diff --git a/include/camera/Camera.h b/include/camera/Camera.h
index 964700b..e6d84ba 100644
--- a/include/camera/Camera.h
+++ b/include/camera/Camera.h
@@ -83,6 +83,18 @@
 enum {
     CAMERA_CMD_START_SMOOTH_ZOOM     = 1,
     CAMERA_CMD_STOP_SMOOTH_ZOOM      = 2,
+    // Set the clockwise rotation of preview display (setPreviewDisplay) in
+    // degrees. This affects the preview frames and the picture displayed after
+    // snapshot. This method is useful for portrait mode applications. Note that
+    // preview display of front-facing cameras is flipped horizontally before
+    // the rotation, that is, the image is reflected along the central vertical
+    // axis of the camera sensor. So the users can see themselves as looking
+    // into a mirror.
+    //
+    // This does not affect the order of byte array of CAMERA_MSG_PREVIEW_FRAME,
+    // CAMERA_MSG_VIDEO_FRAME, CAMERA_MSG_POSTVIEW_FRAME, CAMERA_MSG_RAW_IMAGE,
+    // or CAMERA_MSG_COMPRESSED_IMAGE. This is not allowed to be set during
+    // preview.
     CAMERA_CMD_SET_DISPLAY_ORIENTATION = 3,
 };