diff --git a/lib/README.md b/lib/README.md
index a35b9ae..6f113ee 100644
--- a/lib/README.md
+++ b/lib/README.md
@@ -1,4 +1,25 @@
-# Floatimg, the base library
+# Floatimg, the base/core library
+
+## Data structures
+
+Image's pixels (floating point number, a `float` for C)
+are ordonned as an array of lines of single
+componant values. Every componant is in a separate array,
+who are *glued* together by an in-memory image descriptor.
+
+```
+#define MAGIC_FIMG	0x00F11F00
+typedef struct {
+  uint32_t	      magic;
+  int		      width;
+  int		      height;
+  int		      type;
+  float           fval;
+  int		      count;
+  float	          *R, *G, *B, *A;
+  FimgMetaData    mdatas;			// added 20230912
+  int		      reserved;
+	} FloatImg;
+```
 
-Need more explanations...
 
diff --git a/lib/fimg-2gray.c b/lib/fimg-2gray.c
index a83d714..6db15d4 100644
--- a/lib/fimg-2gray.c
+++ b/lib/fimg-2gray.c
@@ -17,6 +17,7 @@ extern int verbosity;		/* must be declared around main() */
 /* --------------------------------------------------------------------- */
 /*
  *	floating resultat img MUST be allocated before calling this func.
+ *	and MUST be of type 'gray image'.
  */
 int fimg_mk_gray_from(FloatImg *src, FloatImg *dst, int k)
 {
@@ -43,15 +44,12 @@ if (FIMG_TYPE_RGB != src->type) {
 if (FIMG_TYPE_GRAY != dst->type) {
 	fprintf(stderr, "%s : bad dst type %d on %p\n", __func__, 
 				dst->type, dst);
-	/*
-	 * may be we can convert dst picture on the fly ?
-	 */
+	/* may be we can convert dst picture on the fly ? */
 	return -9;
 	}
 
 /* entering the main processing loop */
 nbb = src->width * src->height;
-
 for (foo=0; foo<nbb; foo++) {
 	dst->R[foo] = ( (src->R[foo] * kr) +
 			(src->G[foo] * kg) +
@@ -79,7 +77,6 @@ if (FIMG_TYPE_RGB != src->type || FIMG_TYPE_RGB != dst->type) {
 
 /* entering the main processing loop */
 nbb = src->width * src->height;
-
 for (foo=0; foo<nbb; foo++) {
 	dst->R[foo] = dst->G[foo] = dst->B[foo] =
 			(src->R[foo] + src->G[foo] + src->B[foo]) / 3.0;
diff --git a/lib/fimg-file.c b/lib/fimg-file.c
index 4b1e071..da54676 100644
--- a/lib/fimg-file.c
+++ b/lib/fimg-file.c
@@ -87,15 +87,15 @@ if (FIMG_TYPE_RGB != fimg->type) {
 /* OK, we have to make a fake metadata chunk, if the caller
  * don't have one. Ugly, and nice */
 if (NULL == pmd) {
-	fprintf(stderr, ">>> %s making faked metadata\n", __func__);
+	// fprintf(stderr, ">>> %s making faked metadata\n", __func__);
 	foo = fimg_default_metadata(&fakemd, 3);
-	fprintf(stderr, "fakemd is at %p\n", &fakemd);
+	// fprintf(stderr, "fakemd is at %p\n", &fakemd);
 	pmd = &fakemd;
 	}
 else	{
 	/* OK, get some funky metadatas */
-	// fprintf(stderr, "%s get metadatas\n", __func__);
-	fprintf(stderr, "acqu fval=%f count=%d\n", pmd->fval, pmd->count);
+	fprintf(stderr, "%s: acqu fval=%f count=%d\n", __func__,
+				pmd->fval, pmd->count);
 	}
 
 fp = fopen(fname, "w");
diff --git a/lib/fimg-math.c b/lib/fimg-math.c
index 29cdb73..0122088 100644
--- a/lib/fimg-math.c
+++ b/lib/fimg-math.c
@@ -201,9 +201,7 @@ if (head->type != FIMG_TYPE_RGB) {
 	}
 
 for (foo=0; foo<(head->width*head->height); foo++) {
-	add  = head->R[foo];
-	add += head->G[foo];
-	add += head->B[foo];
+	add  = head->R[foo] + head->G[foo] + head->B[foo];
 	head->R[foo] = head->G[foo] = head->B[foo] = add / 3.0;
 	}
 return 0;
@@ -410,7 +408,6 @@ for (idx=0; idx<surface; idx++) {
 return 0;
 }
 /* ---------------------------------------------------------------- */
-
 /*            Warning: this function is _very_ slow                 */
 void fimg_drand48(FloatImg *fi, float kmul)
 {
@@ -434,7 +431,6 @@ for (idx=0; idx<nbre; idx++) {
 	fi->G[idx] = drand48() * kmul;
 	fi->B[idx] = drand48() * kmul;
 	}
-
 }
 /* ---------------------------------------------------------------- */
 
diff --git a/lib/metadata.c b/lib/metadata.c
index 53a2fba..c01b9a5 100644
--- a/lib/metadata.c
+++ b/lib/metadata.c
@@ -35,7 +35,7 @@ if (notused) {
 				notused, 		__func__);
 	}
 if (NULL != title) {
-	fprintf(stderr, "==== metadate for %s\n", title);
+	fprintf(stderr, "======== metadata for %s\n", title);
 	}
 if (verbosity) {
 	fprintf(stderr, "sizeof(metadata)     = %ld\n",    \
@@ -98,7 +98,7 @@ pmd->magic	   = MAGIC_MDATA;
 pmd->cpid          = getpid();		// we are the creator, no ?
 pmd->count         = 1;			// mmmm...
 pmd->fval          = 255.0;		// Ok
-strcpy(pmd->idcam, "<noname>");
+strcpy(pmd->idcam, "<noname camera>");
 pmd->origin        = 0xdeadbeef;	// classic joke, sorry
 pmd->reserved[0]   = bla;
 pmd->reserved[7]   = 0x55445544;	// magic number is a crime