How to create a Gaussian kernel of arbitrary width? - c++

Question

How to create a Gaussian kernel by only specifying its width w (3,5,7,9...), and without specifying its variance sigma?
In other word, how to adapt sigma so that the Gaussian distribution 'fits well' w?
I would be interested in a C++ implementation:
void create_gaussian_kernel(int w, std::vector<std::vector<float>>& kernel)
{
kernel = std::vector<std::vector<float>>(w, std::vector<float>(w, 0.f)); // 2D array of size w x w
const Scalar sigma = 1.0; // how to adapt sigma to w ???
const int hw = (w-1)/2; // half width
for(int di = -hw; di <= +hw; ++di)
{
const int i = hw + di;
for(int dj = -hw; dj <= +hw; ++dj)
{
const int j = hw + dj;
kernel[i][j] = gauss2D(di, dj, sigma);
}
}
}
Everything I see on the Internet use a fixed size w and a fixed variance sigma :
geeksforgeeks.org/gaussian-filter-generation-c/
tutorialspoint.com/gaussian-filter-generation-in-cplusplus
stackoverflow.com/a/8204880/5317819
stackoverflow.com/q/42186498/5317819
stackoverflow.com/a/54615770/5317819

Answer 1

I found a simple (arbitrary) relation between sigma and w.
I want the next value outside the kernel (along one axis) below a very small value epsilon:
exp( - (half_width + 1)^2 / (2 * sigma^2) ) < epsilon
with half_width the kernel 'half width'.
The result is
sigma^2 = - (half_width + 1)^2 / (2 * log(epsilon))
I use the following c++ code:
#include <vector>
#include <cmath>
#include <cassert>
using Matrix = std::vector<std::vector<float>>;
// compute sigma^2 that 'fit' the kernel half width
float compute_squared_variance(int half_width, float epsilon = 0.001)
{
assert(0 < epsilon && epsilon < 1); // small value required
return - (half_width + 1.0) * (half_width + 1.0) / 2.0 / std::log(epsilon);
}
float gaussian_exp(float y, float x, float sigma2)
{
assert(0 < sigma2);
return std::exp( - (x*x + y*y) / (2 * sigma2) );
}
// create a Gaussian kernel of size 2*half_width+1 x 2*half_width+1
Matrix make_gaussian_kernel(int half_width)
{
if(half_width <= 0)
{
// kernel of size 1 x 1
Matrix kernel(1, std::vector<float>(1, 1.0));
return kernel;
}
Matrix kernel(2*half_width+1, std::vector<float>(2*half_width+1, 0.0));
const float sigma2 = compute_squared_variance(half_width, 0.1);
float sum = 0;
for(int di = -half_width; di <= +half_width; ++di)
{
const int i = half_width + di;
for(int dj = -half_width; dj <= +half_width; ++dj)
{
const int j = half_width + dj;
kernel[i][j] = gaussian_exp(di, dj, sigma2);
sum += kernel[i][j];
}
}
assert(0 < sum);
// normalize
for(int i=0; i<2*half_width+1; ++i)
{
for(int j=0; j<2*half_width+1; ++j)
{
kernel[i][j] /= sum;
}
}
return kernel;
}