avdevice/xcbgrab: Add select_region option

This patch adds a select_region option to the xcbgrab input device.
If set to 1, the user will be prompted to select the grabbing area
graphically by clicking and dragging. A rectangle will be drawn to
mark the grabbing area. A single click with no dragging will select
the whole screen. The option overwrites the video_size, grab_x, and
grab_y options if set by the user.

For testing, just set the select_region option as follows:

ffmpeg -f x11grab -select_region 1 -i :0.0 output.mp4

The drawing happens directly on the root window using standard rubber
banding techniques, so it is very efficient and doesn't depend on any
X extensions or compositors.

Reviewed-by: Andriy Gelman <andriy.gelman@gmail.com>
Signed-off-by: Omar Emara <mail@OmarEmara.dev>

doc/indevs.texi

@@ -1503,6 +1503,14 @@ ffmpeg -f x11grab -framerate 25 -video_size cif -i :0.0+10,20 out.mpg
 @subsection Options
 
 @table @option
+@item select_region
+Specify whether to select the grabbing area graphically using the pointer.
+A value of @code{1} prompts the user to select the grabbing area graphically
+by clicking and dragging. A single click with no dragging will select the
+whole screen. A region with zero width or height will also select the whole
+screen. This option overwrites the @var{video_size}, @var{grab_x}, and
+@var{grab_y} options. Default value is @code{0}.
+
 @item draw_mouse
 Specify whether to draw the mouse pointer. A value of @code{0} specifies
 not to draw the pointer. Default value is @code{1}.

libavdevice/xcbgrab.c

@@ -22,6 +22,7 @@
 #include "config.h"
 
 #include <stdlib.h>
+#include <string.h>
 #include <xcb/xcb.h>
 
 #if CONFIG_LIBXCB_XFIXES
@@ -69,6 +70,7 @@ typedef struct XCBGrabContext {
     int show_region;
     int region_border;
     int centered;
+    int select_region;
 
     const char *framerate;
 
@@ -92,6 +94,7 @@ static const AVOption options[] = {
     { "centered", "Keep the mouse pointer at the center of grabbing region when following.", 0, AV_OPT_TYPE_CONST, { .i64 = -1 }, INT_MIN, INT_MAX, D, "follow_mouse" },
     { "show_region", "Show the grabbing region.", OFFSET(show_region), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, 1, D },
     { "region_border", "Set the region border thickness.", OFFSET(region_border), AV_OPT_TYPE_INT, { .i64 = 3 }, 1, 128, D },
+    { "select_region", "Select the grabbing region graphically using the pointer.", OFFSET(select_region), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1, D },
     { NULL },
 };
 
@@ -677,6 +680,117 @@ static void setup_window(AVFormatContext *s)
     draw_rectangle(s);
 }
 
+#define CROSSHAIR_CURSOR 34
+
+static xcb_rectangle_t rectangle_from_corners(xcb_point_t *corner_a,
+                                              xcb_point_t *corner_b)
+{
+    xcb_rectangle_t rectangle;
+    rectangle.x = FFMIN(corner_a->x, corner_b->x);
+    rectangle.y = FFMIN(corner_a->y, corner_b->y);
+    rectangle.width = FFABS(corner_a->x - corner_b->x);
+    rectangle.height = FFABS(corner_a->y - corner_b->y);
+    return rectangle;
+}
+
+static int select_region(AVFormatContext *s)
+{
+    XCBGrabContext *c = s->priv_data;
+    xcb_connection_t *conn = c->conn;
+    xcb_screen_t *screen = c->screen;
+
+    int ret = 0, done = 0, was_pressed = 0;
+    xcb_cursor_t cursor;
+    xcb_font_t cursor_font;
+    xcb_point_t press_position;
+    xcb_generic_event_t *event;
+    xcb_rectangle_t rectangle = { 0 };
+    xcb_grab_pointer_reply_t *reply;
+    xcb_grab_pointer_cookie_t cookie;
+
+    xcb_window_t root_window = screen->root;
+    xcb_gcontext_t gc = xcb_generate_id(conn);
+    uint32_t mask = XCB_GC_FUNCTION | XCB_GC_SUBWINDOW_MODE;
+    uint32_t values[] = { XCB_GX_INVERT, XCB_SUBWINDOW_MODE_INCLUDE_INFERIORS };
+    xcb_create_gc(conn, gc, root_window, mask, values);
+
+    cursor_font = xcb_generate_id(conn);
+    xcb_open_font(conn, cursor_font, strlen("cursor"), "cursor");
+    cursor = xcb_generate_id(conn);
+    xcb_create_glyph_cursor(conn, cursor, cursor_font, cursor_font,
+                            CROSSHAIR_CURSOR, CROSSHAIR_CURSOR + 1, 0, 0, 0,
+                            0xFFFF, 0xFFFF, 0xFFFF);
+    cookie = xcb_grab_pointer(conn, 0, root_window,
+                              XCB_EVENT_MASK_BUTTON_PRESS |
+                              XCB_EVENT_MASK_BUTTON_RELEASE |
+                              XCB_EVENT_MASK_BUTTON_MOTION,
+                              XCB_GRAB_MODE_ASYNC, XCB_GRAB_MODE_ASYNC,
+                              root_window, cursor, XCB_CURRENT_TIME);
+    reply = xcb_grab_pointer_reply(conn, cookie, NULL);
+    if (!reply || reply->status != XCB_GRAB_STATUS_SUCCESS) {
+        av_log(s, AV_LOG_ERROR,
+               "Failed to select region. Could not grab pointer.\n");
+        ret = AVERROR(EIO);
+        free(reply);
+        goto fail;
+    }
+    free(reply);
+
+    xcb_grab_server(conn);
+
+    while (!done && (event = xcb_wait_for_event(conn))) {
+        switch (event->response_type & ~0x80) {
+        case XCB_BUTTON_PRESS: {
+            xcb_button_press_event_t *press = (xcb_button_press_event_t *)event;
+            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);
+            was_pressed = 1;
+            break;
+        }
+        case XCB_MOTION_NOTIFY: {
+            if (was_pressed) {
+                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);
+                rectangle = rectangle_from_corners(&press_position, &cursor_position);
+                xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
+            }
+            break;
+        }
+        case XCB_BUTTON_RELEASE: {
+            xcb_poly_rectangle(conn, root_window, gc, 1, &rectangle);
+            done = 1;
+            break;
+        }
+        default:
+            break;
+        }
+        xcb_flush(conn);
+        free(event);
+    }
+    c->width  = rectangle.width;
+    c->height = rectangle.height;
+    if (c->width && c->height) {
+        c->x = rectangle.x;
+        c->y = rectangle.y;
+    } else {
+        c->x = 0;
+        c->y = 0;
+    }
+    xcb_ungrab_server(conn);
+    xcb_ungrab_pointer(conn, XCB_CURRENT_TIME);
+    xcb_flush(conn);
+
+fail:
+    xcb_free_cursor(conn, cursor);
+    xcb_close_font(conn, cursor_font);
+    xcb_free_gc(conn, gc);
+    return ret;
+}
+
 static av_cold int xcbgrab_read_header(AVFormatContext *s)
 {
     XCBGrabContext *c = s->priv_data;
@@ -711,6 +825,14 @@ static av_cold int xcbgrab_read_header(AVFormatContext *s)
         return AVERROR(EIO);
     }
 
+    if (c->select_region) {
+        ret = select_region(s);
+        if (ret < 0) {
+            xcbgrab_read_close(s);
+            return ret;
+        }
+    }
+
     ret = create_stream(s);
 
     if (ret < 0) {