triple's values.
    for(int i = 0; i < random_sphere_point.size(); ++i) {
        std::cout << random_sphere_point.at(i) << ' ';
    }
    std::cout << '\n';

    return 0;
}

When I run this in the console, I get seemingly correct stuff:

ely@AMDESK:~/Desktop/Programming/C++/RandOnSphere$ g++ -I /usr/include/boost_1_49/ -L    /usr/include/boost_1_49/stage/lib randomOnSphere.cpp -o ros
ely@AMDESK:~/Desktop/Programming/C++/RandOnSphere$ ./ros 
0.876326 -0.0441729 0.479689 
ely@AMDESK:~/Desktop/Programming/C++/RandOnSphere$ ./ros 
-0.039037 -0.17792 0.98327 
ely@AMDESK:~/Desktop/Programming/C++/RandOnSphere$ ./ros 
-0.896734 0.419589 -0.14076

N/m I see. In my top code section I did have an errant (now edited). It's the code at the bottom that works, though. Thanks for the tip off. — ely, Apr 18 '12 at 04:23
And where in the documentation does it specifies what distribution can be used directly and what needs this variate_generator? All the samples in the tutorial uses the distribution directly with the generator. — lvella, Apr 18 '12 at 04:48
[Here is a doc page](http://www.boost.org/doc/libs/1_46_1/doc/html/boost_random/tutorial.html) showing use of `variate_generator`.. I've only ever seen this way of doing it, so I can't really comment on the difference or if it is important. — ely, Apr 18 '12 at 04:53

How to use boost::uniform_on_sphere?

1 Answers1