#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;
	cv::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 j=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 transformation
		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;
				p[base+0]=gray;
				p[base+1]=gray;
				p[base+2]=gray;
			}
		}//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];
			}
		}
		writer.write(frame);
		delete[] p;
	} //EOF WHILE
	cap.release();
	writer.release();
	cout<< "Video transform success! \n";
	return 0;
}
//EOF MAIN