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Here's one solution. Do a binary inverse threshold so the letters are white and background is dark. Use findContours function with the RETR_EXTERNAL setting to get a contour for each of the 8 letters. Sort these contours by their x value so that they are in order left to right. Do this for 2 different font images so that you have 2 sets of 8 contours. For each contour pair, get their boundingRects. Extract the 2 roi's and resize one if they are not the same size. Then use template matching to compare the two roi's (I used TM_CCOEFF_NORMED). In the match or score image find the max score with minMaxLoc Notice the scores below are much higher for pairs from the same font than for a different font.

    vector<vector<Point>> srcContours;
vector<vector<Point>> src2Contours;
Mat templImg, testImg;
Mat scoreImg;
double maxScore;

Mat templClrImg, testClrImg;
Mat compImg;

findContours(srcImg, srcContours,   RETR_EXTERNAL, CHAIN_APPROX_NONE);
findContours(src2Img, src2Contours, RETR_EXTERNAL, CHAIN_APPROX_NONE);

sort(srcContours.begin(), srcContours.end(), [](const vector<Point>& a, const vector<Point>& b) {
    Point2f actr = ContourFindCenter(a);
    Point2f bctr = ContourFindCenter(b);
    return actr.x < bctr.x;
    });

sort(src2Contours.begin(), src2Contours.end(), [](const vector<Point>& a, const vector<Point>& b) {
    Point2f actr = ContourFindCenter(a);
    Point2f bctr = ContourFindCenter(b);
    return actr.x < bctr.x;
    });

for (int i = 0; i < srcContours.size(); ++i) {
    Rect rect = boundingRect(srcContours[i]);
    Rect rect2 = boundingRect(src2Contours[i]);

    templImg = Mat(srcImg, rect);
    testImg = Mat(src2Img, rect2);
    if (rect != rect2) {
        resize(testImg, testImg, rect.size());
    }

    matchTemplate(testImg, templImg, scoreImg, TM_CCOEFF_NORMED);
    minMaxLoc(scoreImg, 0, &maxScore, 0, 0);
}