opencv/samples/cpp/tutorial_code/HighGUI/video-write/video-write.cpp

#include <iostream>	// for standard I/O
#include <string>   // for strings

#include <opencv2/core/core.hpp>        // Basic OpenCV structures (cv::Mat)
#include <opencv2/highgui/highgui.hpp>  // Video write

using namespace std;
using namespace cv;

void help()
{
    cout
        << "\n--------------------------------------------------------------------------" << endl
        << "This program shows how to write video files. You can extract the R or G or B color channel "
        << " of the input video.write "  << endl
        << "Usage:"                                                               << endl
        << "./video-write inputvideoName [ R | G | B] [Y | N]"                   << endl
        << "--------------------------------------------------------------------------"   << endl
        << endl;
}
int main(int argc, char *argv[], char *window_name)
{
    help();
    if (argc != 4)
    {
        cout << "Not enough parameters" << endl;
        return -1;
    }

    const string source      = argv[1];            // the source file name
    const bool askOutputType = argv[3][0] =='Y';  // If false it will use the inputs codec type
   
    VideoCapture inputVideo(source);        // Open input
    if ( !inputVideo.isOpened())
    {
        cout  << "Could not open the input video." << source << endl;
        return -1;
    }

    string::size_type pAt = source.find_last_of('.');   // Find extension point
    const string NAME = source.substr(0, pAt) + argv[2][0] + ".avi";   // Form the new name with container
    int ex = static_cast<int>(inputVideo.get(CV_CAP_PROP_FOURCC));     // Get Codec Type- Int form

    // Transform from int to char via Bitwise operators
    char EXT[] = {ex & 0XFF , (ex & 0XFF00) >> 8,(ex & 0XFF0000) >> 16,(ex & 0XFF000000) >> 24, 0};

    Size S = Size((int) inputVideo.get(CV_CAP_PROP_FRAME_WIDTH),    //Acquire input size
                  (int) inputVideo.get(CV_CAP_PROP_FRAME_HEIGHT));    

    VideoWriter outputVideo;                                        // Open the output
    if (askOutputType)
            outputVideo.open(NAME  , ex=-1, inputVideo.get(CV_CAP_PROP_FPS),S, true);  
    else
        outputVideo.open(NAME , ex, inputVideo.get(CV_CAP_PROP_FPS),S, true);
    
    if (!outputVideo.isOpened())
    {
        cout  << "Could not open the output video for write: " << source << endl;
        return -1;
    }

    union { int v; char c[5];} uEx ;
    uEx.v = ex;                              // From Int to char via union
    uEx.c[4]='\0';

    cout << "Input frame resolution: Width=" << S.width << "  Height=" << S.height
        << " of nr#: " << inputVideo.get(CV_CAP_PROP_FRAME_COUNT) << endl;
    cout << "Input codec type: " << EXT << endl;

    int channel = 2;    // Select the channel to save
    switch(argv[2][0])
    {
    case 'R' : {channel = 2; break;}
    case 'G' : {channel = 1; break;}
    case 'B' : {channel = 0; break;}
    }
    Mat src,res;
    vector<Mat> spl; 

    while( true) //Show the image captured in the window and repeat
    {
        inputVideo >> src;              // read
        if( src.empty()) break;         // check if at end

       split(src, spl);                 // process - extract only the correct channel
       for( int i =0; i < 3; ++i)       
        if (i != channel)
           spl[i] = Mat::zeros(S, spl[0].type());
       merge(spl, res);

       //outputVideo.write(res); //save or
       outputVideo << res;
    }

    cout << "Finished writing" << endl;
    return 0;
}