cvCreateVideoWriter -- can't change filename on subsequent invocations

Question

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 "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

Answer 1

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