# Problem with meanStdDev

I seem to be having a problem trying to compute the mean and standard deviation of pixels in an image. Can someone look at my code and point out what I am doing wrong? I am unable to get correct results.

```
cv::Mat img;
cv::Mat mean;
cv::Mat stddev;
cv::meanStdDev ( img, mean, stddev );
uchar mean_pxl = mean.data[0];
uchar stddev_pxl = stddev.data[0];
```

I have also tried to get the data using `mean.at<uchar>(0)`

without success.

Thanks for your help.