openCV and Threads issue - c++

Question

I'm trying to fit facedetect from openCV on my QT code, all run fine until i decided to create a thread for my openCV code so I can run another things while the face detect is on.
the problem is if i call class->start(); my program breaks in the while loop in the run() but if i call the class.run(); (like a normal function) it runs as usual! what can be wrong?
code:
faceTracker::faceTracker()
{
qDebug("teste1");
filename = "/Users/marcomartins/Documents/QT/DisplUM/haarcascades/haarcascade_frontalface_alt_tree.xml";
/* load the classifier
note that I put the file in the same directory with this code */
cascade = ( CvHaarClassifierCascade* )cvLoad( filename, 0, 0, 0 );
/* setup memory buffer; needed by the face detector */
storage = cvCreateMemStorage( 0 );
/* initialize camera */
capture = cvCaptureFromCAM( 0 );
/* always check */
assert( cascade && storage && capture );
/* create a window */
cvNamedWindow( "video DisplUM", 1 );
}
void faceTracker::detectFaces( IplImage *img )
{
/* detect faces */
faces = cvHaarDetectObjects(
img,
cascade,
storage,
1.1,
3,
0 /*CV_HAAR_DO_CANNY_PRUNNING*/,
cvSize( 40, 40 ) );
/* for each face found, draw a red box */
for( i = 0 ; i < ( faces ? faces->total : 0 ) ; i++ ) {
CvRect *r = ( CvRect* )cvGetSeqElem( faces, i );
cvRectangle( img,
cvPoint( r->x, r->y ),
cvPoint( r->x + r->width, r->y + r->height ),
CV_RGB( 255, 0, 0 ), 1, 8, 0 );
qDebug("caras: %d", faces->total);
}
/* display video */
cvShowImage( "video", img );
}
void faceTracker::run( )
{
qDebug("teste2");
while( key != 'q' ) {
/* get a frame */
frame = cvQueryFrame( capture );
qDebug("teste3");
/* always check */
if( !frame ) break;
/* 'fix' frame */
cvFlip( frame, frame, 1 );
frame->origin = 0;
/* detect faces and display video */
detectFaces( frame );
/* quit if user press 'q' */
key = cvWaitKey( 10 );
}
/* free memory */
cvReleaseCapture( &capture );
cvDestroyWindow( "video" );
cvReleaseHaarClassifierCascade( &cascade );
cvReleaseMemStorage( &storage );
}
main code:
int main(int argc, char *argv[])
{
faceTracker * ft = new faceTracker();
ft->start();
}
thanks a lot!

Answer 1

The loop will only break if cvQueryFrame() returns a NULL frame or if the user presses q on the keyboard.
Add a debug so you know when the first situation happens:
frame = cvQueryFrame( capture );
if( !frame )
{
qDebug("cvQueryFrame failed!");
break;
}
Are you sure cvCaptureFromCAM(0) works ? Depending on the OS I have to pass -1 for it. But the fact is that you'll never know if cvCaptureFromCAM(0)succeeded because you don't check the return, and this might be the problem!
capture = cvCaptureFromCAM(0);
if (!capture)
{
qDebug("cvCaptureFromCAM failed!");
//exit(0); or whatever
}
EDIT:
Pay huge attention to this: you are creating a window named "video DisplUM" but you are trying to display the frames on a different window, named "video".
Anyway, it's also better if you change the window creation function to use the appropriate enum:
cvNamedWindow("video DisplUM", CV_WINDOW_AUTOSIZE);
and on faceTracker::run( ) comment detectFaces() for now and add a call to cvShowImage( "video DisplUM", frame );
Always be sure your application works with the minimum requirements before adding fancy stuffs, like face detection. My final suggestion is: write enough code just to capture the images from one thread and display them on a window, and then go from there.

Answer 2

Solution:
I can't create windows outside the main thread, that is why it was crashing. If i comment the window creation all works good (face detection included)

Answer 3

I ran into a similar issue. What I found is that I had to rebuild OpenCV and include the TBB libraries. This adds threading support to OpenCV. Once I did this, I was able to pop up windows in any thread I chose. I've tested this on versions 2.1 and 2.2, using both C and C++ implementations.