Merging channels in OpenCV

c++, opencv

Solution

Why are you converting the image if you have a grayscale image present?

Just create two empty matrix of the same size for Blue and Green.

And you have defined your output matrix as 1 channel matrix. Your output matrix must contain at least 3 channels. (Blue, Green and Red). Where Blue and Green will be completely empty and you put your grayscale image as Red channel of the output image.

#include <opencv2/highgui/highgui.hpp> 
#include <stdio.h>
using namespace std;
using namespace cv;

int main()
{
    Mat img, g, fin_img;
    img = imread("Lenna.png",CV_LOAD_IMAGE_GRAYSCALE);
    vector<Mat> channels;

    g = Mat::zeros(Size(img.rows, img.cols), CV_8UC1);

    channels.push_back(g);
    channels.push_back(g);
    channels.push_back(img);

    merge(channels, fin_img);
    imshow("img", fin_img);
    waitKey(0);
    return 0;
}

Problem

I need to create a 'red' image from a grayscale image. I am using this code: ``` void build_red(const cv::Mat& in, cv::Mat& out) { out = Mat::zeros(in.rows, in.cols, CV_8UC1); Mat zeros = Mat::zeros(in.rows, in.cols, CV_8UC1); Mat tmp; in.convertTo(tmp, CV_8UC1); vector<Mat> ch; ch.push_back(zeros); ch.push_back(zeros); ch.push_back(tmp); cout << "Using " << ch.size() << " channels" << endl; merge(ch, out); } // build_red ``` With some explanations: ``` void build_red(const cv::Mat& in, cv::Mat& out) { ``` in is the input matrix, out the output. ``` out = Mat::zeros(in.rows, in.cols, CV_8UC1); ``` allocate some space for out (may be useless, but part of my attempts) ``` Mat zeros = Mat::zeros(in.rows, in.cols, CV_8UC1); Mat tmp; in.convertTo(tmp, CV_8UC1); ``` Create an empty matrix with the same size and convert the input image to single-channel uchar. ``` vector<Mat> ch; ch.push_back(zeros); ch.push_back(zeros); ch.push_back(tmp); cout << "Using " << ch.size() << " channels" << endl; merge(ch, out); ``` Create a vector with three channels, then merge them into 'out'. However, when I run the code I get the following message: ``` Using 3 channels ``` and the following exception: ``` OpenCV Error: Bad number of channels (Source image must have 1, 3 or 4 channels) in cvConvertImage, file /[...]/libs/OpenCV-2.4.0/modules/highgui/src/utils.cpp, line 611 terminate called after throwing an instance of 'cv::Exception' what(): /[...]/libs/OpenCV-2.4.0/modules/highgui/src/utils.cpp:611: error: (-15) Source image must have 1, 3 or 4 channels in function cvConvertImage ``` Could you please help me? From my inexperienced point of view, the type of the images is the same and the number of channels is correct.

Original source