Camera Rotation SolvePnp

Question

Could somebody help me with the math on calculating the Y rotation of my camera? I use SolvePnP to get a rvec. From this rvec I want to know how much my camera is rotated around its own Y axis.

PointF[] ImagePointsCam = GetImagePointsCam(imgThres);
MCvPoint3D32f[] ObjectPointsCam = GetObjectPointsCam();
float f = 1.0f / (float)(Math.Tan(CameraFOV / 360.0 * 3.1415));
Matrix<float> CamMat = new Matrix<float>(3, 3);
CamMat.Data[0, 0] = (float)(f * 0.5 * ResolutionWidth);
CamMat.Data[0, 1] = 0;
CamMat.Data[0, 2] = (float)(ResolutionWidth / 2);
CamMat.Data[1, 0] = 0;
CamMat.Data[1, 1] = (float)(f * 0.5 * ResolutionHeight);
CamMat.Data[1, 2] = (float)(ResolutionHeight / 2);
CamMat.Data[2, 0] = 0;
CamMat.Data[2, 1] = 0;
CamMat.Data[2, 2] = 1;
Mat DistMat = new Mat();
Mat rvec = new Mat();
Mat tvec = new Mat();
CvInvoke.SolvePnP(ObjectPointsCam, ImagePointsCam, CamMat, DistMat, rvec, tvec, false, SolvePnpMethod.Iterative);
Mat R = new Mat();
CvInvoke.Rodrigues(rvec, R);

I'm not sure how to continue so that I can get a single value which represents the Y rotation of the camera.

EDIT

I managed to get it working in C# with DllImport and the answer of @Tahera.T. Now I get an error which I think has to do with the variables that I provide for the function. Could somebody have a look if I do it the right way?

[DllImport("opencv_world320.dll")]

private static extern void cvDecomposeProjectionMatrix(Mat projMatr, out Mat calibMatr, 
out Mat rotMatr, out Mat posVect, out Mat rotMatrX, out Mat rotMatrY, out Mat rotMatrZ,
out MCvPoint3D64f eulerAngles);

public MCvPoint3D64f GetEulerAngles(Mat R)
{
    Matrix<float> _r = new Matrix<float>(3,4);
    Matrix<float> _R = new Matrix<float>(R.Rows, R.Cols);
    R.CopyTo(_R);
    _r.Data[0, 0] = _R.Data[0, 0];
    _r.Data[0, 1] = _R.Data[0, 1];
    _r.Data[0, 2] = _R.Data[0, 2];
    _r.Data[0, 3] = 0;
    _r.Data[1, 0] = _R.Data[1, 0];
    _r.Data[1, 1] = _R.Data[1, 1];
    _r.Data[1, 2] = _R.Data[1, 2];
    _r.Data[1, 3] = 0;
    _r.Data[2, 0] = _R.Data[2, 0];
    _r.Data[2, 1] = _R.Data[2, 1];
    _r.Data[2, 2] = _R.Data[2, 2];
    _r.Data[2, 3] = 0;
    Mat projMatr = _r.Mat;

    cvDecomposeProjectionMatrix(projMatr, out Mat calibMatr, out Mat rotMatr, out Mat posVect, 
    out Mat rotMatrX, out Mat rotMatrY, out Mat rotMatrZ, out MCvPoint3D64f eulerAngles);

    return eulerAngles;
}

The error that I get now is:

System.Runtime.InteropServices.SEHException: 'External component has thrown an exception.'

With error code:

-2147467259

Do I call the function with the wrong variables?

Answer 1

Solvepnp does not give actual rotation values, it used to get real world point so in order to get actual rotation angles you will have to do few more steps : First use solvepnp to get rotation vector. Second use rodrigues to convert it to rotation matrix. Now use decomposeProjectionMatrix to compute eulers angle . These eulers angle give you the rotation along axis. You can try this :

void getEulerAngles(Mat &rotCamerMatrix,Vec3d &eulerAngles){

Mat cameraMatrix,rotMatrix,transVect,rotMatrixX,rotMatrixY,rotMatrixZ;
double* _r = rotCamerMatrix.ptr();
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);
}

Call the function like:

Vec3d eulerAngles;
getEulerAngles(rotCamerMatrix1,eulerAngles);

Where your rotation angles will be :

yaw = eulerAngles[1];
pitch = eulerAngles[0];
roll = eulerAngles[2];

Where pitch is your rotation along Y axis.