draw a vertical line in a videocapture opencv - c++

Question

i followed a tutorial about facedetection using c++ and visual studio 2012 it worked well for , but then i wanted to add vertical lines to the video capture (from webcam) but nothing happened i dont know what exactly went wrong, i could really appreciate your help with this .here is the code i'm working on :
int main() {
VideoCapture cap(0); // Open default camera
Mat frame;
cap.set(CV_CAP_PROP_FRAME_WIDTH, 640);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 480);
line(frame, Point(frame.cols / 2 + 1, 0),
Point(frame.cols / 2 + 1, frame.rows - 1),
Scalar(255, 0, 128));
// Load preconstructed classifier
face_cascade.load("C:\\opencv24\\opencv\\sources\\data\\haarcascades\\haarcascade_frontalface_alt.xml");
while (cap.read(frame)) {
detectFaces(frame); // Call function to detect faces
if (waitKey(30) >= 0) // Pause key
break;
}
return 0;
}

Answer 1

after some modification in the code i finally arrived to get the line drawn ,here is the running code
while (cap.read(frame)) {
// Call function to detect faces
Mat frame;
cap >> frame; // get a new frame from camera
//cvtColor(frame, frame, COLOR_BGR2GRAY);
cap.set(CV_CAP_PROP_FRAME_WIDTH, 640);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 480);
line(frame, Point(frame.cols / 2 + 1, 0),
Point(frame.cols / 2 + 1, frame.rows - 1),
Scalar(255, 0, 0));
imshow("edges", frame);
detectFaces(frame);
if (waitKey(30) >= 0) // Pause key
break;
}
return 0;
}