[FFmpeg-devel] x11grab: capture a window instead of the whole screen

In my previous email the patch got mangled by the web mail interface, so I am
sending it again. Sorry for the duplicate.

This patch allows ffmpeg to capture a specific window instead of the whole
screen. An example:

ffmpeg -f x11grab -window_id 0xa00000a -i :0 output.mp4

---
  doc/indevs.texi       |  3 +++
  libavdevice/xcbgrab.c | 19 ++++++++++++-------
  2 files changed, 15 insertions(+), 7 deletions(-)

Looks fine.


On Tue, Jan 26, 2021 at 11:24 AM <sgerwk-at-aol.com@ffmpeg.org> wrote:

> In my previous email the patch got mangled by the web mail interface, so I
> am
> sending it again. Sorry for the duplicate.
>
> This patch allows ffmpeg to capture a specific window instead of the whole
> screen. An example:
>
> ffmpeg -f x11grab -window_id 0xa00000a -i :0 output.mp4
>
> ---
>   doc/indevs.texi       |  3 +++
>   libavdevice/xcbgrab.c | 19 ++++++++++++-------
>   2 files changed, 15 insertions(+), 7 deletions(-)
>
> diff --git a/doc/indevs.texi b/doc/indevs.texi
> index 3924d03..0738919 100644
> --- a/doc/indevs.texi
> +++ b/doc/indevs.texi
> @@ -1564,6 +1564,9 @@ 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.
> +
>   @item video_size
>   Set the video frame size. Default is the full desktop.
>
> diff --git a/libavdevice/xcbgrab.c b/libavdevice/xcbgrab.c
> index 95bdc8a..6e4ef5d 100644
> --- a/libavdevice/xcbgrab.c
> +++ b/libavdevice/xcbgrab.c
> @@ -60,6 +60,7 @@ typedef struct XCBGrabContext {
>       AVRational time_base;
>       int64_t frame_duration;
>
> +    xcb_window_t window_id;
>       int x, y;
>       int width, height;
>       int frame_size;
> @@ -82,6 +83,7 @@ typedef struct XCBGrabContext {
>   #define OFFSET(x) offsetof(XCBGrabContext, x)
>   #define D AV_OPT_FLAG_DECODING_PARAM
>   static const AVOption options[] = {
> +    { "window_id", "Window to capture", OFFSET(window_id),
> AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, D },

      { "x", "Initial x coordinate.", OFFSET(x), AV_OPT_TYPE_INT, { .i64 =
> 0 }, 0, INT_MAX, D },
>       { "y", "Initial y coordinate.", OFFSET(y), AV_OPT_TYPE_INT, { .i64 =
> 0 }, 0, INT_MAX, D },
>       { "grab_x", "Initial x coordinate.", OFFSET(x), AV_OPT_TYPE_INT, {
> .i64 = 0 }, 0, INT_MAX, D },
> @@ -157,7 +159,7 @@ static int xcbgrab_frame(AVFormatContext *s, AVPacket
> *pkt)
>       XCBGrabContext *c = s->priv_data;
>       xcb_get_image_cookie_t iq;
>       xcb_get_image_reply_t *img;
> -    xcb_drawable_t drawable = c->screen->root;
> +    xcb_drawable_t drawable = c->window_id;
>       xcb_generic_error_t *e = NULL;
>       uint8_t *data;
>       int length;
> @@ -267,7 +269,7 @@ static int xcbgrab_frame_shm(AVFormatContext *s,
> AVPacket *pkt)
>       XCBGrabContext *c = s->priv_data;
>       xcb_shm_get_image_cookie_t iq;
>       xcb_shm_get_image_reply_t *img;
> -    xcb_drawable_t drawable = c->screen->root;
> +    xcb_drawable_t drawable = c->window_id;
>       xcb_generic_error_t *e = NULL;
>       AVBufferRef *buf;
>       xcb_shm_seg_t segment;
> @@ -566,7 +568,7 @@ static int create_stream(AVFormatContext *s)
>
>       avpriv_set_pts_info(st, 64, 1, 1000000);
>
> -    gc  = xcb_get_geometry(c->conn, c->screen->root);
> +    gc  = xcb_get_geometry(c->conn, c->window_id);
>       geo = xcb_get_geometry_reply(c->conn, gc, NULL);
>       if (!geo)
>           return AVERROR_EXTERNAL;
> @@ -745,7 +747,7 @@ static int select_region(AVFormatContext *s)
>               press_position = (xcb_point_t){ press->event_x,
> press->event_y };
>               rectangle.x = press_position.x;
>               rectangle.y = press_position.y;
> -            xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
> +            xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
>               was_pressed = 1;
>               break;
>           }
> @@ -754,14 +756,14 @@ static int select_region(AVFormatContext *s)
>                   xcb_motion_notify_event_t *motion =
>                       (xcb_motion_notify_event_t *)event;
>                   xcb_point_t cursor_position = { motion->event_x,
> motion->event_y };
> -                xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
> +                xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
>                   rectangle = rectangle_from_corners(&press_position,
> &cursor_position);
> -                xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
> +                xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
>               }
>               break;
>           }
>           case XCB_BUTTON_RELEASE: {
> -            xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
> +            xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
>               done = 1;
>               break;
>           }
> @@ -825,6 +827,9 @@ static av_cold int xcbgrab_read_header(AVFormatContext
> *s)
>           return AVERROR(EIO);
>       }
>
> +    if (c->window_id == 0)
> +        c->window_id = c->screen->root;
> +
>       if (c->select_region) {
>           ret = select_region(s);
>           if (ret < 0) {
> --
> 2.29.2
>
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".

What happens if you use non-existing window?

Hi,

On Tue, 26. Jan 11:23, sgerwk-at-aol.com@ffmpeg.org wrote:
> In my previous email the patch got mangled by the web mail interface, so I am
> sending it again. Sorry for the duplicate.

I couldn't apply this version, but previous one applied ok:
https://patchwork.ffmpeg.org/project/ffmpeg/patch/942317827.2397172.1611160298506@mail.yahoo.com/

The author name appears as sgerwk-at-aol.com@ffmpeg.org. You can update
if you want something else.

Also the commit title should start with avdevice/xcbgrab: or lavd/xcbgrab:

> 
> This patch allows ffmpeg to capture a specific window instead of the whole
> screen. An example:

> 
> ffmpeg -f x11grab -window_id 0xa00000a -i :0 output.mp4

If you try with -show_region 1, the border gets drawn in the wrong place.

> 
> ---
>  doc/indevs.texi       |  3 +++
>  libavdevice/xcbgrab.c | 19 ++++++++++++-------
>  2 files changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/doc/indevs.texi b/doc/indevs.texi
> index 3924d03..0738919 100644
> --- a/doc/indevs.texi
> +++ b/doc/indevs.texi
> @@ -1564,6 +1564,9 @@ 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.
> +

I think it would be useful to say how to get a window id.
Probably worth adding what happens if the window size changes or it disappears.

>  @item video_size
>  Set the video frame size. Default is the full desktop.
> 
> diff --git a/libavdevice/xcbgrab.c b/libavdevice/xcbgrab.c
> index 95bdc8a..6e4ef5d 100644
> --- a/libavdevice/xcbgrab.c
> +++ b/libavdevice/xcbgrab.c
> @@ -60,6 +60,7 @@ typedef struct XCBGrabContext {
>      AVRational time_base;
>      int64_t frame_duration;
> 
> +    xcb_window_t window_id;
>      int x, y;
>      int width, height;
>      int frame_size;
> @@ -82,6 +83,7 @@ typedef struct XCBGrabContext {
>  #define OFFSET(x) offsetof(XCBGrabContext, x)
>  #define D AV_OPT_FLAG_DECODING_PARAM
>  static const AVOption options[] = {

> +    { "window_id", "Window to capture", OFFSET(window_id), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, D },

xcb_window_t is uint32_t, so max should be UINT32_MAX? 

>      { "x", "Initial x coordinate.", OFFSET(x), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, D },
>      { "y", "Initial y coordinate.", OFFSET(y), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, D },
>      { "grab_x", "Initial x coordinate.", OFFSET(x), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, D },
> @@ -157,7 +159,7 @@ static int xcbgrab_frame(AVFormatContext *s, AVPacket *pkt)
>      XCBGrabContext *c = s->priv_data;
>      xcb_get_image_cookie_t iq;
>      xcb_get_image_reply_t *img;
> -    xcb_drawable_t drawable = c->screen->root;
> +    xcb_drawable_t drawable = c->window_id;
>      xcb_generic_error_t *e = NULL;
>      uint8_t *data;
>      int length;
> @@ -267,7 +269,7 @@ static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
>      XCBGrabContext *c = s->priv_data;
>      xcb_shm_get_image_cookie_t iq;
>      xcb_shm_get_image_reply_t *img;
> -    xcb_drawable_t drawable = c->screen->root;
> +    xcb_drawable_t drawable = c->window_id;
>      xcb_generic_error_t *e = NULL;
>      AVBufferRef *buf;
>      xcb_shm_seg_t segment;
> @@ -566,7 +568,7 @@ static int create_stream(AVFormatContext *s)
> 
>      avpriv_set_pts_info(st, 64, 1, 1000000);
> 
> -    gc  = xcb_get_geometry(c->conn, c->screen->root);
> +    gc  = xcb_get_geometry(c->conn, c->window_id);
>      geo = xcb_get_geometry_reply(c->conn, gc, NULL);
>      if (!geo)
>          return AVERROR_EXTERNAL;
> @@ -745,7 +747,7 @@ static int select_region(AVFormatContext *s)
>              press_position = (xcb_point_t){ press->event_x, press->event_y };
>              rectangle.x = press_position.x;
>              rectangle.y = press_position.y;
> -            xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
> +            xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
>              was_pressed = 1;
>              break;
>          }
> @@ -754,14 +756,14 @@ static int select_region(AVFormatContext *s)
>                  xcb_motion_notify_event_t *motion =
>                      (xcb_motion_notify_event_t *)event;
>                  xcb_point_t cursor_position = { motion->event_x, motion->event_y };
> -                xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
> +                xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
>                  rectangle = rectangle_from_corners(&press_position, &cursor_position);
> -                xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
> +                xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
>              }
>              break;
>          }
>          case XCB_BUTTON_RELEASE: {
> -            xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
> +            xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
>              done = 1;
>              break;
>          }
> @@ -825,6 +827,9 @@ static av_cold int xcbgrab_read_header(AVFormatContext *s)
>          return AVERROR(EIO);
>      }
> 
> +    if (c->window_id == 0)
> +        c->window_id = c->screen->root;
> +

Can 0 be a valid window_id? 
Maybe it's better to use -1 for default. I saw the function xcb_generate_id() returns -1 on error. 

Thanks,

On Sat, 6 Feb 2021, Paul B Mahol wrote:

> What happens if you use non-existing window?

I added an error message for this case, but the output video is still created
(empty). Is there a value I can return to avoid this?

On Sun, 7 Feb 2021, Andriy Gelman wrote:

> Also the commit title should start with avdevice/xcbgrab: or lavd/xcbgrab:
>

done

>
> If you try with -show_region 1, the border gets drawn in the wrong place.
>

fixed

I also fixed the misalignement of draw_mouse I didn't notice last time.

>> 
>> +@item window_id
>> +Grab this window, instead of the whole screen.
>> +
>
> I think it would be useful to say how to get a window id.
> Probably worth adding what happens if the window size changes or it
> disappears.
>

Done. Recording also ends when the window is iconified or reduced in size beyond
the video size.

For now I disabled follow_mouse and select_region when window_id is given.

> xcb_window_t is uint32_t, so max should be UINT32_MAX?

Indeed.

>> @@ -825,6 +827,9 @@ static av_cold int xcbgrab_read_header(AVFormatContext
>> *s)
>>          return AVERROR(EIO);
>>      }
>> 
>> +    if (c->window_id == 0)
>> +        c->window_id = c->screen->root;
>> +
>
> Can 0 be a valid window_id? Maybe it's better to use -1 for default. I saw the
> function xcb_generate_id() returns -1 on error. 
>

0 is not a valid x11 identifier, but it's better to use the XCB_NONE macro for
it (in Xlib and higher-level libraries, it's "None")

Thanks!

> Thanks,
> -- 
> Andriy
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
From e13c1be7abd6989b3ad80fd8086fe6a0819fd810 Mon Sep 17 00:00:00 2001
From: sgerwk <sgerwk@aol.com>
Date: Wed, 10 Feb 2021 17:36:00 +0100
Subject: [PATCH] libavdevice/xcbgrab: option for grabbing a window instead of
 desktop

---
 doc/indevs.texi       | 14 +++++++-
 libavdevice/xcbgrab.c | 79 ++++++++++++++++++++++++++++++++-----------
 2 files changed, 72 insertions(+), 21 deletions(-)

diff --git a/doc/indevs.texi b/doc/indevs.texi
index 3924d03..48fd2b1 100644
--- a/doc/indevs.texi
+++ b/doc/indevs.texi
@@ -1564,8 +1564,20 @@ 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.
+
+The id of a window can be found by xwininfo(1), 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
diff --git a/libavdevice/xcbgrab.c b/libavdevice/xcbgrab.c
index be5d5ea..7697090 100644
--- a/libavdevice/xcbgrab.c
+++ b/libavdevice/xcbgrab.c
@@ -60,6 +60,8 @@ typedef struct XCBGrabContext {
     AVRational time_base;
     int64_t frame_duration;
 
+    xcb_window_t window_id;
+    int win_x, win_y;
     int x, y;
     int width, height;
     int frame_size;
@@ -82,6 +84,7 @@ typedef struct XCBGrabContext {
 #define OFFSET(x) offsetof(XCBGrabContext, x)
 #define D AV_OPT_FLAG_DECODING_PARAM
 static const AVOption options[] = {
+    { "window_id", "Window to capture", OFFSET(window_id), AV_OPT_TYPE_INT, { .i64 = XCB_NONE }, 0, UINT32_MAX, D },
     { "x", "Initial x coordinate.", OFFSET(x), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, D },
     { "y", "Initial y coordinate.", OFFSET(y), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, D },
     { "grab_x", "Initial x coordinate.", OFFSET(x), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, D },
@@ -157,7 +160,7 @@ static int xcbgrab_frame(AVFormatContext *s, AVPacket *pkt)
     XCBGrabContext *c = s->priv_data;
     xcb_get_image_cookie_t iq;
     xcb_get_image_reply_t *img;
-    xcb_drawable_t drawable = c->screen->root;
+    xcb_drawable_t drawable = c->window_id;
     xcb_generic_error_t *e = NULL;
     uint8_t *data;
     int length;
@@ -267,7 +270,7 @@ static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
     XCBGrabContext *c = s->priv_data;
     xcb_shm_get_image_cookie_t iq;
     xcb_shm_get_image_reply_t *img;
-    xcb_drawable_t drawable = c->screen->root;
+    xcb_drawable_t drawable = c->window_id;
     xcb_generic_error_t *e = NULL;
     AVBufferRef *buf;
     xcb_shm_seg_t segment;
@@ -355,17 +358,17 @@ static void xcbgrab_draw_mouse(AVFormatContext *s, AVPacket *pkt,
     cx = ci->x - ci->xhot;
     cy = ci->y - ci->yhot;
 
-    x = FFMAX(cx, gr->x);
-    y = FFMAX(cy, gr->y);
+    x = FFMAX(cx, gr->win_x + gr->x);
+    y = FFMAX(cy, gr->win_y + gr->y);
 
-    w = FFMIN(cx + ci->width,  gr->x + gr->width)  - x;
-    h = FFMIN(cy + ci->height, gr->y + gr->height) - y;
+    w = FFMIN(cx + ci->width,  gr->win_x + gr->x + gr->width)  - x;
+    h = FFMIN(cy + ci->height, gr->win_y + gr->y + gr->height) - y;
 
     c_off = x - cx;
-    i_off = x - gr->x;
+    i_off = x - gr->x - gr->win_x;
 
     cursor += (y - cy) * ci->width;
-    image  += (y - gr->y) * gr->width * stride;
+    image  += (y - gr->y - gr->win_y) * gr->width * stride;
 
     for (y = 0; y < h; y++) {
         cursor += c_off;
@@ -403,8 +406,8 @@ static void xcbgrab_draw_mouse(AVFormatContext *s, AVPacket *pkt,
 static void xcbgrab_update_region(AVFormatContext *s)
 {
     XCBGrabContext *c     = s->priv_data;
-    const uint32_t args[] = { c->x - c->region_border,
-                              c->y - c->region_border };
+    const uint32_t args[] = { c->win_x + c->x - c->region_border,
+                              c->win_y + c->y - c->region_border };
 
     xcb_configure_window(c->conn,
                          c->window,
@@ -417,16 +420,22 @@ static int xcbgrab_read_packet(AVFormatContext *s, AVPacket *pkt)
     XCBGrabContext *c = s->priv_data;
     xcb_query_pointer_cookie_t pc;
     xcb_get_geometry_cookie_t gc;
+    xcb_translate_coordinates_cookie_t tc;
     xcb_query_pointer_reply_t *p  = NULL;
     xcb_get_geometry_reply_t *geo = NULL;
+    xcb_translate_coordinates_reply_t *translate = NULL;
     int ret = 0;
     int64_t pts;
 
     pts = wait_frame(s, pkt);
 
-    if (c->follow_mouse || c->draw_mouse) {
-        pc  = xcb_query_pointer(c->conn, c->screen->root);
-        gc  = xcb_get_geometry(c->conn, c->screen->root);
+    if (c->window_id == c->screen->root) {
+        c->win_x = 0;
+	c->win_y = 0;
+    }
+    if (c->follow_mouse || c->draw_mouse || c->window_id != c->screen->root) {
+        pc  = xcb_query_pointer(c->conn, c->window_id);
+        gc  = xcb_get_geometry(c->conn, c->window_id);
         p   = xcb_query_pointer_reply(c->conn, pc, NULL);
         if (!p) {
             av_log(s, AV_LOG_ERROR, "Failed to query xcb pointer\n");
@@ -438,6 +447,12 @@ static int xcbgrab_read_packet(AVFormatContext *s, AVPacket *pkt)
             free(p);
             return AVERROR_EXTERNAL;
         }
+        tc = xcb_translate_coordinates(c->conn, c->window_id, c->screen->root, geo->x, geo->y);
+        translate = xcb_translate_coordinates_reply(c->conn, tc, NULL);
+        if (!translate)
+            return AVERROR_EXTERNAL;
+        c->win_x = translate->dst_x;
+        c->win_y = translate->dst_y;
     }
 
     if (c->follow_mouse && p->same_screen)
@@ -447,7 +462,7 @@ static int xcbgrab_read_packet(AVFormatContext *s, AVPacket *pkt)
         xcbgrab_update_region(s);
 
 #if CONFIG_LIBXCB_SHM
-    if (c->has_shm && xcbgrab_frame_shm(s, pkt) < 0) {
+    if (c->has_shm && (ret = xcbgrab_frame_shm(s, pkt)) == AVERROR(ENOMEM)) {
         av_log(s, AV_LOG_WARNING, "Continuing without shared memory.\n");
         c->has_shm = 0;
     }
@@ -558,7 +573,9 @@ static int create_stream(AVFormatContext *s)
     XCBGrabContext *c = s->priv_data;
     AVStream *st      = avformat_new_stream(s, NULL);
     xcb_get_geometry_cookie_t gc;
+    xcb_translate_coordinates_cookie_t tc;
     xcb_get_geometry_reply_t *geo;
+    xcb_translate_coordinates_reply_t *translate;
     int64_t frame_size_bits;
     int ret;
 
@@ -571,11 +588,20 @@ static int create_stream(AVFormatContext *s)
 
     avpriv_set_pts_info(st, 64, 1, 1000000);
 
-    gc  = xcb_get_geometry(c->conn, c->screen->root);
+    gc  = xcb_get_geometry(c->conn, c->window_id);
     geo = xcb_get_geometry_reply(c->conn, gc, NULL);
-    if (!geo)
+    if (!geo) {
+        av_log(s, AV_LOG_ERROR, "Can't find window '0x%x', aborting.\n", c->window_id);
+        return AVERROR_EXTERNAL;
+    }
+
+    tc = xcb_translate_coordinates(c->conn, c->window_id, c->screen->root, geo->x, geo->y);
+    translate = xcb_translate_coordinates_reply(c->conn, tc, NULL);
+    if (!translate)
         return AVERROR_EXTERNAL;
 
+    c->win_x = translate->dst_x;
+    c->win_y = translate->dst_y;
     if (!c->width || !c->height) {
         c->width = geo->width;
         c->height = geo->height;
@@ -750,7 +776,7 @@ static int select_region(AVFormatContext *s)
             press_position = (xcb_point_t){ press->event_x, press->event_y };
             rectangle.x = press_position.x;
             rectangle.y = press_position.y;
-            xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
+            xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
             was_pressed = 1;
             break;
         }
@@ -759,14 +785,14 @@ static int select_region(AVFormatContext *s)
                 xcb_motion_notify_event_t *motion =
                     (xcb_motion_notify_event_t *)event;
                 xcb_point_t cursor_position = { motion->event_x, motion->event_y };
-                xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
+                xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
                 rectangle = rectangle_from_corners(&press_position, &cursor_position);
-                xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
+                xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
             }
             break;
         }
         case XCB_BUTTON_RELEASE: {
-            xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
+            xcb_poly_rectangle(conn, c->window_id, gc, 1, &rectangle);
             done = 1;
             break;
         }
@@ -830,6 +856,19 @@ static av_cold int xcbgrab_read_header(AVFormatContext *s)
         return AVERROR(EIO);
     }
 
+    if (c->window_id == XCB_NONE)
+        c->window_id = c->screen->root;
+    else {
+        if (c->select_region) {
+            av_log(s, AV_LOG_WARNING, "select_region ignored with window_id.\n");
+            c->select_region = 0;
+        }
+        if (c->follow_mouse) {
+            av_log(s, AV_LOG_WARNING, "follow_mouse ignored with window_id.\n");
+            c->follow_mouse = 0;
+        }
+    }
+
     if (c->select_region) {
         ret = select_region(s);
         if (ret < 0) {