From d3f56d9c0d98873a1dcafcd8c1983de9aac8c04b Mon Sep 17 00:00:00 2001
From: Ben Blain <mail@servc.eu>
Date: Sun, 3 Jun 2018 11:27:51 +0100
Subject: Blank pixels outside monitor bounds (Bug #12177)

For multiple monitor configurations, this sets the pixels outside
the monitor geometries to black.

diff --git a/lib/screenshooter-capture.c b/lib/screenshooter-capture.c
index c610750..43cfa38 100644
--- a/lib/screenshooter-capture.c
+++ b/lib/screenshooter-capture.c
@@ -79,6 +79,12 @@ static GdkFilterReturn  region_filter_func                  (GdkXEvent      *xev
                                                              GdkEvent       *event,
                                                              RbData         *rbdata);
 static GdkPixbuf       *get_rectangle_screenshot            (gint delay);
+static void             mask_outside_monitor_bounds         (GdkPixbuf      *screenshot,
+                                                             gint           x_orig,
+                                                             gint           y_orig,
+                                                             gint           width,
+                                                             gint           height,
+                                                             GdkWindow      *root_window);
 static gboolean         cb_key_pressed                      (GtkWidget      *widget,
                                                              GdkEventKey    *event,
                                                              RubberBandData *rbdata);
@@ -350,6 +356,9 @@ static GdkPixbuf
 
   screenshot = gdk_pixbuf_get_from_window (root, x_orig, y_orig, width, height);
 
+  if (window == root)
+    mask_outside_monitor_bounds (screenshot, x_orig, y_orig, width, height, root);
+
   /* Code adapted from gnome-screenshot:
    * Copyright (C) 2001-2006  Jonathan Blandford <jrb@alum.mit.edu>
    * Copyright (C) 2008 Cosimo Cecchi <cosimoc@gnome.org>
@@ -793,7 +802,91 @@ static GdkPixbuf
   else
     sleep (delay);
 
-  return gdk_pixbuf_get_from_window (root, x, y, w, h);
+  GdkPixbuf *screenshot = gdk_pixbuf_get_from_window (root, x, y, w, h);
+  mask_outside_monitor_bounds (screenshot, x, y, w, h, root);
+  return screenshot;
+}
+
+
+
+static void mask_outside_monitor_bounds (GdkPixbuf  *screenshot,
+                                         gint       x_orig,
+                                         gint       y_orig,
+                                         gint       width,
+                                         gint       height,
+                                         GdkWindow  *root_window)
+{
+  GdkDisplay *display = gdk_window_get_display (root_window);
+
+  gint monitor_count = gdk_display_get_n_monitors (display);
+  if (monitor_count < 2)
+    return;
+
+  TRACE ("Blanking pixels outside monitor bounds");
+
+  cairo_rectangle_int_t rect;
+  cairo_region_t *region, *monitor_region;
+  gint i, n_rects;
+
+  rect.x = x_orig;
+  rect.y = y_orig;
+  rect.width = width;
+  rect.height = height;
+  region = cairo_region_create_rectangle (&rect);
+
+  for (i = 0; i < monitor_count; i++)
+    {
+      gdk_monitor_get_geometry (gdk_display_get_monitor (display, i),
+                                &rect);
+
+      /* Subtract each of the monitor geometries from the mask */
+      monitor_region = cairo_region_create_rectangle (&rect);
+      cairo_region_subtract (region, monitor_region);
+    }
+  cairo_region_destroy (monitor_region);
+
+  /* Region dimensions are relative to display, not image */
+  cairo_region_translate (region, -x_orig, -y_orig);
+
+  n_rects = cairo_region_num_rectangles (region);
+
+  if (n_rects > 0)
+    {
+      guchar *pixels = gdk_pixbuf_get_pixels (screenshot);
+      gint rowstride = gdk_pixbuf_get_rowstride (screenshot);
+      gint n_channels = gdk_pixbuf_get_n_channels (screenshot);
+      gboolean has_alpha = gdk_pixbuf_get_has_alpha (screenshot);
+
+      for (i = 0; i < n_rects; i++)
+        {
+          gint x, y, x2, y2;
+          guchar *row;
+
+          cairo_region_get_rectangle (region, i, &rect);
+
+          x2 = rect.x + rect.width;
+          y2 = rect.y + rect.height;
+
+          for (y = rect.y; y < y2; y++)
+            {
+              guchar *p;
+
+              row = pixels + y * rowstride;
+              p = row + rect.x * n_channels;
+
+              for (x = rect.x; x < x2; x++)
+                {
+                  *p++ = *p++ = *p++ = 0;
+
+                  if (has_alpha)
+                    *p++ = 255; /* Opaque black */
+                }
+            }
+
+        }
+    }
+
+  cairo_region_destroy (region);
 }