diff --git a/libs/geom/include/psemek/geom/quaternion.hpp b/libs/geom/include/psemek/geom/quaternion.hpp
index c7abc87e..ade080aa 100644
--- a/libs/geom/include/psemek/geom/quaternion.hpp
+++ b/libs/geom/include/psemek/geom/quaternion.hpp
@@ -246,6 +246,21 @@ namespace psemek::geom
 		return std::exp(q[3]) * quaternion<T>({v[0] * s, v[1] * s, v[2] * s, std::cos(l)});
 	}
 
+	template <typename T>
+	quaternion<T> log(quaternion<T> const & q)
+	{
+		auto v = vector{q[0], q[1], q[2]};
+		auto l = length(v);
+		auto n = length(q);
+
+		if (l == T{})
+			v = v.zero();
+		else
+			v = (v / l) * std::acos(q[3] / n);
+
+		return quaternion<T>{{v[0], v[1], v[2], std::log(n)}};
+	}
+
 	template <typename T>
 	std::ostream & operator << (std::ostream & os, quaternion<T> const & q)
 	{