将8位图像转换为4位图像OpenCv

Question

如何手动将8位（CV_8U）图像转换为4位图像？这是加载图像的简单示例。

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    if (argc != 2)
    {
        cout << " Usage: display_image ImageToLoadAndDisplay" << endl;
        return -1;
    }

    Mat image;
    image = imread(argv[1], CV_LOAD_IMAGE_COLOR);

    for (int r = 0; r < image.rows; ++r)
    {
        for (int c = 0; c < image.cols; ++c)
        {
            // What to do here??
        }
    }

    return 0;
}

或许还有另一种转换方式。