diff --git a/util/ansi-print.hh b/util/ansi-print.hh
index 6476668a8..5439081be 100644
--- a/util/ansi-print.hh
+++ b/util/ansi-print.hh
@@ -196,10 +196,10 @@ struct biimage_t
     color_t bgc = color_t::from_ansi (bg);
     color_t fgc = color_t::from_ansi (fg);
     color_diff_t diff = fgc.diff (bgc);
-    int dd = diff.dot (diff);
+    int dd = sqrt (diff.dot (diff));
     for (unsigned int y = 0; y < height; y++)
       for (unsigned int x = 0; x < width; x++) {
-	int d = diff.dot (image (x, y).diff (bgc));
+	int d = sqrt (diff.dot (image (x, y).diff (bgc)));
 	(*this)(x, y) = d < 0 ? 0 : d > dd ? 255 : lround (d * 255. / dd);
       }
   }