Created attachment 198386 [details] [review]
Patch to include resolution and size in CoreText _FontHashKey

The font cache in PangoFcFontMapPrivate includes the FcPattern in the key into the hash table of the cache. The FcPattern contains information about the font size.

On the other hand, the font cache in PangoCairoCoreTextFontMap (font_hash) contains no such information.

As a result, the CoreText implementation may return cached fonts with an invalid size in pango_core_text_font_map_load_font:

      best_font = pango_core_text_font_map_lookup (ctfontmap,
                                                   context,
                                                   best_description,
                                                   best_face);

      if (best_font)
        g_object_ref (best_font);
      else
....

This happens for example in the gschem application which uses the pango_cairo_context_set_resolution function to apply a zoom factor:

  pango_cairo_context_set_resolution (context, 1000. * scale_factor);
  font_size_pt = o_text_get_font_size_in_points (w_current->toplevel,
                                                 o_current);

  desc = pango_font_description_from_string (FONT_NAME);
  pango_font_description_set_size (desc, (double)PANGO_SCALE * font_size_pt);

On a Llinux machine with FontConfig backend, the different scale_factor causes a new font instance to be created. On Mac OS X with the CoreText backend, an existing font with the same name and different size is returned.

Attached is a patch which adds the font size and resolution to the _FontHashKey structure. I'm not sure if this is the correct way to solve the issue or may cause side-effects, but gschem works fine with the patch applied to Pango. (In my opinion, the only effect should be that there are less cache-hits, so there should be no side effects.)

Thanks and greetings
Andreas

PS: I've set up a small blog entry with screenshot and some more details, may be this is helpful: http://web.me.com/aschweiz/Website/Blog/Entries/2011/10/6_CoreText_vs._FontConfig.html