Matrix4 from Quaternion

bjadamson · 2017-07-12 02:27:52 UTC

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:

brendanzab/cgmath/blob/master/src/quaternion.rs#L589-L638




impl<S: BaseFloat> ApproxEq for Quaternion<S> {
type Epsilon = S::Epsilon;


#[inline]
fn default_epsilon() -> S::Epsilon {
    S::default_epsilon()
}


#[inline]
fn default_max_relative() -> S::Epsilon {
    S::default_max_relative()
}


#[inline]
fn default_max_ulps() -> u32 {
    S::default_max_ulps()
}


#[inline]

This file has been truncated. show original

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!!

bjadamson · 2017-07-12 05:56:38 UTC

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!

pengowen123 · 2017-07-16 19:37:14 UTC

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.

sebcrozet · 2017-07-19 20:38:00 UTC

@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.