It doesn't matter if cvReleaseImage is before cvWaitKey?
I change the order of cvReleaseImage and cvWaitKey, and it does work too.
#include "stdafx.h"
int main(int argc, char** argv)
{
cvNamedWindow("testWindow",0);
IplImage *img=cvLoadImage("E:/lena.jpg");
cvShowImage("testWindow",img);
Sleep(10);
cvReleaseImage(&img);
cvWaitKey(0);
cvDestroyWindow("testWindow");
return 0;
}
Then I stepped into the program ,and cvReleaseImage did release the image, and Image Watch showed that img pointed to NULL,while when cvWaitKey was called, The img was shown on the window correctly!
To find the reason, I refered to the source code of cvShowImage:
CV_IMPL void
cvShowImage( const char* name, const CvArr* arr )
{
CV_FUNCNAME( "cvShowImage" );
__BEGIN__;
CvWindow* window;
SIZE size = { 0, 0 };
int channels = 0;
void* dst_ptr = 0;
const int channels0 = 3;
int origin = 0;
CvMat stub, dst, *image;
bool changed_size = false; // philipg
if( !name )
CV_ERROR( CV_StsNullPtr, "NULL name" );
window = icvFindWindowByName(name);
if( !window || !arr )
EXIT; // keep silence here.
if( CV_IS_IMAGE_HDR( arr ))
origin = ((IplImage*)arr)->origin;
CV_CALL( image = cvGetMat( arr, &stub ));
if (window->image)
// if there is something wrong with these system calls, we cannot display image...
if (icvGetBitmapData( window, &size, &channels, &dst_ptr ))
return;
if( size.cx != image->width || size.cy != image->height || channels != channels0 )
{
changed_size = true;
uchar buffer[sizeof(BITMAPINFO) + 255*sizeof(RGBQUAD)];
BITMAPINFO* binfo = (BITMAPINFO*)buffer;
DeleteObject( SelectObject( window->dc, window->image ));
window->image = 0;
size.cx = image->width;
size.cy = image->height;
channels = channels0;
FillBitmapInfo( binfo, size.cx, size.cy, channels*8, 1 );
window->image = SelectObject( window->dc, CreateDIBSection(window->dc, binfo,
DIB_RGB_COLORS, &dst_ptr, 0, 0));
}
cvInitMatHeader( &dst, size.cy, size.cx, CV_8UC3,
dst_ptr, (size.cx * channels + 3) & -4 );
cvConvertImage( image, &dst, origin == 0 ? CV_CVTIMG_FLIP : 0 );
// ony resize window if needed
if (changed_size)
icvUpdateWindowPos(window);
InvalidateRect(window->hwnd, 0, 0);
// philipg: this is not needed and just slows things down
// UpdateWindow(window->hwnd);
__END__;
}
Note that window->image is linked with dst_ptr which is the MatHeader of dst, and dst is coverted by image. So ,we got that it is the coverted image "dst" rather than image itself shown on the window.
Another problem rises to my sight, when is dst released?
The only possible released time is cvDestroyWindow, while the source code doesn't show such a step.
So, there is a memory leak for cvShowImage, right？
Similarly, some other functions like cvCloneImage have the problem of memory leak
http://hi.baidu.com/gilbertjuly/item/aa13b19223a303b983d29550
Learning when and how to use release function is very important to avoid memory leak.
Basically, if we have applied for a memory using functions like cvLoadImage, cvCreatImage, it is required that a functional call of cvReleaseImage must be called. A special attention should be paid to cvRetrieveFrame , as the documentation says:
OpenCV 1.x functions cvRetrieveFrame and cv.RetrieveFrame return image stored inside the video capturing structure. It is not allowed to modify or release the image! You can copy the frame using cvCloneImage() and then do whatever you want with the copy.
（cvCopy may be better than cvCloneImage()）
don't use cvReleaseImage to release the image each time you get a frame by cvRetrieveFrame.
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