diff --git a/lib/fimg-math.c b/lib/fimg-math.c
index 4647a25..097a5b8 100644
--- a/lib/fimg-math.c
+++ b/lib/fimg-math.c
@@ -66,6 +66,9 @@ for (idx=0; idx<4; idx++)	means[idx] /= (float)surface;
 return 0;
 }
 /* ---------------------------------------------------------------- */
+/*
+ *	more elaborate functions are in fimg-2gray.c
+ */
 int fimg_to_gray(FloatImg *head)
 {
 float		add;