Index: xfsettingsd/registry.c
===================================================================
--- xfsettingsd/registry.c	(revision 29498)
+++ xfsettingsd/registry.c	(working copy)
@@ -74,6 +74,10 @@
                                | G_PARAM_STATIC_NICK \
                                | G_PARAM_STATIC_BLURB)
 
+#define INCH_MM      25.4
+#define FALLBACK_DPI 96
+#define MAX_DPI      1000
+
 G_DEFINE_TYPE(XSettingsRegistry, xsettings_registry, G_TYPE_OBJECT);
 
 enum
@@ -399,6 +403,39 @@
     g_error_free (error);
 }
 
+static int
+compute_xsettings_dpi (XSettingsRegistry *registry, int value)
+{
+    Screen *xscreen;
+    int width_mm, height_mm;
+    int width, height;
+    int dpi;
+    
+    xscreen = ScreenOfDisplay (registry->priv->display,
+                                registry->priv->screen);
+    width_mm  = WidthMMOfScreen (xscreen);
+    height_mm = HeightMMOfScreen (xscreen);
+    dpi = 1024 * FALLBACK_DPI;
+    
+    if (value > 0)
+    {
+        dpi = 1024 * value;
+    }
+    else if (width_mm > 0 && height_mm > 0)
+    {
+        width     = WidthOfScreen (xscreen);
+        height    = HeightOfScreen (xscreen);
+        dpi = MIN (1024 * INCH_MM * width  / width_mm,
+                   1024 * INCH_MM * height / height_mm);
+    }
+    if (dpi > MAX_DPI * 1024)
+    {
+        dpi = 1024 * FALLBACK_DPI;
+    }
+
+    return dpi;
+}
+
 void
 xsettings_registry_notify(XSettingsRegistry *registry)
 {
@@ -541,8 +578,8 @@
                  * font sizes with -1 DPI and a forced setting equal to the X
                  * server's calculated DPI don't match.  Need to look into
                  * this a bit more. */
-                if (strcmp (entry->name, "Xft/DPI") == 0 && g_value_get_int(&entry->value) != -1)
-                    *(CARD32 *)pos = g_value_get_int(&entry->value) * 1024;
+                if (strcmp (entry->name, "Xft/DPI") == 0)
+                    *(CARD32 *)pos = compute_xsettings_dpi (registry, g_value_get_int(&entry->value));
                 else
                     *(CARD32 *)pos = g_value_get_int(&entry->value);
                 pos += 4;