Debug assert error caused by normal distribution sigma equals 0.0 - MSVC 14.40

[robbietuk]

Building a cmake project ontop of STIR with CMake (because of #1483).

I am using the example https://github.com/UCL/STIR/blob/master/examples/C%2B%2B/using_installed_STIR/demo_create_image.cxx but building in Debug.

Modifying the program to:

#include <iostream>
#include "stir/Scanner.h"
using namespace std;

int main()
{
	cout << "Here are all the scanner names:\n" << stir::Scanner::list_all_names() << endl;
	return 0;
}

I get an error in C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.40.33807\include\random, specifically l:3371

        void _Init(_Ty _Mean0, _Ty _Sigma0) noexcept { // set internal state
            _STL_ASSERT(0.0 < _Sigma0, "invalid sigma argument for normal_distribution");
            _Mean  = _Mean0;
            _Sigma = _Sigma0;
        }

Where the _STL_ASSERT is throwing an exception because 0.0 == _Sigma0. This stems from

distributions[0] = std::normal_distribution<double>(0.0, sigma);

in

STIR/src/include/stir/DetectorCoordinateMap.h

Lines 122 to 136 in 8c57bc9

	protected:
	explicit DetectorCoordinateMap(double sigma = 0.0)
	: sigma(sigma)
	{
	# ifdef STIR_OPENMP
	generators.resize(omp_get_max_threads());
	distributions.resize(omp_get_max_threads());
	for (auto& distribution : distributions)
	distribution = std::normal_distribution<double>(0.0, sigma);
	# else
	generators.resize(1);
	distributions.resize(1);
	distributions[0] = std::normal_distribution<double>(0.0, sigma);
	# endif
	}

My guess is this hasn't been caught because CI only tests Release on MSVC. This is the only issue I have found so far.

---

What is the fix?

I am not sure what the logic behind the use of sigma=0.0 to define distributions[0] as a normal distribution with mean=0 and sigma=0 is not a well-defined probability distribution. It essentially collapses to a single point at 0. Likely, the plan was to set distributions[0]=0. Does this seem correct?

[robbietuk]

As far as I can tell, the distributions is providing a way to add noise to the detector coordinates.

STIR/src/include/stir/DetectorCoordinateMap.h

Lines 92 to 108 in 8c57bc9

	stir::CartesianCoordinate3D<float> get_coordinate_for_det_pos(const stir::DetectionPosition<>& det_pos) const
	{
	auto coord = det_pos_to_coord.at(det_pos);
	if (sigma == 0.0)
	return coord;
	# ifdef STIR_OPENMP
	auto thread_id = omp_get_thread_num();
	# else
	auto thread_id = 0;
	# endif
	coord.x() += static_cast<float>(distributions[thread_id](generators[thread_id]));
	coord.y() += static_cast<float>(distributions[thread_id](generators[thread_id]));
	coord.z() += static_cast<float>(distributions[thread_id](generators[thread_id]));
	return coord;
	}

In its usage, there is a if (sigma == 0.0) return; check. I think the best course of action is to add this to the constructor.