How to get pointer of cv::Mat.data?

Question

I read an image. Then, I would like to access 39th element of the image.

cv::Mat img = cv::imread( "lena.png",cv::IMREAD_GRAYSCALE);
float * ptr = (float*)refImage.data;

printf(" %d \n", refImage.data[39]);
printf(" %f \n", *(ptr + 39));

Output is shown below:

110 
274415818481933370785792.000000 

I've read that we can access using float pointer. Can you tell me what is wrong?

Answer 1

I found out that it is not always float pointer:

According to the matrix type, the pointer will have a different template.

For CV_8UC1: uchar* ptr = image.ptr(r);
For CV_8UC3: cv::Vec3b* ptr = image.ptr<cv::Vec3b>(r);
For CV_32FC1: float* ptr = image.ptr(r);
For CV_32FC3: cv::Vec3f* ptr = image.ptr<cv::Vec3f>(r);

You can read more.