How to fill an array of Vec3b - c++

Question

I am working with images in C++ with OpenCV.
I wrote code with an uchar array of two dimensions where I can read pixel values of an image, uploaded with imread in grayscale using .at< uchar>(i,j).
However I would like to do the same thing for color images. Since I know that to access the pixels values I now need .at< Vec3b>(i,j)[0], .at< Vec3b>(i,j)[1] and .at< Vec3b>(i,j)[2], I made a similar Vec3b 2d arrays.
But I don't know how to fill this array with the pixel values. It has to be a 2D array.
I tried:
array[width][height].val[0]=img.at< Vec3b>(i,j)[0]
but that didn't work.
Didn't find an answer on the OpenCV doc or here neither.
Anybody has an idea?
I've included some of my code. I need an array because I already have my whole algorithm working, using an array, for the images in grayscale with only one channel.
The grayscale code is like that:
for(int i=0;i<height;i++){
for(int j=0;j<width;j++){
image_data[i*width+j]=all_images[nb_image-1].at< uchar>(i,j);
}
}
Where I read from:
std::vector< cv::Mat> all_images
each image (I have a long sequence), retrieves the pixel values in the uchar array image_data, and processes them.
I want now to do the same but for RGB images, and I can't manage to read the data pixel of each channel and put them in an array.
This time image_data is a Vec3b array, and the code I'm trying looks like this:
for(int i=0;i<height;i++){
for(int j=0;j<width;j++){
image_data[0][i*width+j]=all_images[nb_image-1].at<cv::Vec3b>(i,j)[2];
image_data[1][i*width+j]=all_images[nb_image-1].at<cv::Vec3b>(i,j)[1];
image_data[2][i*width+j]=all_images[nb_image-1].at<cv::Vec3b>(i,j)[0];
}
}
But this doesn't work, so I am now at loss I don't know how to succeed to fill the image_data array with the values of all three channels, without changing the code structure as this array is then used on my image processing algorithm.

Answer 1

I don't understand exactly what you are trying to do.
You can directly read a color image with:
cv::Mat img = cv::imread("image.jpeg",1);
Your matrix (img) type will be CV_8UC3, then you can access to each pixel like you said using:
img.at<cv::Vec3b>(row,col)[channel].
If you have a 2D array of Vec3b as Vec3b myArray[n][m];
You can access the values like that:
myArray[i][j](k) where k={1,2,3} since Vec3b is a row matrix.

Answer 2

Here is the code I just tested, and it works.
#include <iostream>
#include <cstdlib>
#include <opencv/cv.h>
#include <opencv/highgui.h>
int main(int argc, char**argv){
cv::Mat img = cv::imread("image.jpg",1);
cv::imshow("image",img);
cv::waitKey(0);
cv::Vec3b firstline[img.cols];
for(int i=0;i<img.cols;i++){
// access to matrix
cv::Vec3b tmp = img.at<cv::Vec3b>(0,i);
std::cout << (int)tmp(0) << " " << (int)tmp(1) << " " << (int)tmp(2) << std::endl;
// access to my array
firstline[i] = tmp;
std::cout << (int)firstline[i](0) << " " << (int)firstline[i](0) << " " << (int)firstline[i](0) << std::endl;
}
return EXIT_SUCCESS;
}

Answer 3

In you edited first message, this line is strange:
image_data[0][i*width+j]=all_images[nb_image-1].at<cv::Vec3b>(i,j)[2];
If image data is your colored image, then it should be written like this:
image_data[i][j] = all_images[nb_image-1].at<cv::Vec3b>(i,j);