diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 963c9dd97e..68a7888686 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -601,6 +601,8 @@ typedef struct AVPanScan{
 \
     /**\
      * is this picture used as reference\
+     * The values for this are the same as the MpegEncContext.picture_structure\
+     * variable, that is 1->top field, 2->bottom field, 3->frame/both fields.\
      * - encoding: unused\
      * - decoding: Set by libavcodec. (before get_buffer() call)).\
      */\