Displaying an outline on top of an image - opencv

Question

I am trying to draw a face outline and overlay it on top of a webcam image.
But towards the end, I think I am using addWeighted in a wrong way, because my program crashes.
Could you please help me understand what I am doing wrong with imshow and addWeighted?
int main( int argc, const char** argv )
{
VideoCapture camera;
camera.open(0);
if( !camera.isOpened() )
{
cerr << "Could not access the camera!" << endl;
return 1;
}
while( true )
{
Mat cameraFrame;
camera >> cameraFrame;
if( cameraFrame.empty() )
{
cerr << "Could not grab a camera frame!" << endl;
return 1;
}
Mat gray;
cvtColor( cameraFrame, gray, CV_BGR2GRAY );
Size size = cameraFrame.size();
Mat faceOutline = Mat::zeros( size, CV_8UC3 ); // Draw a black canvas.
Scalar color = CV_RGB( 255, 255, 0 ); // Yellow
int thickness = 4;
ellipse( faceOutline, Point(320, 240), Size(320, 240), 0, 0, 360, color, thickness, CV_AA );
addWeighted( gray, 1.0, faceOutline, 0.7, 0, gray, CV_8UC3 );
imshow( "final image", gray );
char keypress = waitKey(20);
if( keypress == 27 ) break;
}
}

Answer 1

This works fine:
int main( int argc, const char** argv )
{
VideoCapture camera;
camera.open(0);
if( !camera.isOpened() )
{
cerr << "Could not access the camera!" << endl;
return 1;
}
while( true )
{
Mat cameraFrame;
camera >> cameraFrame;
if( cameraFrame.empty() )
{
cerr << "Could not grab a camera frame!" << endl;
return 1;
}
Mat gray;
cvtColor( cameraFrame, gray, cv::COLOR_BGR2GRAY );
Size size = cameraFrame.size();
Mat faceOutline = Mat::zeros( size, CV_8UC3 ); // Draw a black canvas.
Scalar color = Scalar( 255, 255, 0 ); // Yellow
int thickness = 4;
cvtColor( gray, gray, cv::COLOR_GRAY2BGR );
ellipse( faceOutline, Point(320, 240), Size(320, 240), 0, 0, 360, color, thickness );
addWeighted( gray, 1.0, faceOutline, 0.7, 0, gray, CV_8UC3 );
imshow( "final image", gray );
char keypress = waitKey(20);
if( keypress == 27 ) break;
}
}

Answer 2

why not just draw the ellipse into the cameraFrame ?
ellipse( cameraFrame , Point(320, 240), Size(320, 240), 0, 0, 360, color, thickness, CV_AA );
and if you want to use addWeighted,
the type of both input images has to match ( you can't add a color to a grayscale image )
the factors have to sum up to 1.0
the last argument is depth, not type ( i.e you could convert it to float here, but not change the number of channels)
addWeighted( cameraFrame , 0.7, faceOutline, 0.3, 0, cameraFrame );

Answer 3

I suppose your gray image is single-channel and your faceOutline image has 3 channels.
From the documentation:
src2 – second input array of the same size and channel number as src1.
Try mixChannels to switch a single channel of a multichannel image