How to get the means OTSU threshold level in openCV?

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The documentation (http://docs.opencv.org/modules/imgproc/doc/miscellaneous_transformations.html?highlight=threshold#threshold) states that the function cv::threshold() returns the computed threshold value.

So you can simply retrieve the value with something like: threshold_value = cv::threshold(img1, img2, 0, 255, CV_THRESH_OTSU);