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;
 }

clutter/clutter-private.h

@@ -193,7 +193,8 @@ void          _clutter_backend_init_events    (ClutterBackend  *backend);
 
 ClutterFeatureFlags _clutter_backend_get_features (ClutterBackend *backend);
 
-gfloat        _clutter_backend_get_units_per_em   (ClutterBackend *backend);
+gfloat        _clutter_backend_get_units_per_em   (ClutterBackend       *backend,
+                                                   PangoFontDescription *font_desc);
 
 void          _clutter_feature_init (void);
 

clutter/clutter-units.c

@@ -169,11 +169,52 @@ clutter_units_pt (gdouble pt)
 ClutterUnit
 clutter_units_em (gdouble em)
 {
-  ClutterBackend *backend;
+  ClutterBackend *backend = clutter_get_default_backend ();
 
-  backend = clutter_get_default_backend ();
+  return em * _clutter_backend_get_units_per_em (backend, NULL);
+}
 
-  return em * _clutter_backend_get_units_per_em (backend);
+/**
+ * clutter_units_em_for_font:
+ * @font_name: the font name and size
+ * @em: em to convert
+ *
+ * Converts a value in em to #ClutterUnit<!-- -->s at the
+ * current DPI for the given font name.
+ *
+ * The @font_name string must be in a format that
+ * pango_font_description_from_string() can parse, like
+ * for clutter_text_set_font_name() or clutter_backend_set_font_name().
+ *
+ * Return value: the value in units
+ *
+ * Since: 1.0
+ */
+ClutterUnit
+clutter_units_em_for_font (const gchar *font_name,
+                           gdouble      em)
+{
+  ClutterBackend *backend = clutter_get_default_backend ();
+
+  if (font_name == NULL || *font_name == '\0')
+    return em * _clutter_backend_get_units_per_em (backend, NULL);
+  else
+    {
+      PangoFontDescription *font_desc;
+      gfloat res;
+
+      font_desc = pango_font_description_from_string (font_name);
+      if (font_desc == NULL)
+        res = -1.0;
+      else
+        {
+          res = em * _clutter_backend_get_units_per_em (backend, font_desc);
+
+          pango_font_description_free (font_desc);
+        }
+
+      return res;
+    }
 }
 
 /**

clutter/clutter-units.h

@@ -32,7 +32,8 @@
 #define __CLUTTER_UNITS_H__
 
 #include <glib-object.h>
-#include <clutter/clutter-fixed.h>
+
+#include <cogl/cogl.h>
 
 G_BEGIN_DECLS
 
@@ -192,10 +193,12 @@ typedef float ClutterUnit;
  */
 #define CLUTTER_UNITS_FROM_EM(x)        (clutter_units_em (x))
 
-ClutterUnit clutter_units_mm        (gdouble     mm);
-ClutterUnit clutter_units_pt        (gdouble     pt);
-ClutterUnit clutter_units_em        (gdouble     em);
-ClutterUnit clutter_units_pixels    (gint        px);
+ClutterUnit clutter_units_mm          (gdouble      mm);
+ClutterUnit clutter_units_pt          (gdouble      pt);
+ClutterUnit clutter_units_em          (gdouble      em);
+ClutterUnit clutter_units_em_for_font (const gchar *font_name,
+                                       gdouble      em);
+ClutterUnit clutter_units_pixels      (gint         px);
 
 gint        clutter_units_to_pixels (ClutterUnit units);