Add some math verifying relationship between get_viewer_pose and get_pose

1 files changed, 8 insertions, 2 deletions

diff --git a/components/script/dom/xrreferencespace.rs b/components/script/dom/xrreferencespace.rs
index 804768eee69..664dc4f0e23 100644
--- a/components/script/dom/xrreferencespace.rs
+++ b/components/script/dom/xrreferencespace.rs
@@ -56,12 +56,18 @@ impl XRReferenceSpaceMethods for XRReferenceSpace {
 impl XRReferenceSpace {
     /// Gets pose of the viewer with respect to this space
     ///
-    /// This is equivalent to `get_pose(self).inverse() * get_pose(viewerSpace)`, however
-    /// we specialize it to be efficient
+    /// This is equivalent to `get_pose(self).inverse() * get_pose(viewerSpace)` (in column vector notation),
+    /// however we specialize it to be efficient
     pub fn get_viewer_pose(&self, base_pose: &WebVRFrameData) -> RigidTransform3D<f64> {
         let pose = self.get_unoffset_viewer_pose(base_pose);
 
         // This may change, see https://github.com/immersive-web/webxr/issues/567
+
+        // in column-vector notation,
+        // get_viewer_pose(space) = get_pose(space).inverse() * get_pose(viewer_space)
+        //                        = (get_unoffset_pose(space) * offset).inverse() * get_pose(viewer_space)
+        //                        = offset.inverse() * get_unoffset_pose(space).inverse() * get_pose(viewer_space)
+        //                        = offset.inverse() * get_unoffset_viewer_pose(space)
         let offset = self.transform.get().transform();
         let inverse = offset.inverse();
         inverse.pre_mul(&pose)