Difference in accessing pixel by at and by data OpenCV - c++

Question

Today I observed one interesting thing: if I access image pixel using the function 'at' I received different result then if I access image pixel using image member 'data'.
Does anybody know why it happened?
int main()
{
double sigma = 1.0;
cv::Mat verticalGaussianKernel = getGaussianKernel(7, sigma);
printImg(verticalGaussianKernel);
return 0;
}
void printImg(cv::Mat &img)
{
cout << "---------//------\n";
if (img.empty())
{
cout << "Empty Image\n";
return;
}
for (int i = 0; i < img.size().height; i++)
{
for (int j = 0; j < img.size().width; j++)
{
cout << int(img.data[i * img.size().height + j]) << " " << img.at<double>(i, j) << endl;
}
cout << endl;
}
cout << "---------//------\n";
}
it code gives results:
data-------at
48------0.00443305
63------0.0540056
171-----0.242036
251-----0.39905
10------0.242036
12------0.0540056
84------0.00443305
Firstly I thought that values in data normalizing to 0-255, but the last string refute my guess

Answer 1

Your casting is wrong. The .data member is an uchar*, you're dereferencing it and casting that value (a single uchar) to int thats why you're not getting the correct values.
The proper way to do it would be to cast it to a double* and then dereferencing it. The following code does that.
#include <iostream>
#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;
void printImg(cv::Mat &img) {
cout << "---------//------\n";
if (img.empty()) {
cout << "Empty Image\n";
return;
}
for (int i = 0; i < img.rows; i++) {
for (int j = 0; j < img.cols; j++) {
cout << reinterpret_cast<double *>(img.data)[i * img.cols + j]
<< " " << img.at<double>(i, j) << endl;
}
cout << endl;
}
cout << "---------//------\n";
}
int main() {
double sigma = 1.0;
cv::Mat verticalGaussianKernel = getGaussianKernel(7, sigma);
cout << verticalGaussianKernel << endl;
printImg(verticalGaussianKernel);
return 0;
}
Output:
[0.004433048175243745;
0.05400558262241448;
0.2420362293761143;
0.3990502796524549;
0.2420362293761143;
0.05400558262241448;
0.004433048175243745]
---------//------
0.00443305 0.00443305
0.0540056 0.0540056
0.242036 0.242036
0.39905 0.39905
0.242036 0.242036
0.0540056 0.0540056
0.00443305 0.00443305

Answer 2

You are reading the data as a char . Instead, read it as a double
int main()
{
double sigma = 1.0;
cv::Mat verticalGaussianKernel = getGaussianKernel(7, sigma);
printImg(verticalGaussianKernel);
return 0;
}
void printImg(cv::Mat &img)
{
cout << "---------//------\n";
if (img.empty())
{
cout << "Empty Image\n";
return;
}
for (int i = 0; i < img.size().height; i++)
{
for (int j = 0; j < img.size().width; j++)
{
cout << double(img.data[i * img.size().height*sizeof(double) + j*sizeof(double)]) << " " << img.at<double>(i, j) << endl;
}
cout << endl;
}
cout << "---------//------\n";
}