#include <opencv2/opencv.hpp>
#include <iostream>
#include <cstdint>
using namespace std;
int main(int argc,char **argv) {
        if (argc<3) {
                cout << "Usage: ./grayvideo input.mp4 output.mp4 " << endl;
                return -1;
       }
       cv::VideoCapture cap(argv[1]);
       if (!cap.isOpened()) {
               cout << "Error opening file \n";
               return -1;
      }
      int width=cap.get(cv::CAP_PROP_FRAME_WIDTH);
      int height=cap.get(cv::CAP_PROP_FRAME_HEIGHT);
      double fps=cap.get(cv::CAP_PROP_FPS);
      if (fps<=0) fps=30;
      cd::VideoWriter writer (argv[2], cv::VideoWriter::fourcc('a','v','c','1'),fps,cv::Size(width,height));
      if (!writer.isOpened()) {
              cout << "Error writing file! \n";
              return -1;
     }
     cv::Mat frame;
     while (cap.read(frame)) {
             int rows=frame.rows;
             int cols=frame.cols;
             int channels=3;
             //Allocate continuous memory chunk
             uint8_t *p=new uint8_t[rows*cols*channels];
             //Copy frame into continuous memory
             for (int i=0;i<rows;i++)
             {
                      for(int =0;j<cols;j++)
                      {
                                 cv::Vec3b pixel=frame.at<cv::Vec3b>(i,j)
                                 int base=(i*cols+j)*3;
                                 p[base+0]=pixel[0];
                                 p[base+1]=pixel[1];
                                 p[base+2]=pixel[2];
                      }
             }
             //Grayscale transform
             for (int i=0;i<rows;i++)
             {
                          for(int j=0;j<cols;j++)
                          {
                                  int base=(i*cols+j)*3;
                                  uint8_t B=p[base+0];
                                  uint8_t G=p[base+1];
                                  uint8_t R=p[base+2];
                                  uint8_t gray=(uint8_t) 0.299*R+0.587*G+0.114*B;
                          }
             }
             //copy back output.mp4
             for (int i=0;i<rows;i++)
             {
                      for(int j=0;j<cols;j++)
                      {
                              int base=(i*cols+j)*3;
                              frame.at<cv::vec3b>(i,j)[0]=p[base+0];
                              frame.at<cv::vec3b>(i,j)[1]=p[base+1];
                              frame.at<cv::vec3b>(i,j)[2]=p[base+2];
                      }
             }
             weiter.write(frame);
             delete[] p;         
       }//EOF while
       cap.release();
       writer.release();
       cout << "video transform success! \n";
       return 0;
}//EOF main