[font] Add hb_font_set_variation()

docs/harfbuzz-sections.txt

@@ -421,6 +421,7 @@ hb_font_set_synthetic_bold
 hb_font_set_synthetic_slant
 hb_font_get_synthetic_slant
 hb_font_set_variations
+hb_font_set_variation
 HB_FONT_NO_VAR_NAMED_INSTANCE
 hb_font_set_var_named_instance
 hb_font_get_var_named_instance

src/hb-font.cc

@@ -2649,6 +2649,70 @@ hb_font_set_variations (hb_font_t            *font,
   _hb_font_adopt_var_coords (font, normalized, design_coords, coords_length);
 }
 
+/**
+ * hb_font_set_variation:
+ * @font: #hb_font_t to work upon
+ * @tag: The #hb_tag_t tag of the variation-axis name
+ * @value: The value of the variation axis
+ *
+ * Change the value of one variation axis on the font.
+ *
+ * Note: This function is expensive to be called repeatedly.
+ *   If you want to set multipl variation axes at the same time,
+ *   use hb_font_set_variations() instead.
+ *
+ * XSince: REPLACEME
+ */
+void
+hb_font_set_variation (hb_font_t *font,
+		       hb_tag_t tag,
+		       float    value)
+{
+  if (hb_object_is_immutable (font))
+    return;
+
+  font->serial_coords = ++font->serial;
+
+  // TODO Share some of this code with set_variations()
+
+  const OT::fvar &fvar = *font->face->table.fvar;
+  auto axes = fvar.get_axes ();
+  const unsigned coords_length = axes.length;
+
+  int *normalized = coords_length ? (int *) hb_calloc (coords_length, sizeof (int)) : nullptr;
+  float *design_coords = coords_length ? (float *) hb_calloc (coords_length, sizeof (float)) : nullptr;
+
+  if (unlikely (coords_length && !(normalized && design_coords)))
+  {
+    hb_free (normalized);
+    hb_free (design_coords);
+    return;
+  }
+
+  /* Initialize design coords. */
+  for (unsigned int i = 0; i < coords_length; i++)
+    design_coords[i] = axes[i].get_default ();
+  if (font->instance_index != HB_FONT_NO_VAR_NAMED_INSTANCE)
+  {
+    unsigned count = coords_length;
+    /* This may fail if index is out-of-range;
+     * That's why we initialize design_coords from fvar above
+     * unconditionally. */
+    hb_ot_var_named_instance_get_design_coords (font->face, font->instance_index,
+						&count, design_coords);
+  }
+
+  for (unsigned axis_index = 0; axis_index < coords_length; axis_index++)
+    if (axes[axis_index].axisTag == tag)
+      design_coords[axis_index] = value;
+
+  font->face->table.avar->map_coords (normalized, coords_length);
+
+  hb_ot_var_normalize_coords (font->face, coords_length, design_coords, normalized);
+  _hb_font_adopt_var_coords (font, normalized, design_coords, coords_length);
+
+}
+
 /**
  * hb_font_set_var_coords_design:
  * @font: #hb_font_t to work upon

src/hb-font.h

@@ -1150,6 +1150,11 @@ hb_font_set_variations (hb_font_t *font,
 			const hb_variation_t *variations,
 			unsigned int variations_length);
 
+HB_EXTERN void
+hb_font_set_variation (hb_font_t *font,
+		       hb_tag_t tag,
+		       float    value);
+
 HB_EXTERN void
 hb_font_set_var_coords_design (hb_font_t *font,
 			       const float *coords,