Add windowid option to draw to an existing window

This lets `feh` draw the background pixmap of an existing window,
opening the door for use with tools like `xscreensaver` or `xsecurelock`

5 files changed, 37 insertions, 5 deletions

diff --git a/man/feh.pre b/man/feh.pre
index 680a3f4..da6fafa 100644
--- a/man/feh.pre
+++ b/man/feh.pre
@@ -896,6 +896,11 @@ comes before
 Note that this option only has an effect when a sort mode is set using
 .Cm --sort .
 .
+.It Cm --windowid Ar windowid
+.
+Draw to an existing X11 window by its ID
+.Ar windowid .
+.
 .It Cm --xinerama-index Ar screen
 .
 .Pq optional feature, $MAN_XINERAMA$ in this build
diff --git a/src/help.raw b/src/help.raw
index e0fb62a..2369c8d 100644
--- a/src/help.raw
+++ b/src/help.raw
@@ -100,6 +100,7 @@ OPTIONS
      --scroll-step COUNT   scroll COUNT pixels when movement key is pressed
      --cache-size NUM      imlib cache size in mebibytes (0 .. 2048)
      --auto-reload         automatically reload shown image if file was changed
+     --windowid            Draw to an existing X11 window by its ID
 
 MONTAGE MODE OPTIONS
  -X, --ignore-aspect       Set thumbnail to specified width/height without
diff --git a/src/options.c b/src/options.c
index 04f02c5..65e5a10 100644
--- a/src/options.c
+++ b/src/options.c
@@ -433,6 +433,7 @@ static void feh_parse_option_array(int argc, char **argv, int finalrun)
 #endif
 		{"class"         , 1, 0, 249},
 		{"no-conversion-cache", 0, 0, 250},
+		{"windowid", 1, 0, 251},
 		{0, 0, 0, 0}
 	};
 	int optch = 0, cmdx = 0;
@@ -833,6 +834,9 @@ static void feh_parse_option_array(int argc, char **argv, int finalrun)
 		case 250:
 			opt.use_conversion_cache = 0;
 			break;
+		case 251:
+			opt.x11_windowid = atoi(optarg);
+			break;
 		default:
 			break;
 		}
diff --git a/src/options.h b/src/options.h
index 4906004..5270f38 100644
--- a/src/options.h
+++ b/src/options.h
@@ -130,6 +130,7 @@ struct __fehoptions {
 	unsigned char adjust_reload;
 	int xinerama_index;
 	char *x11_class;
+	unsigned int *x11_windowid;
 
 	/* signed in case someone wants to invert scrolling real quick */
 	int scroll_step;
diff --git a/src/winwidget.c b/src/winwidget.c
index 6ff90ed..696f3db 100644
--- a/src/winwidget.c
+++ b/src/winwidget.c
@@ -143,6 +143,7 @@ winwidget winwidget_create_from_file(gib_list * list, char type)
 
 void winwidget_create_window(winwidget ret, int w, int h)
 {
+	XWindowAttributes window_attr;
 	XSetWindowAttributes attr;
 	XEvent ev;
 	XClassHint *xch;
@@ -249,11 +250,31 @@ void winwidget_create_window(winwidget ret, int w, int h)
 		}
 	}
 
-	ret->win =
-	    XCreateWindow(disp, DefaultRootWindow(disp), x, y, w, h, 0,
-			  depth, InputOutput, vis,
-			  CWOverrideRedirect | CWSaveUnder | CWBackingStore
-			  | CWColormap | CWBackPixel | CWBorderPixel | CWEventMask, &attr);
+	if (opt.x11_windowid == 0) {
+		ret->win =
+		  XCreateWindow(disp, DefaultRootWindow(disp), x, y, w, h, 0,
+		                depth, InputOutput, vis,
+		                CWOverrideRedirect | CWSaveUnder | CWBackingStore
+		                | CWColormap | CWBackPixel | CWBorderPixel | CWEventMask,
+		                &attr);
+	} else {
+		/* x11_windowid is a pointer to the window */
+		ret->win = (Window) opt.x11_windowid;
+
+		/* set our attributes on the window */
+		XChangeWindowAttributes(disp, ret->win,
+                            CWOverrideRedirect | CWSaveUnder | CWBackingStore
+                            | CWColormap | CWBackPixel | CWBorderPixel
+                            | CWEventMask, &attr);
+
+		/* determine the size and visibility of the window */
+		XGetWindowAttributes(disp, ret->win, &window_attr);
+		ret->visible = (window_attr.map_state == IsViewable);
+		ret->x = window_attr.x;
+		ret->y = window_attr.y;
+		ret->w = window_attr.width;
+		ret->h = window_attr.height;
+	}
 
 	if (mwmhints.flags) {
 		XChangeProperty(disp, ret->win, prop, prop, 32,