diff --git a/lib/contrast.c b/lib/contrast.c
index 377ae575..22323893 100644
--- a/lib/contrast.c
+++ b/lib/contrast.c
@@ -1,3 +1,7 @@
+/*
+ *		contrast.c - part of libfloatimg
+ */
+
 #include  <stdio.h>
 #include  <stdlib.h>
 #include  <unistd.h>
@@ -9,6 +13,9 @@
 extern int		verbosity;
 
 /* ---------------------------------------------------------------- */
+/*
+ *	if the second parameter is NULL, operate 'in-place'
+ */
 int fimg_square_root(FloatImg *s, FloatImg *d, double maxval)
 {
 int		nbre, idx;
@@ -23,6 +30,13 @@ if (s->type != FIMG_TYPE_RGB) {
 if (NULL==d) {
 	d = s;
 	}
+else	{
+	if (d->type != FIMG_TYPE_RGB) {
+		fprintf(stderr, "%s : dst type %d invalide\n",
+					__func__, d->type);
+		return -4;
+		}
+	}
 
 
 nbre = s->width * s->height * 3;
@@ -52,7 +66,7 @@ if (NULL==d) {
 else	{
 	if (d->type != FIMG_TYPE_RGB) {
 		fprintf(stderr, "%s : dst type %d invalide\n",
-					__func__, s->type);
+					__func__, d->type);
 		return -4;
 		}
 	}
@@ -73,7 +87,6 @@ return 0;
 		#macro Cos_01( X )
 		  (0.5-0.5*cos( 3.141592654 * X))
 		#end
-
  */
 int fimg_cos_01(FloatImg *s, FloatImg *d, double maxval)
 {
@@ -89,6 +102,13 @@ if (s->type != FIMG_TYPE_RGB) {
 if (NULL==d) {
 	d = s;
 	}
+else	{
+	if (d->type != FIMG_TYPE_RGB) {
+		fprintf(stderr, "%s : dst type %d invalide\n",
+					__func__, d->type);
+		return -4;
+		}
+	}
 
 nbre = s->width * s->height * 3;