# Rotation matrix to euler angles with opencv c++ correct value wrong place

I am working on a project wich involves Aruco markers and opencv. I am quite far in the project progress. I can read the rotation vectors and convert them to a rodrigues matrix using rodrigues() from opencv.

This is a example of a rodrigues matrix I get:

[0,1,0;

1,0,0;

0,0,-1]

I use the following code.

Mat m33(3, 3, CV_64F);
Mat measured_eulers(3, 1, CV_64F);
Rodrigues(rotationVectors, m33);

measured_eulers = rot2euler(m33);

Degree_euler = measured_eulers * 180 / CV_PI;


I use the predefined rot2euler to convert from rodrigues matrix to euler angles. And I convert the received radians to degrees.

rot2euler looks like the following.

Mat rot2euler(const Mat & rotationMatrix)
{
Mat euler(3, 1, CV_64F);

double m00 = rotationMatrix.at<double>(0, 0);
double m02 = rotationMatrix.at<double>(0, 2);
double m10 = rotationMatrix.at<double>(1, 0);
double m11 = rotationMatrix.at<double>(1, 1);
double m12 = rotationMatrix.at<double>(1, 2);
double m20 = rotationMatrix.at<double>(2, 0);
double m22 = rotationMatrix.at<double>(2, 2);

double x, y, z;

// Assuming the angles are in radians.
if (m10 > 0.998) { // singularity at north pole
x = 0;
y = CV_PI / 2;
z = atan2(m02, m22);
}
else if (m10 < -0.998) { // singularity at south pole
x = 0;
y = -CV_PI / 2;
z = atan2(m02, m22);
}
else
{
x = atan2(-m12, m11);
y = asin(m10);
z = atan2(-m20, m00);
}

euler.at<double>(0) = x;
euler.at<double>(1) = y;
euler.at<double>(2) = z;

return euler;
}


If I use the rodrigues matrix I give as an example I get the following euler angles.

[0; 90; -180]

But I am suppose to get the following.

[-180; 0; 90]

When is use this tool

[danceswithcode.net/engineeringnotes/rotations_in_3d/demo3D/rotations_in_3d_tool.html]

You can see that [0; 90; -180] doesn't match the rodrigues matrix but [-180; 0; 90] does. (I am aware of the fact that the tool works with ZYX coordinates)

So the problem is I get the correct values but in a wrong order.

Another problem is that this isn't always the case. For example rodrigues matrix:

[1,0,0;

0,-1,0;

0,0,-1]

Provides me the correct euler angles.

If someone knows a solution to the problem or can provide me with a explanation how the rot2euler function works exactly. It will be higly appreciated.

Kind Regards

Brent Convens

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