Retrieving frames from .avi videos using OpenCV

Question

#include "cv.h"
#include "highgui.h"
int main(int argc, char** argv)
{
CvCapture* capture=0;
IplImage* frame=0;

capture = cvCaptureFromAVI("C:\\boy walking back.avi"); // read AVI video
if( !capture )
    throw "Error when reading steam_avi";

cvNamedWindow( "w", 1);

for( ; ; )
{
/*  int cvGrabFrame (CvCapture* capture);
    IplImage* cvRetrieveFrame (CvCapture* capture)*/
    frame = cvQueryFrame( capture );
if(!frame)
        break;
    cvShowImage("w", frame);

}
cvWaitKey(0); // key press to close window
cvDestroyWindow("w");
cvReleaseImage(&frame);
}

I am using openCV with VS2008. I read in the video file and used CV_CAP_PROP_FRAME_COUNT to get the number of frames, which was about 130 for a 4 second video clip. I do walking motion recognition, so I need to get all the other 5 frames, because between 5 frames, there is a slight change in body movement. So far I have a program that allows me to get one frame of a video clip. However, I cannot get different frames, and also, I'm not sure how to get all the other 5 frames. The above code is used to get one frame of video.

Answer 1

4 , 5- . , , :

IplImage* skipNFrames(CvCapture* capture, int n)
{
    for(int i = 0; i < n; ++i)
    {
        if(cvQueryFrame(capture) == NULL)
        {
            return NULL;
        }
    }

    return cvQueryFrame(capture);
}


int main(int argc, char* argv[])
{
    CvCapture* capture = cvCaptureFromFile("../opencv-root/samples/c/tree.avi");

    IplImage* frame = NULL;
    do
    {
        frame = skipNFrames(capture, 4);
        cvNamedWindow("frame", CV_WINDOW_AUTOSIZE);
        cvShowImage("frame", frame);
        cvWaitKey(100);
    } while( frame != NULL );

    cvReleaseCapture(&capture);
    cvDestroyWindow("frame");
    cvReleaseImage(&frame);

    return 0;
}

, :)