diff --git a/libs/geom/include/psemek/geom/interval.hpp b/libs/geom/include/psemek/geom/interval.hpp
index d247b18c..77baf57b 100644
--- a/libs/geom/include/psemek/geom/interval.hpp
+++ b/libs/geom/include/psemek/geom/interval.hpp
@@ -263,6 +263,12 @@ namespace psemek::geom
 		return i.min + i.length() * t;
 	}
 
+	template <typename T>
+	T unlerp(interval<T> const & i, T t)
+	{
+		return (t - i.min) / i.length();
+	}
+
 	template <typename T>
 	std::ostream & operator << (std::ostream & os, interval<T> const & i)
 	{