Re: font size problem with pango

[Owen Taylor <otaylor redhat com>]

On Tue, 2003-07-22 at 01:16, Noah Levitt wrote:
> I was wondering the same thing today. Probably your best bet
> is to calculate the conversion using gdk_screen_get_height_mm ()
> something like
> 
> font_size_pt = font_size_px * (gdk_screen_get_height_mm/gdk_screen_get_height) * 72.27pt/1in * 1in/25.4mm

Actually, this doesn't work. The problem is that height_mm/height gives
you the "physical DPI" of the screen ... the best approximation to
the size of the pixels on the screen that the X server knows.

This is distinct from the "logical DPI" used by the Pango backend
to convert a font size into a pixel size.

There is no current portable way of retrieving the logical DPI. 
If you know that you are using Xft+fontconfig, then you can
do something like the following:

 * Check if there is a gtk-xft-dpi setting

        GtkSettings *settings = gtk_widget_get_settings (widget);
        GObjectClass *klass;

        klass = G_OBJECT_CLASS(GTK_SETTINGS_GET_CLASS(settings));
        if (g_object_class_find_property(klass, "gtk-xft-dpi"))
          {
            int int_dpi;
            g_object_get (settings, "gtk-xft-dpi", &int_dpi);
            dpi = int_dpi / PANGO_SCALE;
            g_signal_connect (settings, "notify::gtk-xft-dpi",
                              G_CALLBACK (dpi_changed_callback), NULL);
          }

 * If not, call XftDefaultSubstitute() on an empty pattern
   and check the FC_DPI value of the result:

       else
         {
           GdkScreen *screen = gdk_widget_get_screen (widget);
           FcPattern *pattern;

           dpi = 96.; /* following will not normally fail, but 
                         in case it does */

           pattern = FcPatternCreate();
           if (pattern)
             {
               XftDefaultSubstitute (GDK_SCREEN_XDISPLAY (screen),
                                     GDK_SCREEN_XNUMBER (screen),
                                     pattern);
               FcPatternGetDouble (pattern, FC_DPI, 0, &dpi); 
               FcPatternDestroy (pattern);
             }
         }

The above isn't tested, but it should be about right.

I've filed:
 
 http://bugzilla.gnome.org/show_bug.cgi?id=119081
 
because clearly this should not be necessary.

Regards,
						Owen