cvReleaseCapture() error - c++

Question

To make my question clearer, please view the following codes below:
For snapping image:
void CameraTest ::on_snapButton_clicked()
{
CvCapture* capture = cvCaptureFromCAM(0); // capture from video device #0
cvSetCaptureProperty(capture ,CV_CAP_PROP_FRAME_WIDTH , 800);
cvSetCaptureProperty(capture ,CV_CAP_PROP_FRAME_HEIGHT , 600);
if(!cvGrabFrame(capture)) //if no webcam detected or failed to capture anything
{ // capture a frame
cout << "Could not grab a frame\n\7";
exit(0);
}
IplImage* img=cvRetrieveFrame(capture); // retrieve the captured frame
cv::Mat imageContainer(img);
image=imageContainer;
cv::imshow("Mat",image);
//cvReleaseCapture(&capture); When I enable this, and run the programming calling this, there will be an error.
}
Now, the program to display the image:
void CameraTest ::on_processButton_clicked()
{
cv::imshow("image snapped", image);
//my image processing steps...
}
When I enable the cvReleaseCapture(&capture) line, I recieve the following error:
Unhandled exception at 0x00fc3ff5 in CameraTest.exe: 0xC0000005: Access violation reading location 0x042e1030.
When I comment/remove the line, I am able to display image properly upon clicking the other button, but when I want to snap new images, I have to click the button a few times, which is a major flaw in the program. Is there anyway to go around it?

Answer 1

avoid the outdated c-api (IplImages, Cv*functions) stick with the c++ api.
the images you get from a capture point to memory inside the cam-driver. if you don't clone() the image, and release the capture, you got a dangling pointer.
don't create a new capture for each shot. (cam needs some 'warm-up' time so it'll be slow as hell).
keep one instance around in your class instead
class CameraTest
{
VideoCapture capture; // make it a class - member
CameraTest () : capture(0) // capture from video device #0
{
capture.set(CV_CAP_PROP_FRAME_WIDTH , 800);
capture.set(CV_CAP_PROP_FRAME_HEIGHT , 600);
}
// ...
};
void CameraTest ::on_snapButton_clicked()
{
Mat img; // temp var pointing todriver mem
if(!capture.read(img)) //if no webcam detected or failed to capture anything
{
cout << "Could not grab a frame\n\7";
exit(0);
}
image = img.clone(); // keep our member var alive
cv::imshow("Mat",image);
}

Answer 2

Replace :
if(!cvGrabFrame(capture)) //if no webcam detected or failed to capture anything
{ // capture a frame
cout << "Could not grab a frame\n\7";
exit(0);
}
by
if ( !capture )
{
cout << "Could not grab a frame\n\7";
exit(0);
}
and replace
IplImage* img=cvRetrieveFrame(capture);
by
IplImage* img = cvQueryFrame( capture );
cvQueryFrame Grabs and returns a frame from video or camera. This function is a combination of cvGrabFrame and cvRetrieveFrame in one call. The returned image should not be released or modified by user.