Eigen: convert Matrix3d rotation to Quaternion

Question

I'm trying to convert a `Matrix3d` rotation to a `Quaternion`, but I got only weird compiler errors so far. The code I'm using is: ``` Quaternion getQuaternionFromRotationMatrix(const Matrix3d& mat) { AngleAxisd aa; aa = mat; Quaternion q = aa;// conversion error return q; } ``` And the compiler errors: ``` path/src/Utils.cpp: In function ‘Eigen::Quaternion Utils::getQuaternionFromRotationMatrix(const Matrix3d&)’: path/src/Utils.cpp:55:26: error: conversion from ‘Eigen::AngleAxisd {aka Eigen::AngleAxis}’ to non-scalar type ‘Eigen::Quaternion’ requested In file included from /usr/local/include/eigen3/Eigen/Core:283:0, from /usr/local/include/eigen3/Eigen/Dense:1, from path/include/Utils.h:4, from path/src/Utils.cpp:1: /usr/local/include/eigen3/Eigen/src/Core/Assign.h: In member function ‘Derived& Eigen::DenseBase::lazyAssign(const Eigen::DenseBase&) [with OtherDerived = Eigen::Matrix, Derived = Eigen::…

Accepted Answer

The constructors from an `AngleAxis` or `Matrix` are explicit meaning you have to write the conversion as follow:

```
Matrix3f mat;
Quaternionf q(mat);
```

or

```
Quaternionf q;
q = mat;
```

Same for `AngleAxis`.

Eigen: convert Matrix3d rotation to Quaternion

Solution

Problem