视频抓取对OpenCV不起作用

Question

I am using this piece of code to grab frames off a video :

#include <stdio.h>
#include <stdlib.h>
#include <opencv/cv.h>
#include <opencv/highgui.h>
#include <iostream>

using namespace cv;
using namespace std;

int main (int argc, char** argv)
{
//initializing capture from file
CvCapture * capture = cvCaptureFromAVI ("/home/<some_file>.avi");

//Capturing a frame
IplImage* img = 0;
if(!cvGrabFrame(capture))      //capture a frame
{
cout << Could not grab a frame\n\7";
exit(0);
}
img=cvRetrieveFrame(capture);    //retrieve the captured frame



//free resources
cvReleaseCapture(&capture);

}

Which is returning :

Could not grab a frame

Additional details : I had used code to save webcam video feed to the file from which i want to grab frames . I used this code :

#include <opencv2/opencv.hpp>
#include <opencv2/highgui/highgui.hpp>

int main( int argc, char** argv ) {
CvCapture* capture;

capture = cvCreateCameraCapture(0);

assert( capture != NULL );

IplImage* bgr_frame = cvQueryFrame( capture );

CvSize size = cvSize(
                     (int)cvGetCaptureProperty( capture,
                                               CV_CAP_PROP_FRAME_WIDTH),
                     (int)cvGetCaptureProperty( capture,
                                               CV_CAP_PROP_FRAME_HEIGHT)
                     );

cvNamedWindow( "Webcam", CV_WINDOW_AUTOSIZE );

CvVideoWriter *writer = cvCreateVideoWriter(    "/Users/user/Desktop/OpenCV_trial/OpenCV_trial/vidtry.AVI",
                                            CV_FOURCC('D','I','V','X'),
                                            30,
                                            size
                                            );

while( (bgr_frame = cvQueryFrame( capture )) != NULL ) 
{
    cvWriteFrame(writer, bgr_frame );
    cvShowImage( "Webcam", bgr_frame );
    char c = cvWaitKey( 33 );
    if( c == 27 ) break;
}
cvReleaseVideoWriter( &writer );
cvReleaseCapture( &capture );
cvDestroyWindow( "Webcam" );
return( 0 );
}

Does anyone know where I might be going wrong ? I am running OpenCV-2.4.3 on a Beagleboard -xM with Ubuntu Quantal.

Answer 1

I am not quite sure what your exactly question is, but if you want to grab frames from a video, you should at least have a loop.
A reason for your error could be, that your video file is not available. Have you tried another one? The full path of the file? Or put the file directly into your working directory and check it.
Another reason could be a problem with the first frame (this sometimes happens). So try to remove your exit and enclose your code with a loop over all frames.
Here is an example that shows the given video file (Consider to use the C++-interface):

#include <opencv/cv.h>
#include <opencv/highgui.h>
#include <iostream>

using namespace cv;
using namespace std;

int main (int argc, char** argv)
{
//initializing capture from file
Mat img;
VideoCapture capture("a.avi");  
if(!capture.isOpened())
{
    cout<<"Could not open video!\n";
    return 1;
}       

while(true)
{
    //Capturing a frame
    capture >> img;
    if(!img.empty())
    {
        imshow("Video",img);
    }       
    else    
    {   
        cout <<"Could not grab a frame\n";
        break;
    }

    if(waitKey(5) >= 0) 
        break;
}   
return 0;
}

This program runs on my PC if the file "a.avi" is in the current working directory of the program.