[var] Add a new API, hb_font_set_var_named_instance

docs/harfbuzz-sections.txt

@@ -342,6 +342,7 @@ hb_font_set_user_data
 hb_font_set_variations
 hb_font_set_var_coords_design
 hb_font_set_var_coords_normalized
+hb_font_set_var_named_instance
 hb_font_subtract_glyph_origin_for_direction
 hb_font_t
 hb_reference_table_func_t

src/hb-font.cc

@@ -1861,6 +1861,7 @@ hb_font_set_variations (hb_font_t *font,
 				  normalized, coords_length);
   _hb_font_adopt_var_coords_normalized (font, normalized, coords_length);
 }
+
 /**
  * hb_font_set_var_coords_design:
  *
@@ -1881,6 +1882,33 @@ hb_font_set_var_coords_design (hb_font_t *font,
   hb_ot_var_normalize_coords (font->face, coords_length, coords, normalized);
   _hb_font_adopt_var_coords_normalized (font, normalized, coords_length);
 }
+
+/**
+ * hb_font_set_var_named_instance:
+ * @font: a font.
+ * @instance_index: named instance index.
+ *
+ * Sets design coords of a font from a named instance index.
+ *
+ * Since: REPLACEME
+ */
+void
+hb_font_set_var_named_instance (hb_font_t *font,
+				unsigned instance_index)
+{
+  if (hb_object_is_immutable (font))
+    return;
+
+  unsigned int coords_length = hb_ot_var_named_instance_get_design_coords (font->face, instance_index, nullptr, nullptr);
+
+  float *coords = coords_length ? (float *) calloc (coords_length, sizeof (float)) : nullptr;
+  if (unlikely (coords_length && !coords))
+    return;
+
+  hb_ot_var_named_instance_get_design_coords (font->face, instance_index, &coords_length, coords);
+  hb_font_set_var_coords_design (font, coords, coords_length);
+  free (coords);
+}
 #endif
 
 /**

src/hb-font.h

@@ -705,6 +705,10 @@ HB_EXTERN const int *
 hb_font_get_var_coords_normalized (hb_font_t *font,
 				   unsigned int *length);
 
+HB_EXTERN void
+hb_font_set_var_named_instance (hb_font_t *font,
+				unsigned instance_index);
+
 HB_END_DECLS
 
 #endif /* HB_FONT_H */

test/api/test-ot-extents-cff.c

@@ -184,6 +184,37 @@ test_extents_cff2_vsindex (void)
   hb_font_destroy (font);
 }
 
+static void
+test_extents_cff2_vsindex_named_instance (void)
+{
+  hb_face_t *face = hb_test_open_font_file ("fonts/AdobeVFPrototype_vsindex.otf");
+  g_assert (face);
+  hb_font_t *font = hb_font_create (face);
+  hb_face_destroy (face);
+  g_assert (font);
+  hb_ot_font_set_funcs (font);
+
+  hb_font_set_var_named_instance (font, 6); // 6 (BlackMediumContrast): 900, 50
+  hb_glyph_extents_t  extents;
+  hb_bool_t result = hb_font_get_glyph_extents (font, 1, &extents);
+  g_assert (result);
+
+  g_assert_cmpint (extents.x_bearing, ==, 12);
+  g_assert_cmpint (extents.y_bearing, ==, 652);
+  g_assert_cmpint (extents.width, ==, 653);
+  g_assert_cmpint (extents.height, ==, -652);
+
+  result = hb_font_get_glyph_extents (font, 2, &extents);
+  g_assert (result);
+
+  g_assert_cmpint (extents.x_bearing, ==, 6);
+  g_assert_cmpint (extents.y_bearing, ==, 675);
+  g_assert_cmpint (extents.width, ==, 647);
+  g_assert_cmpint (extents.height, ==, -675);
+
+  hb_font_destroy (font);
+}
+
 int
 main (int argc, char **argv)
 {
@@ -194,6 +225,7 @@ main (int argc, char **argv)
   hb_test_add (test_extents_cff1_seac);
   hb_test_add (test_extents_cff2);
   hb_test_add (test_extents_cff2_vsindex);
+  hb_test_add (test_extents_cff2_vsindex_named_instance);
 
   return hb_test_run ();
 }