cvCreateVideoWriter -- can't change filename on subsequent invocations

OpenCV 2.2 Windows 7

I have a C++ method (below) which creates an avi file from an array of frames (IplImage *). The file name is based on which of two threads (let's call them "开发者_Python百科left" and "right") is doing the writing.

Code:

void AvtCameraCapture::save_as_video(IplImage **frames,const char * fname, int num_frames, int playback_fps,CvSize &size)
{
    //EnterCriticalSection(&cs); 

    CvVideoWriter *writer = cvCreateVideoWriter(fname,CV_FOURCC('D', 'I', 'B', ' '),playback_fps,size);
    printf("Writer is %x\n", writer);
    for (int i=0; i < num_frames; i++) 
    {
        printf("Wrote frame %d to file %s\n",i,fname);
        cvWriteFrame(writer,frames[i]);
    }
    cvReleaseVideoWriter(&writer);

    //LeaveCriticalSection(&cs);
}

The file gets created fine for the first thread that executes the method. But, no file gets created for the second thread.

I see the prints for both threads, with the correct filenames for each. Note the CvVideoWriter * is non-null.

writing to file c:\flyball_passes\Left_dog_2.avi
Writer is 4dabf60
Wrote frame 0 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 1 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 2 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 3 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 4 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 5 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 6 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 7 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 8 to file c:\flyball_passes\Left_dog_2.avi
writing to file c:\flyball_passes\Right_dog_2.avi
Writer is 4d2a930
Wrote frame 0 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 1 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 2 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 3 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 4 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 5 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 6 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 7 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 8 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 9 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 10 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 11 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 12 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 13 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 14 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 15 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 16 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 17 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 18 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 19 to file c:\flyball_passes\Right_dog_2.avi
video written to file c:\flyball_passes\Right_dog_2.avi
Masking for 2.696000 seconds in lane Right.
Wrote frame 9 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 10 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 11 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 12 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 13 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 14 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 15 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 16 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 17 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 18 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 19 to file c:\flyball_passes\Left_dog_2.avi
video written to file c:\flyball_passes\Left_dog_2.avi

If I uncomment the Enter/Exit critical sections, one thread finishes writing before the other starts, but still no file is created for the thread that gets there last:

writing to file c:\flyball_passes\Left_dog_2.avi
Writer is 4f5d298
Wrote frame 0 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 1 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 2 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 3 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 4 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 5 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 6 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 7 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 8 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 9 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 10 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 11 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 12 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 13 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 14 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 15 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 16 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 17 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 18 to file c:\flyball_passes\Left_dog_2.avi
Wrote frame 19 to file c:\flyball_passes\Left_dog_2.avi
video written to file c:\flyball_passes\Left_dog_2.avi
Masking for 2.570000 seconds in lane Left.
writing to file c:\flyball_passes\Right_dog_2.avi
Writer is 4f5d298
Wrote frame 0 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 1 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 2 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 3 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 4 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 5 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 6 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 7 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 8 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 9 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 10 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 11 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 12 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 13 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 14 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 15 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 16 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 17 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 18 to file c:\flyball_passes\Right_dog_2.avi
Wrote frame 19 to file c:\flyball_passes\Right_dog_2.avi
video written to file c:\flyball_passes\Right_dog_2.avi

Even if I wait several seconds to kick off an event that triggers write of the second file, it never shows up.

If I only fire events that trigger writing of the file for the right lane (no prior call cvCreateVideoWriter on the left lane), I see the avi file for the right lane.

Any insight would be greatly appreciated!

Thanks,

Dave Thomas

You are calling OpenCV functions from separate threads. There is no guarantee that your system will work properly since OpenCV is not thread-safe.

Use mutexes or some other mechanism to protect OpenCV calls that are being done on the threads.

You should protect this entire block of code from being executed simultaneously:

// lock global mutex

CvVideoWriter *writer = cvCreateVideoWriter(fname,CV_FOURCC('D', 'I', 'B', ' '),playback_fps,size);
printf("Writer is %x\n", writer);
for (int i=0; i < num_frames; i++) 
{
    printf("Wrote frame %d to file %s\n",i,fname);
    cvWriteFrame(writer,frames[i]);
}
cvReleaseVideoWriter(&writer);

// unlock mutex