From dfdeb9b28474f117d05f7358df6f51311b8b9ae6 Mon Sep 17 00:00:00 2001
From: Daniel Scharrer <daniel@constexpr.org>
Date: Wed, 24 Jun 2020 23:32:48 +0200
Subject: [PATCH] Fix singularity in quaternion to euler angle roll conversion

---
 glm/gtc/quaternion.inl | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/glm/gtc/quaternion.inl b/glm/gtc/quaternion.inl
index 06f9f020..e1ef0321 100644
--- a/glm/gtc/quaternion.inl
+++ b/glm/gtc/quaternion.inl
@@ -15,7 +15,13 @@ namespace glm
 	template<typename T, qualifier Q>
 	GLM_FUNC_QUALIFIER T roll(qua<T, Q> const& q)
 	{
-		return static_cast<T>(atan(static_cast<T>(2) * (q.x * q.y + q.w * q.z), q.w * q.w + q.x * q.x - q.y * q.y - q.z * q.z));
+		T const y = static_cast<T>(2) * (q.x * q.y + q.w * q.z);
+		T const x = q.w * q.w + q.x * q.x - q.y * q.y - q.z * q.z;
+
+		if(all(equal(vec<2, T, Q>(x, y), vec<2, T, Q>(0), epsilon<T>()))) //avoid atan2(0,0) - handle singularity - Matiis
+			return static_cast<T>(0);
+
+		return static_cast<T>(atan(y, x));
 	}
 
 	template<typename T, qualifier Q>