camera / cvRetrieveFrame problem

Hi,

It seems there is an error in the following code example, but I don't
know where to start searching. Maybe one of you could be so kind to
help me finding the bug.

- - - - - - - - - - - - 
#include <stdafx.h>
#include "cv.h"
#include "highgui.h"
#include "cvcam.h"
#include "cxcore.h"
#include <time.h>
#include <math.h>
#include <ctype.h>

int main(int argc, char* argv[])
{
  CvCapture* capture;
  int cameraSelected = -1;
  capture = cvCaptureFromCAM(cameraSelected);
  // capture = cvCaptureFromAVI("office101.avi");
  cvWaitKey(0);

  if( capture )
  {
    IplImage* motion = 0;
    cvNamedWindow( "Motion", CV_WINDOW_AUTOSIZE );
    for(;;)
    {
        IplImage* image = 0;
        if( !cvGrabFrame( capture ))
        break;
        image = cvRetrieveFrame( capture );
        if( image )
        {
          if( !motion )
          {
            motion = cvCreateImage(cvSize(image->width,image->height), 8, 3 );
            cvZero( motion );
            motion->origin = image->origin;
          }
        }

        cvShowImage( "Motion", image );
        if( cvWaitKey(10) >= 0 )
        break;
     }
        
     cvReleaseCapture( &capture );
     cvDestroyWindow( "Motion" );
  }

   return 0;
}
- - - - - - - - - - - 

First problem:

On execution, the camera gets initialized the motion frame opens but
it doesn't show any images. No exceptions, no error messages. The usb
webcam works fine using cvcam (cvcamInit(), cvcamStart(),...).

While stepping through the code, I found out that
cvRetrieveFrame(capture) doesn't return any image, but I don't
understand why.

Second problem:

I tried the same example with cvCaptureFromAVI instead of
cvCaptureFormCAM with an example video from openAVCSR which works
fine, but only if I use cvShowImage( "Motion", image ) instead of
cvShowImage( "Motion", motion ). That seems strange to me as motion is
just a copy of the image.

Any help would be appreciated.

Helge







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