Allow specifying the font for the em conversion

Currently, the conversion from em to units is done by using the
default font name inside the backend. For actors using their own
font/text layout we need a way to specify the font name along
with the quantity we wish to transform.

clutter/clutter-backend.c

@@ -103,55 +103,74 @@ clutter_backend_dispose (GObject *gobject)
   G_OBJECT_CLASS (clutter_backend_parent_class)->dispose (gobject);
 }
 
-static inline void
-update_units_per_em (ClutterBackend *backend)
+static ClutterUnit
+get_units_per_em (ClutterBackend       *backend,
+                  PangoFontDescription *font_desc)
 {
-  ClutterBackendPrivate *priv = backend->priv;
-  const gchar *font_name;
+  ClutterUnit units_per_em = -1.0;
+  gboolean free_font_desc = FALSE;
   gdouble dpi;
 
-  font_name = clutter_backend_get_font_name (backend);
   dpi = clutter_backend_get_resolution (backend);
 
-  if (G_LIKELY (font_name != NULL && *font_name != '\0'))
+  if (font_desc == NULL)
     {
-      PangoFontDescription *font_desc;
-      gdouble font_size = 0;
+      const gchar *font_name = clutter_backend_get_font_name (backend);
 
-      font_desc = pango_font_description_from_string (font_name);
-      if (G_LIKELY (font_desc != NULL))
+      if (G_LIKELY (font_name != NULL && *font_name != '\0'))
         {
-          gint pango_size;
-          gboolean is_absolute;
-
-          pango_size = pango_font_description_get_size (font_desc);
-          is_absolute =
-            pango_font_description_get_size_is_absolute (font_desc);
-          if (!is_absolute)
-            font_size = ((gdouble) font_size) / PANGO_SCALE;
-
-          pango_font_description_free (font_desc);
+          font_desc = pango_font_description_from_string (font_name);
+          free_font_desc = TRUE;
         }
+    }
 
-      /* 10 points at 96 DPI is 12 pixels */
-      priv->units_per_em = 1.2f * font_size
-                         * dpi
-                         / 96.0f;
+  if (font_desc != NULL)
+    {
+      gdouble font_size = 0;
+      gint pango_size;
+      gboolean is_absolute;
+
+      pango_size = pango_font_description_get_size (font_desc);
+      is_absolute = pango_font_description_get_size_is_absolute (font_desc);
+
+      /* "absolute" means "device units" (usually, pixels); otherwise,
+       * it means logical units (points)
+       */
+      if (is_absolute)
+        font_size = (gdouble) pango_size / PANGO_SCALE;
+      else
+        font_size = (gdouble) pango_size / PANGO_SCALE
+                  * dpi
+                  / 96.0f;
+
+      /* 10 points at 96 DPI is 13.3 pixels */
+      units_per_em = (1.2f * font_size)
+                   * dpi
+                   / 96.0f;
     }
   else
-    priv->units_per_em = -1.0f;
+    units_per_em = -1.0f;
+
+  if (free_font_desc)
+    pango_font_description_free (font_desc);
+
+  return units_per_em;
 }
 
 static void
 clutter_backend_real_resolution_changed (ClutterBackend *backend)
 {
-  update_units_per_em (backend);
+  backend->priv->units_per_em = get_units_per_em (backend, NULL);
+
+  CLUTTER_NOTE (BACKEND, "Units per em: %.2f", backend->priv->units_per_em);
 }
 
 static void
 clutter_backend_real_font_changed (ClutterBackend *backend)
 {
-  update_units_per_em (backend);
+  backend->priv->units_per_em = get_units_per_em (backend, NULL);
+
+  CLUTTER_NOTE (BACKEND, "Units per em: %.2f", backend->priv->units_per_em);
 }
 
 static void
@@ -361,7 +380,8 @@ _clutter_backend_init_events (ClutterBackend *backend)
 }
 
 gfloat
-_clutter_backend_get_units_per_em (ClutterBackend *backend)
+_clutter_backend_get_units_per_em (ClutterBackend       *backend,
+                                   PangoFontDescription *font_desc)
 {
   ClutterBackendPrivate *priv;
 
@@ -369,8 +389,12 @@ _clutter_backend_get_units_per_em (ClutterBackend *backend)
 
   priv = backend->priv;
 
-  if (G_UNLIKELY (priv->units_per_em < 0))
-    update_units_per_em (backend);
+  /* recompute for the font description, but do not cache the result */
+  if (font_desc != NULL)
+    return get_units_per_em (backend, font_desc);
+
+  if (priv->units_per_em < 0)
+    priv->units_per_em = get_units_per_em (backend, NULL);
 
   return priv->units_per_em;
 }
@@ -603,6 +627,8 @@ clutter_backend_get_font_options (ClutterBackend *backend)
   cairo_font_options_set_antialias (priv->font_options,
                                     CAIRO_ANTIALIAS_DEFAULT);
 
+  g_signal_emit (backend, backend_signals[FONT_CHANGED], 0);
+
   return priv->font_options;
 }
 
@@ -662,7 +688,12 @@ clutter_backend_get_font_name (ClutterBackend *backend)
   if (G_LIKELY (priv->font_name))
     return priv->font_name;
 
+  /* if we have never been called then we need to set the
+   * default font and update everything that relies on the
+   * ::font-changed signal
+   */
   priv->font_name = g_strdup (DEFAULT_FONT_NAME);
+  g_signal_emit (backend, backend_signals[FONT_CHANGED], 0);
 
   return priv->font_name;
 }