How to improve thresholding

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Hello,

I have an image and I would like apply threshold operation and detect the coin in my image. Right now I'm using Otsu's method, but even with different thresholding values and smoothing types, I couldn't get any clean output.

Is there any way that I can improve this outcome?

#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"

using namespace std;
using namespace cv;

int main(int argc, char** argv)
{
    namedWindow("Threshold", CV_WINDOW_AUTOSIZE);

    /// Load the source image
    Mat src;
    src = imread("1cent.png", 1); 

    Mat src_gray;
    cvtColor(src, src_gray, CV_RGB2GRAY);

    Mat smooth;
    for (int i = 1; i < 32; i = i + 2)
    {
        GaussianBlur(src_gray, smooth, Size(i, i), 0, 0);
    }   

    Mat thresh;
    threshold(smooth, thresh, 0, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);

    imshow("Threshold", thresh);

    waitKey(0);
    return 0;
}

Source image:

Threshold output: