points-projects.cpp

vector<Point3f> objectPointsForReprojection {
  objectPoints[2],                   // tip of nose
  objectPoints[2] + Point3f(0,0,15), // nose and Z-axis
  objectPoints[2] + Point3f(0,15,0), // nose and Y-axis
  objectPoints[2] + Point3f(15,0,0)  // nose and X-axis
};
//...
vector<Point2f> projectionOutput(objectPointsForReprojection.size());
projectPoints(objectPointsForReprojection, rvec, tvec, K, Mat(), projectionOutput);
arrowedLine(out, projectionOutput[0], projectionOutput[1], Scalar(255,255,0));
arrowedLine(out, projectionOutput[0], projectionOutput[2], Scalar(0,255,255));
arrowedLine(out, projectionOutput[0], projectionOutput[3], Scalar(255,0,255));