Rodrigues into Eulerangles and vice versa

Question

I am useing solvePnP and i am getting a translation vector. Now i need to compare some euler angles with those results from solvePnP. And i want/need to transfer the euler

Answer 1

Adding a more concrete answer to supplement the other answers here. If you desire a direction vector instead of Euler angles, the process can indeed be simplified with a matrix multiplication, here's a quick solution:

// The output is a direction vector in OpenGL coordinate system:
// +X is Right on the screen, +Y is Up, +Z is INTO the screen
static Vector3 ToDirectionVectorGL(const Mat& rodrigues1x3) noexcept
{
    Mat rotation3x3;
    cv::Rodrigues(rodrigues1x3, rotation3x3);

    // direction OUT of the screen in CV coordinate system, because we care
    // about objects facing towards us - you can change this to anything
    // OpenCV coordsys: +X is Right on the screen, +Y is Down on the screen,
    //                  +Z is INTO the screen
    Vec3d axis{ 0, 0, -1 }; 
    Mat direction = rotation3x3 * Mat(axis, false);

    // normalize to a unit vector
    double dirX = direction.at(0);
    double dirY = direction.at(1);
    double dirZ = direction.at(2);
    double len = sqrt(dirX*dirX + dirY*dirY + dirZ*dirZ);
    dirX /= len;
    dirY /= len;
    dirZ /= len;
    // Convert from OpenCV to OpenGL 3D coordinate system
    return { float(dirX), float(-dirY), float(dirZ) };
}

If you are using this for head pose estimation, ensure the Rodrigues 1x3 rotation is formed properly around {0,0,0} or you might get odd results.