cv::meanStdDev

cv::cvtColor(matROI_Brightness2, matROI_Brightness2, COLOR_BGR2GRAY);
scalar = cv::mean(matROI_Brightness2);
Mat tmp_m, tmp_sd;
cv::meanStdDev(matROI_Brightness2, tmp_m, tmp_sd);
double tmpCountMinVal = 0, tmpCountMaxVal = 0;
minMaxLoc(matROI_Brightness2, &tmpCountMinVal, &tmpCountMaxVal, NULL, NULL);
//avg = scalar.val[0];
double sd = 0;
avg = tmp_m.at<double>(0, 0);
sd = tmp_sd.at<double>(0, 0);