[trak] Allow disabling tracking for ranges of text

Fixes https://github.com/harfbuzz/harfbuzz/issues/1303

src/hb-aat-layout-trak-table.hh

@@ -184,6 +184,8 @@ struct trak
   {
     TRACE_APPLY (this);
 
+    hb_mask_t trak_mask = c->plan->trak_mask;
+
     const float ptem = c->font->ptem;
     if (unlikely (ptem <= 0.f))
       return_trace (false);
@@ -197,6 +199,7 @@ struct trak
       hb_position_t advance_to_add = c->font->em_scalef_x (tracking);
       foreach_grapheme (buffer, start, end)
       {
+        if (!(buffer->info[start].mask & trak_mask)) continue;
 	buffer->pos[start].x_advance += advance_to_add;
 	buffer->pos[start].x_offset += offset_to_add;
       }
@@ -209,6 +212,7 @@ struct trak
       hb_position_t advance_to_add = c->font->em_scalef_y (tracking);
       foreach_grapheme (buffer, start, end)
       {
+        if (!(buffer->info[start].mask & trak_mask)) continue;
 	buffer->pos[start].y_advance += advance_to_add;
 	buffer->pos[start].y_offset += offset_to_add;
       }

src/hb-ot-shape.cc

@@ -62,16 +62,18 @@ hb_ot_shape_planner_t::compile (hb_ot_shape_plan_t &plan,
   plan.shaper = shaper;
   map.compile (plan.map, coords, num_coords);
 
-  plan.rtlm_mask = plan.map.get_1_mask (HB_TAG ('r','t','l','m'));
   plan.frac_mask = plan.map.get_1_mask (HB_TAG ('f','r','a','c'));
   plan.numr_mask = plan.map.get_1_mask (HB_TAG ('n','u','m','r'));
   plan.dnom_mask = plan.map.get_1_mask (HB_TAG ('d','n','o','m'));
   plan.has_frac = plan.frac_mask || (plan.numr_mask && plan.dnom_mask);
+  plan.rtlm_mask = plan.map.get_1_mask (HB_TAG ('r','t','l','m'));
   hb_tag_t kern_tag = HB_DIRECTION_IS_HORIZONTAL (plan.props.direction) ?
 		      HB_TAG ('k','e','r','n') : HB_TAG ('v','k','r','n');
   plan.kern_mask = plan.map.get_mask (kern_tag);
+  plan.trak_mask = plan.map.get_mask (HB_TAG ('t','r','a','k'));
 
   plan.requested_kerning = !!plan.kern_mask;
+  plan.requested_tracking = !!plan.trak_mask;
   bool has_gpos_kern = plan.map.get_feature_index (1, kern_tag) != HB_OT_LAYOUT_NO_FEATURE_INDEX;
   bool disable_gpos = plan.shaper->gpos_tag &&
 		      plan.shaper->gpos_tag != plan.map.chosen_script[1];
@@ -116,8 +118,7 @@ hb_ot_shape_planner_t::compile (hb_ot_shape_plan_t &plan,
     plan.fallback_mark_positioning = true;
 
   /* Currently we always apply trak. */
-  plan.apply_trak = hb_aat_layout_has_tracking (face) &&
-		    plan.map.needs_fallback (HB_TAG ('t','r','a','k'));
+  plan.apply_trak = plan.requested_tracking && hb_aat_layout_has_tracking (face);
 }
 
 
@@ -178,7 +179,9 @@ hb_ot_shape_collect_features (hb_ot_shape_planner_t          *planner,
   /* Random! */
   map->enable_feature (HB_TAG ('r','a','n','d'), F_RANDOM, HB_OT_MAP_MAX_VALUE);
 
-  /* Tracking. */
+  /* Tracking.  We enable dummy feature here just to allow disabling
+   * AAT 'trak' table using features.
+   * https://github.com/harfbuzz/harfbuzz/issues/1303 */
   map->enable_feature (HB_TAG ('t','r','a','k'), F_HAS_FALLBACK);
 
   map->enable_feature (HB_TAG ('H','A','R','F'));

src/hb-ot-shape.hh

@@ -40,10 +40,13 @@ struct hb_ot_shape_plan_t
   const struct hb_ot_complex_shaper_t *shaper;
   hb_ot_map_t map;
   const void *data;
-  hb_mask_t rtlm_mask, frac_mask, numr_mask, dnom_mask;
+  hb_mask_t frac_mask, numr_mask, dnom_mask;
+  hb_mask_t rtlm_mask;
   hb_mask_t kern_mask;
+  hb_mask_t trak_mask;
 
   bool requested_kerning : 1;
+  bool requested_tracking : 1;
   bool has_frac : 1;
   bool has_gpos_mark : 1;
   bool fallback_glyph_classes : 1;

test/shaping/data/in-house/tests/aat-trak.tests

@@ -8,3 +8,4 @@
 ../fonts/TestTRAK.ttf:--font-ptem=144:U+0041,U+0042,U+0043:[A.alt=0@-107,0+786|B=1@-107,0+786|C.alt=2@-107,0+786]
 ../fonts/TestTRAK.ttf:--font-ptem=144:U+0041,U+0042,U+0043:[A.alt=0@-107,0+786|B=1@-107,0+786|C.alt=2@-107,0+786]
 ../fonts/TestTRAK.ttf:--font-ptem=144 --features=-trak:U+0041,U+0042,U+0043:[A.alt=0+1000|B=1+1000|C.alt=2+1000]
+../fonts/TestTRAK.ttf:--font-ptem=144 --features=-trak[1;3]:U+0041,U+0042,U+0043,U+0041,U+0042,U+0043:[A.alt=0@-107,0+786|B=1+1000|C.alt=2+1000|A.alt=3@-107,0+786|B=4@-107,0+786|C.alt=5@-107,0+786]