diff --git a/libavcodec/vp6.c b/libavcodec/vp6.c
index 6d5d46b0f0..139105de19 100644
--- a/libavcodec/vp6.c
+++ b/libavcodec/vp6.c
@@ -19,6 +19,11 @@
  * You should have received a copy of the GNU Lesser General Public
  * License along with FFmpeg; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ *
+ * The VP6F decoder accept an optional 1 byte extradata. It is composed of:
+ *  - upper 4bits: difference between encoded width and visible width
+ *  - lower 4bits: difference between encoded height and visible height
  */
 
 #include <stdlib.h>
diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
index 87984b576d..ae89850eca 100644
--- a/libavformat/flvdec.c
+++ b/libavformat/flvdec.c
@@ -17,6 +17,12 @@
  * You should have received a copy of the GNU Lesser General Public
  * License along with FFmpeg; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ *
+ * This demuxer will generate a 1 byte extradata for VP6F content.
+ * It is composed of:
+ *  - upper 4bits: difference between encoded width and visible width
+ *  - lower 4bits: difference between encoded height and visible height
  */
 #include "avformat.h"