Matrix4 from Quaternion


Hi, hopefully this is an easy question!

I’m converting some code from cgmath to nalgebra, and I can’t find a matching use case.

I’m trying to convert a quaternion to a mat4.

In cgmath, there is this implementation of From<> here:

I can’t find matching functionality in the docs or the code for nalgebra, does this functionality exist under a function I can’t find? Basically, I can’t figure out how to (using the library, I could do it by hand) convert a quat -> mat4.

Anyone know how I would do that using this library? Thanks a bunch!!


A little sad, I just tried to implement this for myself but it seems the coherence rules prevent me. I’m still pretty new so I hope this doesn’t turn out to be too easy!


Not every quaternion can be used as a rotation, you must use a UnitQuaternionBase. Some methods are only implemented for it, including the one you want: to_rotation_matrix. This method can be found here. You can get a UnitQuaternionBase by calling Unit::new_normalize on a quaternion.


@bjadamson In addition to what @pengowen123 said, the method .to_rotation_matrix() will return a Rotation3, i.e., a 3x3 rotation matrix. If you want a 4x4 matrix, you have to call .to_homogeneous() instead (because a 4x4 rotation matrix is actually the 3D rotation expressed in homogeneous coordinates). For example:

use na::UnitQuaternion;

fn main() {
    let q = UnitQuaternion::<f64>::identity();
    let mat4 = q.to_homogeneous();

Also note that the type UnitQuaternion should be used instead of UnitQuaternionBase. The latter which will be removed in a future release.