move quat rotation to fn

src/action/systems.rs

@@ -22,6 +22,17 @@ fn yaw_to_angle(yaw: f32) -> f32 {
     yaw.powi(3) * FRAC_PI_4
 }
 
+fn rotate_point(pt: &Vec3, rot: &Quat) -> Vec3 {
+    // thanks to https://danceswithcode.net/engineeringnotes/quaternions/quaternions.html
+    let [x, y, z] = pt.to_array();
+    let qpt = Quat::from_xyzw(x, y, z, 0.0);
+    // p' = rot^-1 * qpt * rot
+    let rot_qpt = rot.inverse() * qpt * *rot;
+
+    // why does this need to be inverted???
+    -Vec3::from_array([rot_qpt.x, rot_qpt.y, rot_qpt.z])
+}
+
 /// The gravity vector points from the cyberbike to the center of the planet.
 pub(super) fn gravity(
     mut query: Query<(&Transform, &mut ExternalForce), With<CyberBikeBody>>,
@@ -68,13 +79,9 @@ pub(super) fn falling_cat(
 ) {
     let (xform, mut forces, mut control_vars) = bike_query.single_mut();
     let world_up = xform.translation.normalize();
-    let [x, y, z] = world_up.to_array();
-    // turn our "world up" point into a Quat
-    let qup = Quat::from_xyzw(x, y, z, 0.0);
-    // p' = rot^-1 * qup * rot
     let rot = Quat::from_axis_angle(xform.back(), lean.lean);
-    let rot_qup = rot.inverse() * qup * rot;
-    let target_up = -Vec3::from_array([rot_qup.x, rot_qup.y, rot_qup.z]).normalize();
+    let target_up = rotate_point(&world_up, &rot).normalize();
+
     let bike_right = xform.right();
 
     let world_right = xform.forward().cross(world_up).normalize();