# Head nod detection from pitch angle

I am making an android application that will detect driver's fatigue from head nod pose.

What I did:

1. I have detected 68 facial landmarks from image using dlib library.
2. Used solvePnP to find out rotation matrix and from that matrix I have got roll, pitch and yaw angles. Now, I have to detect head nod from pitch angle.

My Problem:

1. How to set a threshold value so that an angle below or above the threshold can be said as a head nod?
2. It results some negative angles. What does a negative angle mean in 3 dimensional axis?

My code:

 void getEulerAngles(Mat &rotCamerMatrix,Vec3d &eulerAngles)
{
Mat cameraMatrix,rotMatrix,transVect,rotMatrixX,rotMatrixY,rotMatrixZ;
double* _r = rotCamerMatrix.ptr<double>();
double projMatrix[12] = {_r[0],_r[1],_r[2],0,
_r[3],_r[4],_r[5],0,
_r[6],_r[7],_r[8],0};

decomposeProjectionMatrix( Mat(3,4,CV_64FC1,projMatrix),
cameraMatrix,
rotMatrix,
transVect,
rotMatrixX,
rotMatrixY,
rotMatrixZ,
eulerAngles);

}
int renderToMat(std::vector<full_object_detection>& dets, Mat& dst)
{
Scalar color;
std::vector<cv::Point2d> image_points;
std::vector<cv::Point3d> model_points;
string disp;

int sz = 3,l;
color = Scalar(0,255,0);
double p1,p2,p3,leftear,rightear,ear=0,yawn=0.00,yaw=0.00,pitch=0.00,roll=0.00;
l=dets.size();
//I am calculating only for one face.. so assuming l=1
for(unsigned long idx = 0; idx < l; idx++)
{
image_points.push_back(
Point2d(dets[idx].part(30).x(),dets[idx].part(30).x() ) );
image_points.push_back(Point2d(dets[idx].part(8).x(),dets[idx].part(8).x() ) );
image_points.push_back(Point2d(dets[idx].part(36).x(),dets[idx].part(36).x() ) );
image_points.push_back(Point2d(dets[idx].part(45).x(),dets[idx].part(45).x() ) );
image_points.push_back(Point2d(dets[idx].part(48).x(),dets[idx].part(48).x() ) );
image_points.push_back(Point2d(dets[idx].part(54).x(),dets[idx].part(54).x() ) );
}
double focal_length = dst.cols;
Point2d center = cv::Point2d(dst.cols/2.00,dst.rows/2.00);
cv::Mat camera_matrix = (cv::Mat_<double>(3.00,3.00) << focal_length, 0, center.x, 0, focal_length, center.y, 0,
0, 1);
cv::Mat dist_coeffs = cv::Mat::zeros(4,1,cv::DataType<double>::type);
cv::Mat rotation_vector; //s Rotation in axis-angle form
cv::Mat translation_vector;
cv::Mat rotCamerMatrix1;

if(l!=0)
{
model_points.push_back(cv::Point3d(0.0f, 0.0f, 0.0f));
model_points.push_back(cv::Point3d(0.0f, -330.0f, -65.0f));
model_points.push_back(cv::Point3d(-225.0f, 170.0f, -135.0f));
model_points.push_back(cv::Point3d(225.0f, 170.0f, -135.0f));
model_points.push_back(cv::Point3d(-150.0f, -150.0f, -125.0f));
model_points.push_back(cv::Point3d(150.0f, -150.0f, -125.0f));
cv::solvePnP(model_points, image_points, camera_matrix, dist_coeffs,rotation_vector, translation_vector);
Rodrigues(rotation_vector,rotCamerMatrix1);
Vec3d eulerAngles;
getEulerAngles(rotCamerMatrix1,eulerAngles);

yaw   = eulerAngles[1];
pitch = eulerAngles[0];
roll  = eulerAngles[2];
/*My problem begins here. I don't know how to set a threshold value for pitch so that I can say a value below or
above the pitch is a head nod!*/

}
return 0;
}

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