Second Program—AVI Video

Playing a video with OpenCV is almost as easy as displaying a single picture. The only new issue we face is that we need some kind of loop to read each frame in sequence; we may also need some way to get out of that loop if the movie is too boring. See Example 2-2.

Example 2-2. A simple OpenCV program for playing a video file from disk

#include "highgui.h"

int main( int argc, char** argv ) {
    cvNamedWindow( "Example2", CV_WINDOW_AUTOSIZE );
    CvCapture* capture = cvCreateFileCapture( argv[1] );
    IplImage* frame;
    while(1) {
        frame = cvQueryFrame( capture );
        if( !frame ) break;
        cvShowImage( "Example2", frame );
        char c = cvWaitKey(33);
        if( c == 27 ) break;
    }
    cvReleaseCapture( &capture );
    cvDestroyWindow( "Example2" );
}

Here we begin the function main() with the usual creation of a named window, in this case "Example2". Things get a little more interesting after that.

CvCapture* capture = cvCreateFileCapture( argv[1] );

The function cvCreateFileCapture() takes as its argument the name of the AVI file to be loaded and then returns a pointer to a CvCapture structure. This structure contains all of the information about the AVI file being read, including state information. When created in this way, the CvCapture structure is initialized to the beginning of the AVI.

frame = cvQueryFrame( capture );

Once inside of the while(1) loop, we begin reading from the AVI file. cvQueryFrame() takes as its argument a pointer to a CvCapture structure. It then grabs the next video frame into memory (memory that is actually part of the CvCapture structure). A pointer is returned to that frame. Unlike cvLoadImage, which actually allocates memory for the image, cvQueryFrame uses memory already allocated in the CvCapture structure. Thus it will not be necessary (or wise) to call cvReleaseImage() for this "frame" pointer. Instead, the frame image memory will be freed when the CvCapture structure is released.

c = cvWaitKey(33);
if( c == 27 ) break;

Once we have displayed the frame, we then wait for 33 ms.^[11] If the user hits a key, then c will be set to the ASCII value of that key; if not, then it will be set to –1. If the user hits the Esc key (ASCII 27), then we will exit the read loop. Otherwise, 33 ms will pass and we will just execute the loop again.

It is worth noting that, in this simple example, we are not explicitly controlling the speed of the video in any intelligent way. We are relying solely on the timer in cvWaitKey() to pace the loading of frames. In a more sophisticated application it would be wise to read the actual frame rate from the CvCapture structure (from the AVI) and behave accordingly!

cvReleaseCapture( &capture );

When we have exited the read loop—because there was no more video data or because the user hit the Esc key—we can free the memory associated with the CvCapture structure. This will also close any open file handles to the AVI file.

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^[11] You can wait any amount of time you like. In this case, we are simply assuming that it is correct to play the video at 30 frames per second and allow user input to interrupt between each frame (thus we pause for input 33 ms between each frame). In practice, it is better to check the CvCapture structure returned by cvCaptureFromCamera() in order to determine the actual frame rate (more on this in Chapter 4).