better support of gray level

lib/fimg-2gray.c

@@ -13,7 +13,7 @@ extern int verbosity;		/* must be declared around main() */
 
 /* --------------------------------------------------------------------- */
 /*
- *	floating img MUST be allocated.
+ *	floating imgs MUST be allocated before calling this func.
  */
 int fimg_mk_gray_from(FloatImg *src, FloatImg*dst, int k)
 {
@@ -40,14 +40,13 @@ if (FIMG_TYPE_GRAY != dst->type) {
 nbb = src->width * src->height;
 
 for (foo=0; foo<nbb; foo++) {
-
 	dst->R[foo] = ( (src->R[foo] * kr) +
 			(src->G[foo] * kg) +
 			(src->B[foo] * kb) ) /
 				kdiv;
 	}
 
-return -1;
+return 0;
 }
 /* --------------------------------------------------------------------- */