OpenCV Image Mat to 1D CHW(RR...R, GG..G, BB..B) vector

Nvidia's cuDNN for deep learning has a rather interesting format for images called CHW. I have a cv::Mat img; that I want to convert to a one-dimensional vector of floats. The problem that I'm having is that the format of the 1D vector for CHW is (RR...R, GG..G,BB..B).

So I'm curious as to how I can extract the channel values for each pixel and order them for this format.

You can either iterate over the image manually and copy the values into the right place, or you can use something like cv::extractChannel to copy the channels one by one like so:

#include <opencv2/opencv.hpp>

int main()
{
    //create dummy 3 channel float image
    cv::Mat sourceRGB(cv::Size(100,100),CV_32FC3);
    auto size = sourceRGB.size();
    for (int y = 0; y < size.height; ++y)
    {
        for (int x = 0; x < size.width; ++x)
        {
            float* pxl = sourceRGB.ptr<float>(x, y);
            *pxl = x / 100.0f;
            *(pxl+1) = y / 100.0f;
            *(pxl + 2) = (y / 100.0f) * (x / 100.0f);
        }
    }

    cv::imshow("test", sourceRGB);
    cv::waitKey(0);

    //create single image with all 3 channels one after the other
    cv::Size newsize(size.width,size.height*3);
    cv::Mat destination(newsize,CV_32FC1);

    //copy the channels from the source image to the destination
    for (int i = 0; i < sourceRGB.channels(); ++i)
    {
        cv::extractChannel(
            sourceRGB,
            cv::Mat(
                size.height,
                size.width,
                CV_32FC1,
                &(destination.at<float>(size.height*size.width*i))),
            i);
    }

    cv::imshow("test", destination);
    cv::waitKey(0);
    return 0;
}