Pixel values are partially readable - c++

Question

I took an image and I wanted to write the image as its corresponding pixel values and I have done the code and it compiled but the problem is that, I stored those values in a txt file and I copied those values to an excel sheet and what I see is that the pixel values that I got are only for half of the picture i.e when I see it as a whole picture the pixels show me the half the picture or less I dont know, but it isnt showing me the complete picture.
Help me.
#include <opencv2/opencv.hpp>
using namespace cv;
#include <fstream>
using namespace std;
int main()
{
Mat colorImage = imread("/home/bmit/display_image/CIRCLE.jpg");
// Open the file in write mode.
ofstream outputFile("name.txt");
// Iterate through pixels.
int r, c;
for (r = 1; colorImage.rows > r ;r++)
{
for (c = 1; colorImage.cols > c ; c++)
{
int pixel = colorImage.at<uchar>(r,c);
outputFile << pixel << '\t';
}
outputFile << endl;
}
// Close the file.
outputFile.close();
return 0;
}

Answer 1

There are a number of errors which I think are adding up to create this issue. The first is that for a colour image, there isn't really a single "pixel value" - there is a red component, a green component, a blue component (and possibly an alpha channel as well). I'm going to assume from this point onwards that you actually want the BGR values for each pixel sequentially.
cv::Mat is generally a wrapper around a pointer to a large singular block of continuous memory (it isn't always continuous but usually is). The Mat.at<typename>() method is one of the ways of accessing this data, using the typename to interpret it and cast the data accessed.
The issue you are having is the total information stored in this matrix is more than row*cols of uchars. The matrix is storing row*cols*3 trios of blue,green, and red uchars. The line of code int pixel = colorImage.at<uchar>(r,c); is accessing some point in this data sequence based on the size of a uchar, the number of rows in the image, and the values of r & c.
For instance, at some point in the innerloop you will call int pixel = colorImage.at<uchar>(r,c); when r is equal to the number of rows and c equal to the number of columns. You want this value of pixel to be the "pixel value" of the lower right pixel, but what you are actually getting is the value of one of the channel values for a pixel about a 3rd of the width along and a 3rd of the height down the image.
To fix this you have a number of options. I think you'll find reading some of the tutorials on the OpenCV website (this one probably being the most relevant) useful. But if you replace the loop in your code with the following it should work, although I haven't tested it.
for (r=0;r<colorImage.rows; r++)
{
for (r=0; c<colorImage.cols; c++)
{
Point3_<uchar> pixel = colorImage.at<Point3_<uchar>>(r,c);
outputFile << pixel.x << '\t'<< pixel.y << '\t'<< pixel.z << '\t';
}
outputFile << endl;
}
Note that this will be in order BGR, if you require RGB just swap the order of pixel.x & pixel.z