How can i create a "Quaterniond from Matrix4d" and "Matrix4d from Quaterniond"?

Is there any "cast" or something like this?
Else, How can i make it?

Thank you.

you have to extract the linear part and be sure it represents a pure rotation, if so then you can do:

quat = mat44.topLeftCorner<3,3>();

otherwise you have to perform a polar decomposition to extract the rotational part:

Matrix3d mat33;
Afffine3d aff = mat44;
aff.computeRotationScaling(&mat33, 0);
quat = mat33;

see: http://eigen.tuxfamily.org/dox/classEig ... 8581d23227

Thank you. Works perfectly.