Head pose estimation fails with specific image sizes

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I want to find angles of rotation of the head using opencv and dlib. So, I tried to use this code from the tutorial:

cv::Mat im = imread("img.jpg");

matrix<bgr_pixel> dlibImage;
assign_image(dlibImage, cv_image<bgr_pixel>(im));

auto face = detector(dlibImage)[0];
auto shape = sp(dlibImage, face);

// 2D image points.
std::vector<cv::Point2d> image_points;
image_points.push_back(cv::Point2d(shape.part(30).x(), shape.part(30).y()));    // Nose tip
image_points.push_back(cv::Point2d(shape.part(8).x(), shape.part(8).y()));    // Chin
image_points.push_back(cv::Point2d(shape.part(36).x(), shape.part(36).y()));     // Left eye left corner
image_points.push_back(cv::Point2d(shape.part(45).x(), shape.part(45).y()));    // Right eye right corner
image_points.push_back(cv::Point2d(shape.part(48).x(), shape.part(48).y()));    // Left Mouth corner
image_points.push_back(cv::Point2d(shape.part(54).x(), shape.part(54).y()));    // Right mouth corner


// 3D model points.
std::vector<cv::Point3d> model_points;
model_points.push_back(cv::Point3d(0.0f, 0.0f, 0.0f));               // Nose tip
model_points.push_back(cv::Point3d(0.0f, -330.0f, -65.0f));          // Chin
model_points.push_back(cv::Point3d(-225.0f, 170.0f, -135.0f));       // Left eye left corner
model_points.push_back(cv::Point3d(225.0f, 170.0f, -135.0f));        // Right eye right corner
model_points.push_back(cv::Point3d(-150.0f, -150.0f, -125.0f));      // Left Mouth corner
model_points.push_back(cv::Point3d(150.0f, -150.0f, -125.0f));       // Right mouth corner

// Camera internals
double focal_length = im.cols; // Approximate focal length.
Point2d center = cv::Point2d(im.cols/2,im.rows/2);
cv::Mat camera_matrix = (cv::Mat_<double>(3,3) << 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); // Assuming no lens distortion

cout << "Camera Matrix " << endl << camera_matrix << endl ;
// Output rotation and translation
cv::Mat rotation_vector; // Rotation in axis-angle form

cv::Mat translation_vector;

// Solve for pose
cv::solvePnP(model_points, image_points, camera_matrix, dist_coeffs, rotation_vector, translation_vector);


// Project a 3D point (0, 0, 1000.0) onto the image plane.
// We use this to draw a line sticking out of the nose

std::vector<Point3d> nose_end_point3D;
std::vector<Point2d> nose_end_point2D;
nose_end_point3D.push_back(Point3d(0,0,1000.0));

projectPoints(nose_end_point3D, rotation_vector, translation_vector, camera_matrix, dist_coeffs, nose_end_point2D);


for(int i=0; i < image_points.size(); i++)
{
    circle(im, image_points[i], 3, Scalar(0,0,255), -1);
}

cv::line(im,image_points[0], nose_end_point2D[0], cv::Scalar(255,0,0), 2);

cout << "Rotation Vector " << endl << rotation_vector << endl;
cout << "Translation Vector" << endl << translation_vector << endl;

cout <<  nose_end_point2D << endl;

// Display image.
cv::imshow("Output", im);
cv::waitKey(0);

But, unfortunately, I get completely different results depending on the size of the same image!

If I use this img.jpg which has size 299x299 px(many sizes are ok, but we take the nearest), then all ok and I get right result:

Output:

Rotation Vector 
[-0,04450161828760668;
 -2,133664002574712;
 -0,2208024002827168]

But if I use this img.jpg which has size 298x298 px, then I get absolutely wrong result:

Output:

Rotation Vector 
[-2,999117288644056;
 0,0777816930911016;
 -0,7573144061217354]

I also understood that it ... (more)