diff --git a/experiment/mnt.c b/experiment/mnt.c
index c4d66e7..9519123 100644
--- a/experiment/mnt.c
+++ b/experiment/mnt.c
@@ -13,12 +13,34 @@
 
 int		verbosity;
 
+/* ------------------------------------------------------------------- */
+/* for debug purpose */
+int printf_histo_gray(char *fname, int histo[], int nbre)
+{
+FILE		*fp;
+int		idx; 
+
+#if DEBUG_LEVEL
+fprintf(stderr, ">>> %s ( '%s' %p %d )\n", __func__,
+			fname, histo, nbre);
+#endif
+
+if (NULL == (fp = fopen(fname, "w"))) {
+	perror(fname);
+	exit(1);
+	}
+for (idx=0; idx<nbre; idx++) {
+	fprintf(fp, "%6d  %8d\n", idx, histo[idx]);
+	}
+fclose(fp);
+
+return -1;
+}
 /* ------------------------------------------------------------------- */
 /* for debug purpose */
 int calcul_histo_gray(FloatImg *img, char *fname, int nbslots, float *pmax)
 {
 int		offset, nbpix, ival;
-FILE		*fp;
 float		pixel, minp, maxp;
 int		*counts;
 
@@ -59,14 +81,10 @@ for (offset=0; offset<nbpix; offset++) {
 	// fprintf(stderr, "%6d    %10.6f    %i\n", offset, pixel, ival);
 	}
 
-if (NULL == (fp = fopen(fname, "w"))) {
-	perror(fname);
-	exit(1);
+if (NULL != fname)
+	{
+	printf_histo_gray(fname, counts, nbslots);
 	}
-for (ival=0; ival<nbslots; ival++) {
-	fprintf(fp, "%6d  %8d\n", ival, counts[ival]);
-	}
-fclose(fp);
 
 /* garbage collect stuff */
 free(counts);