Learning OpenCV, 2nd Edition

quite well when there is only one camera to pick (see Chapter 4 for more details). Camera capture from file

or from camera is demonstrated in Example 2-10.

Example 2-10: The same object can load videos from a camera or a file

#include <opencv2/opencv.hpp>

#include <iostream>

int main( int argc, char** argv ) {

cv::namedWindow( "Example2_10", cv::WINDOW_AUTOSIZE );

cv::VideoCapture cap;

if (argc==1) {

cap.open(0); // open the default camera

} else {

cap.open(argv[1]);

}

if( !cap.isOpened() ) { // check if we succeeded

std::cerr << "Couldn't open capture." << std::endl;

return -1;

}

// The rest of program proceeds as in Example 2-3

…

In Example 2-10, if a filename is supplied, it opens that file just like in Example 2-3, and if no filename is

given, it attempts to open camera zero (0). We have added a check that something actually opened that

will report an error if not.

Writing to an AVI File

In many applications, we will want to record streaming input or even disparate captured images to an

output video stream, and OpenCV provides a straightforward method for doing this. Just as we are able to

create a capture device that allows us to grab frames one at a time from a video stream, we are able to

create a writer device that allows us to place frames one by one into a video file. The object that allows us

to do this is cv::VideoWriter ...