Answers for "Mirror a FRotator along a plane"
https://answers.unrealengine.com/questions/758012/mirror-a-frotator-along-a-plane.html
The latest answers for the question "Mirror a FRotator along a plane"Answer by Firefly74
https://answers.unrealengine.com/answers/758023/view.html
From an [unity answer talking about UE4][1]
var objectQuat; // from world rotation
var mirrorNormalQuat = new Quaternion(plane.x, plane.y, plane.z, 0);
var reflectedQuat = mirrorNormalQuat * objectQuat * mirrorNormalQuat;
i'm not sur if the plane is represented by it's normal here, but i guess you can give it a try
edit: according to [this][2] plane is indeed represented by it's normal
[1]: https://forum.unity.com/threads/how-to-mirror-a-euler-angle-or-rotation.90650/#post-3359844
[2]: http://www.euclideanspace.com/maths/geometry/affine/reflection/quaternion/index.htmTue, 13 Feb 2018 23:36:36 GMTFirefly74