Convert vec3b into mat

Question

I have color image in im, i want to get the pixel value of 3 channels image using vec3b using the following code

    for (int i = 0; i < im.rows; i++)
{
    for (int j = 0; j < im.cols; j++)
    {
        for (int k = 0; k < nChannels; k++)
        {
            zay[k] = im.at<Vec3b>(i, j)[k]; //get the pixel value and assign to new vec3b variable zay


        }

    }

}

After that, i want to multiply the following mat 3x3 filter with that vec3b in zay

    Filter= (Mat_<double>(3, 3) << 0, 0, 0,
                                0, 1, 0,
                                0, 0, 0);

How to convert vec3b into mat matrix so i can multiplied with mat Filter?? And vec3b is an array 3x1? Thanks

Answer 1

According to your example, what you are trying to accomplish is known as a convolution with a kernel.

You can set up the kernel and invoke cv::filter2D() to apply it for you:

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

int main()
{
    cv::Mat img = cv::imread("input.jpg");
    if (img.empty())
    {
        std::cout << "!!! Failed to open input image" << std::endl;
        return -1;
    }

    cv::Mat kernel = (cv::Mat_<float>(3, 3) << 0, 0, 0,
                                               0, 1, 0,
                                               0, 0, 0);

    cv::Mat dst;
    cv::filter2D(img, dst, -1, kernel, cv::Point(-1, -1), 0, cv::BORDER_DEFAULT);

    cv::imshow("output", dst);
    cv::waitKey(0);

    return 0;
}

Thus, there's no need to iterate on the pixels and perform the computation yourself. OpenCV documentation explains it all at: Making your own linear filters!.

Answer 2

Didnt try but should work:

cv::Mat DoubleMatFromVec3b(cv::Vec3b in)
{
    cv::Mat mat(3,1, CV_64FC1);
    mat.at <double>(0,0) = in [0];
    mat.at <double>(1,0) = in [1];
    mat.at <double>(2,0) = in [2];

    return mat;
};