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You can move center from previos frame to current frame, and then draw line.

cv::Point preCenter = cv::Point(0, 0); 

while(true)
{
    cv::Point Center = cv::Point (preCenter.x+5, preCenter.y+5);

    cv::line(img, preCenter, Center, cv::Scalar(255,0,0));

    preCenter = Point(Center);
}

You can move center from previos frame to current frame, and then draw line.

cv::Point preCenter = cv::Point(0, 0); 

while(true)
{
    cv::Point Center center = cv::Point (preCenter.x+5, preCenter.y+5);

    cv::line(img, preCenter, Center, center, cv::Scalar(255,0,0));

    preCenter = Point(Center);
Point(center);
}