diff --git a/doc/the_floatimg_hack.tex b/doc/the_floatimg_hack.tex
index 63a12c7..1b47989 100644
--- a/doc/the_floatimg_hack.tex
+++ b/doc/the_floatimg_hack.tex
@@ -10,7 +10,7 @@
 
 % \lstset{frame=single}           % dessin d'un cadre autour du listing
 \lstset{basicstyle=\ttfamily\small}
-\lstset{aboveskip=0.333em,belowskip=0.666em}
+\lstset{aboveskip=0.222em,belowskip=0.222em}
 
 \usepackage{babel}
 
@@ -355,9 +355,12 @@ classiques : gray, rgb et rgba.  et expliquées quelques lignes plus haut.
 
 Comme vous allez le voir plus loin, il y a plein de fonctions qui
 prennent en argument deux images: une source et une destination.
+Et dans la plupart des cas, ces deux images doivent être compatibles,
+c'est à dire même type et mêmes dimensions.
 
 \begin{lstlisting}
-int fimg_images_compatible(FloatImg *a, FloatImg *b);
+/* return 0 if compatible */
+int fimg_images_not_compatible(FloatImg *a, FloatImg *b);
 \end{lstlisting}
 
 
diff --git a/floatimg.h b/floatimg.h
index b6bdfda..5732591 100644
--- a/floatimg.h
+++ b/floatimg.h
@@ -68,7 +68,7 @@ int   fimg_add_rgb(FloatImg *head, int x, int y, float r, float g, float b);
 int   fimg_rgb_constant(FloatImg *head, float r, float g, float b);
 
 /*	--> lib/fimg-compare.c		*/
-int fimg_images_compatible(FloatImg *a, FloatImg *b);
+int fimg_images_not_compatible(FloatImg *a, FloatImg *b);
 
 int fimg_interpolate(FloatImg *s1, FloatImg *s2, FloatImg *d, float coef);
 
diff --git a/lib/fimg-compare.c b/lib/fimg-compare.c
index 8576e38..c0b6a72 100644
--- a/lib/fimg-compare.c
+++ b/lib/fimg-compare.c
@@ -16,7 +16,7 @@ extern int verbosity;		/* must be declared around main() */
 /*
  *	return 0 if images are compatibles
  */
-int fimg_images_compatible(FloatImg *a, FloatImg *b)
+int fimg_images_not_compatible(FloatImg *a, FloatImg *b)
 {
 #if DEBUG_LEVEL > 1
 fprintf(stderr, ">>> %s ( %p %p )\n", __func__, a, b);
diff --git a/lib/fimg-core.c b/lib/fimg-core.c
index 3896ced..430837c 100644
--- a/lib/fimg-core.c
+++ b/lib/fimg-core.c
@@ -194,7 +194,7 @@ int		foo;
 fprintf(stderr, ">>> %-25s ( %p %p )\n", __func__, from, to);
 #endif
 
-foo = fimg_images_compatible(from, to);
+foo = fimg_images_not_compatible(from, to);
 if (foo) {
 	fprintf(stderr, "%s: pics not compatible (%d)\n", __func__, foo);
 	return foo;
diff --git a/lib/interpolate.c b/lib/interpolate.c
index 737c48b..b64f111 100644
--- a/lib/interpolate.c
+++ b/lib/interpolate.c
@@ -46,13 +46,13 @@ fprintf(stderr, ">>> %s ( %p %p %p %f )\n", __func__,
 				s1, s2, d, coef);
 #endif
 
-foo = fimg_images_compatible(s1, s2);
+foo = fimg_images_not_compatible(s1, s2);
 if (foo) {
 	fprintf(stderr, "compat s1 s2 -> %d\n", foo);
 	return foo;
 	}
 
-foo = fimg_images_compatible(s1, d);
+foo = fimg_images_not_compatible(s1, d);
 if (foo) {
 	fprintf(stderr, "compat s1 d  -> %d\n", foo);
 	return foo;
diff --git a/tools/fimgops.c b/tools/fimgops.c
index 02f17bf..91bc9b2 100644
--- a/tools/fimgops.c
+++ b/tools/fimgops.c
@@ -153,7 +153,7 @@ if (verbosity > 1) {				/* please, debug me */
 	fimg_describe(&srcB, argv[optind+1]);
 	}
 
-foo = fimg_images_compatible(&srcA, &srcB);
+foo = fimg_images_not_compatible(&srcA, &srcB);
 if (foo) {
 	fprintf(stderr, "images are not compatibles, %d\n", foo);
 	exit(4);