avdevice/xcbgrab: Add option for grabbing a window

The option allows to select a specific window instead of the whole
screen.

Reviewed-by: Andriy Gelman <andriy.gelman@gmail.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>

1 files changed, 14 insertions, 1 deletions

diff --git a/doc/indevs.texi b/doc/indevs.texi
index 3924d03908..b377924c2f 100644
--- a/doc/indevs.texi
+++ b/doc/indevs.texi
@@ -1564,8 +1564,21 @@ With @var{follow_mouse}:
 ffmpeg -f x11grab -follow_mouse centered -show_region 1 -framerate 25 -video_size cif -i :0.0 out.mpg
 @end example
 
+@item window_id
+Grab this window, instead of the whole screen. Default value is 0, which maps to
+the whole screen (root window).
+
+The id of a window can be found using the @command{xwininfo} program, possibly with options -tree and
+-root.
+
+If the window is later enlarged, the new area is not recorded. Video ends when
+the window is closed, unmapped (i.e., iconified) or shrunk beyond the video
+size (which defaults to the initial window size).
+
+This option disables options @option{follow_mouse} and @option{select_region}.
+
 @item video_size
-Set the video frame size. Default is the full desktop.
+Set the video frame size. Default is the full desktop or window.
 
 @item grab_x
 @item grab_y