diff --git a/libs/geom/include/psemek/geom/matrix.hpp b/libs/geom/include/psemek/geom/matrix.hpp
index 83606b4a..99ddb673 100644
--- a/libs/geom/include/psemek/geom/matrix.hpp
+++ b/libs/geom/include/psemek/geom/matrix.hpp
@@ -150,6 +150,15 @@ namespace psemek::geom
 		return *this;
 	}
 
+	template <typename T, std::size_t R, std::size_t C>
+	matrix<T, R, C> operator - (matrix<T, R, C> const & m)
+	{
+		matrix<T, R, C> r;
+		for (std::size_t i = 0; i < R * C; ++i)
+			r.coords[i] = -m.coords[i];
+		return r;
+	}
+
 	template <typename T, std::size_t R, std::size_t C>
 	matrix<T, R, C> operator + (matrix<T, R, C> const & m1, matrix<T, R, C> const & m2)
 	{