diff --git a/lib/fimg-2gray.c b/lib/fimg-2gray.c
index a9a2ea89..5aca461b 100644
--- a/lib/fimg-2gray.c
+++ b/lib/fimg-2gray.c
@@ -52,6 +52,7 @@ for (foo=0; foo<nbb; foo++) {
 return 0;
 }
 /* --------------------------------------------------------------------- */
+/* this function can work 'in place'					 */
 int fimg_desaturate(FloatImg *src, FloatImg *dst, int notused)
 {
 int		foo, nbb;