Avoid NaNs in Rotation::get_angle

src/rigid.rs

@@ -136,7 +136,7 @@ mod float {
             //        = R' * T' * T2
             //        = R' * T''
             //
-            // (R2^-1 * T2 * R2^) = T' = T2 rotated by R2
+            // (R2^-1 * T1 * R2) = T' = T2 rotated by R2
             // R1 * R2  = R'
             // T' * T2 = T'' = vector addition of translations T2 and T'
 

src/rotation.rs

@@ -405,6 +405,8 @@ where
     T: Copy,
 {
     /// Returns the vector part (i, j, k) of this quaternion.
+    ///
+    /// The vector part represents the axis of the rotation.
     #[inline]
     pub fn vector_part(&self) -> Vector3D<T, UnknownUnit> {
         vec3(self.i, self.j, self.k)
@@ -807,9 +809,14 @@ mod rotation3d_float {
         #[inline]
         /// Returns the angle of rotation about the stored axis.
         pub fn get_angle(&self) -> Angle<T> {
-            let two = T::one() + T::one();
-            let angle = two * self.r.acos();
-            return Angle::radians(angle);
+            let one = T::one();
+            let two = one + one;
+            // Clamp r to acos's safe range [-1, 1] to avoid NaNs if float precision
+            // causes the value to be slightly out of range.
+            let r = self.r.max(-one).min(one);
+            let angle = two * r.acos();
+
+            Angle::radians(angle)
         }
     }
 

src/transform3d.rs

@@ -55,7 +55,7 @@ use serde::{Deserialize, Serialize};
 /// ```text
 ///  |x'|   | m11 m12 m13 m14 |   |x|
 ///  |y'|   | m21 m22 m23 m24 |   |y|
-///  |z'| = | m31 m32 m33 m34 | x |y|
+///  |z'| = | m31 m32 m33 m34 | x |z|
 ///  |w |   | m41 m42 m43 m44 |   |1|
 /// ```
 ///