Add XRRigidTransform::Inverse

components/script/dom/xrrigidtransform.rs

@@ -46,7 +46,8 @@ impl XRRigidTransform {
             reflector_: Reflector::new(),
             position: Dom::from_ref(position),
             orientation: Dom::from_ref(orientation),
-            translate, rotate
+            translate,
+            rotate,
         }
     }
 
@@ -98,6 +99,42 @@ impl XRRigidTransformMethods for XRRigidTransform {
     fn Orientation(&self) -> DomRoot<DOMPointReadOnly> {
         DomRoot::from_ref(&self.orientation)
     }
+    // https://immersive-web.github.io/webxr/#dom-xrrigidtransform-inverse
+    fn Inverse(&self) -> DomRoot<XRRigidTransform> {
+        // An XRRigidTransform is a rotation and a translation,
+        // i.e. T * R
+        //
+        // Its inverse is (T * R)^-1
+        //    = R^-1 * T^-1
+        //    = R^-1 * T^-1 * (R * R^-1)
+        //    = (R^-1 * T^-1 * R) * R^-1
+        //    = T' * R^-1
+        //    = T' * R'
+        //
+        //  (R^-1 * T^-1 * R) is a translation matrix, and R^-1 is a
+        //  rotation matrix, so we can use these in the new rigid transform
+        let r_1 = self.rotate.inverse();
+        let t_1 = self
+            .translate
+            .inverse()
+            .expect("translation matrices should be invertible");
+        let t_p = r_1
+            .to_transform()
+            .post_mul(&t_1)
+            .post_mul(&self.rotate.to_transform());
+
+        let global = self.global();
+        let position =
+            DOMPointReadOnly::new(&global, t_p.m41.into(), t_p.m42.into(), t_p.m43.into(), 1.);
+        let orientation = DOMPointReadOnly::new(
+            &global,
+            r_1.i.into(),
+            r_1.j.into(),
+            r_1.k.into(),
+            r_1.r.into(),
+        );
+        XRRigidTransform::new(global.as_window(), &position, &orientation)
+    }
 }
 
 impl XRRigidTransform {