Use bitmap strikes as fallbacks for ftcr font backend

* src/ftfont.h (struct font_info): New member bitmap_strike_index.
* src/ftfont.c (ftfont_open2): Try bitmap strikes as fallbacks.
(ftfont_open): Discard bitmap strikes.
* src/ftcrfont.c (ftcrfont_open): Recalculate metrics for bitmap strikes.
(ftcrfont_get_bitmap, ftcrfont_anchor_point, ftcrfont_shape): New functions.
(struct font_driver): Use them.

3 files changed, 137 insertions, 11 deletions

diff --git a/src/ftcrfont.c b/src/ftcrfont.c
index 4845ee4cf3d..e341c409b1e 100644
--- a/src/ftcrfont.c
+++ b/src/ftcrfont.c
@@ -135,7 +135,10 @@ ftcrfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
       font->driver = &ftcrfont_driver;
       FT_New_Size (ft_face, &ftcrfont_info->ft_size_draw);
       FT_Activate_Size (ftcrfont_info->ft_size_draw);
-      FT_Set_Pixel_Sizes (ft_face, 0, font->pixel_size);
+      if (ftcrfont_info->bitmap_strike_index < 0)
+        FT_Set_Pixel_Sizes (ft_face, 0, font->pixel_size);
+      else
+        FT_Select_Size (ft_face, ftcrfont_info->bitmap_strike_index);
       cairo_font_face_t *font_face =
         cairo_ft_font_face_create_for_ft_face (ft_face, 0);
       cairo_matrix_t font_matrix, ctm;
@@ -148,6 +151,56 @@ ftcrfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
       cairo_font_options_destroy (options);
       ftcrfont_info->metrics = NULL;
       ftcrfont_info->metrics_nrows = 0;
+      if (ftcrfont_info->bitmap_strike_index >= 0)
+        {
+          /* Several members of struct font/font_info set by
+             ftfont_open2 are bogus.  Recalculate them with cairo
+             scaled font functions.  */
+          cairo_font_extents_t extents;
+          cairo_scaled_font_extents (ftcrfont_info->cr_scaled_font, &extents);
+          font->ascent = lround (extents.ascent);
+          font->descent = lround (extents.descent);
+          font->height = lround (extents.height);
+
+          cairo_glyph_t stack_glyph;
+          int n = 0;
+          font->min_width = font->average_width = font->space_width = 0;
+          for (char c = 32; c < 127; c++)
+            {
+              cairo_glyph_t *glyphs = &stack_glyph;
+              int num_glyphs = 1;
+              cairo_status_t status =
+                cairo_scaled_font_text_to_glyphs (ftcrfont_info->cr_scaled_font,
+                                                  0, 0, &c, 1,
+                                                  &glyphs, &num_glyphs,
+                                                  NULL, NULL, NULL);
+
+              if (status == CAIRO_STATUS_SUCCESS)
+                {
+                  if (glyphs != &stack_glyph)
+                    cairo_glyph_free (glyphs);
+                  else
+                    {
+                      int this_width =
+                        ftcrfont_glyph_extents (font, stack_glyph.index, NULL);
+
+                      if (this_width > 0
+                          && (! font->min_width
+                              || font->min_width > this_width))
+                        font->min_width = this_width;
+                      if (c == 32)
+                        font->space_width = this_width;
+                      font->average_width += this_width;
+                      n++;
+                    }
+                }
+            }
+          if (n > 0)
+            font->average_width /= n;
+
+          font->underline_position = -1;
+          font->underline_thickness = 0;
+        }
     }
   unblock_input ();
 
@@ -211,6 +264,43 @@ ftcrfont_text_extents (struct font *font,
 }
 
 static int
+ftcrfont_get_bitmap (struct font *font, unsigned int code,
+                     struct font_bitmap *bitmap, int bits_per_pixel)
+{
+  struct font_info *ftcrfont_info = (struct font_info *) font;
+
+  if (ftcrfont_info->bitmap_strike_index < 0)
+    return ftfont_get_bitmap (font, code, bitmap, bits_per_pixel);
+
+  return -1;
+}
+
+static int
+ftcrfont_anchor_point (struct font *font, unsigned int code, int idx,
+                       int *x, int *y)
+{
+  struct font_info *ftcrfont_info = (struct font_info *) font;
+
+  if (ftcrfont_info->bitmap_strike_index < 0)
+    return ftfont_anchor_point (font, code, idx, x, y);
+
+  return -1;
+}
+
+static Lisp_Object
+ftcrfont_shape (Lisp_Object lgstring)
+{
+#if defined HAVE_M17N_FLT && defined HAVE_LIBOTF
+  struct font_info *ftcrfont_info = (struct font_info *) font;
+
+  if (ftcrfont_info->bitmap_strike_index < 0)
+    return ftfont_shape (lgstring);
+#endif
+
+  return make_fixnum (0);
+}
+
+static int
 ftcrfont_draw (struct glyph_string *s,
                int from, int to, int x, int y, bool with_background)
 {
@@ -286,14 +376,12 @@ struct font_driver const ftcrfont_driver =
   .encode_char = ftfont_encode_char,
   .text_extents = ftcrfont_text_extents,
   .draw = ftcrfont_draw,
-  .get_bitmap = ftfont_get_bitmap,
-  .anchor_point = ftfont_anchor_point,
+  .get_bitmap = ftcrfont_get_bitmap,
+  .anchor_point = ftcrfont_anchor_point,
 #ifdef HAVE_LIBOTF
   .otf_capability = ftfont_otf_capability,
 #endif
-#if defined HAVE_M17N_FLT && defined HAVE_LIBOTF
-  .shape = ftfont_shape,
-#endif
+  .shape = ftcrfont_shape,
 #ifdef HAVE_OTF_GET_VARIATION_GLYPHS
   .get_variation_glyphs = ftfont_variation_glyphs,
 #endif
diff --git a/src/ftfont.c b/src/ftfont.c
index 3e820f583ff..d0078a37701 100644
--- a/src/ftfont.c
+++ b/src/ftfont.c
@@ -1097,6 +1097,7 @@ ftfont_open2 (struct frame *f,
   int spacing;
   int i;
   double upEM;
+  FT_Int strike_index = -1;
 
   val = assq_no_quit (QCfont_entity, AREF (entity, FONT_EXTRA_INDEX));
   if (! CONSP (val))
@@ -1126,12 +1127,32 @@ ftfont_open2 (struct frame *f,
     size = pixel_size;
   if (FT_Set_Pixel_Sizes (ft_face, size, size) != 0)
     {
-      if (cache_data->face_refcount == 0)
+      int min_distance = INT_MAX;
+      bool magnify = true;
+
+      for (FT_Int i = 0; i < ft_face->num_fixed_sizes; i++)
         {
-          FT_Done_Face (ft_face);
-          cache_data->ft_face = NULL;
+          int distance = ft_face->available_sizes[i].height - (int) size;
+
+          /* Prefer down-scaling to upscaling.  */
+          if (magnify == (distance < 0) ? abs (distance) <= min_distance
+              : magnify)
+            {
+              magnify = distance < 0;
+              min_distance = abs (distance);
+              strike_index = i;
+            }
+        }
+
+      if (strike_index < 0 || FT_Select_Size (ft_face, strike_index) != 0)
+        {
+          if (cache_data->face_refcount == 0)
+            {
+              FT_Done_Face (ft_face);
+              cache_data->ft_face = NULL;
+            }
+          return Qnil;
         }
-      return Qnil;
     }
   cache_data->face_refcount++;
 
@@ -1144,6 +1165,7 @@ ftfont_open2 (struct frame *f,
   ftfont_info->maybe_otf = (ft_face->face_flags & FT_FACE_FLAG_SFNT) != 0;
   ftfont_info->otf = NULL;
 #endif  /* HAVE_LIBOTF */
+  ftfont_info->bitmap_strike_index = strike_index;
   /* This means that there's no need of transformation.  */
   ftfont_info->matrix.xx = 0;
   font->pixel_size = size;
@@ -1229,7 +1251,19 @@ ftfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
     size = pixel_size;
   font_object = font_build_object (VECSIZE (struct font_info),
                                    Qfreetype, entity, size);
-  return ftfont_open2 (f, entity, pixel_size, font_object);
+  font_object = ftfont_open2 (f, entity, pixel_size, font_object);
+  if (FONT_OBJECT_P (font_object))
+    {
+      struct font *font = XFONT_OBJECT (font_object);
+      struct font_info *ftfont_info = (struct font_info *) font;
+
+      if (ftfont_info->bitmap_strike_index >= 0)
+        {
+          ftfont_close (font);
+          font_object = Qnil;
+        }
+    }
+  return font_object;
 }
 
 void
diff --git a/src/ftfont.h b/src/ftfont.h
index 327cd085acf..adbda49ff1c 100644
--- a/src/ftfont.h
+++ b/src/ftfont.h
@@ -54,6 +54,10 @@ struct font_info
 #endif  /* HAVE_LIBOTF */
   FT_Size ft_size;
   int index;
+  /* Index of the bitmap strike used as a fallback for
+     FT_Set_Pixel_Sizes failure.  If the value is non-negative, then
+     ft_size is not of the requested size.  Otherwise it is -1.  */
+  FT_Int bitmap_strike_index;
   FT_Matrix matrix;
 
 #ifdef USE_CAIRO