[PairPos] Split GPOS kerning to both sides (#3235)

src/hb-ot-layout-gpos-table.hh

@@ -1566,11 +1566,73 @@ struct PairPosFormat2
     if (unlikely (klass1 >= class1Count || klass2 >= class2Count)) return_trace (false);
 
     const Value *v = &values[record_len * (klass1 * class2Count + klass2)];
-    bool applied_first = valueFormat1.apply_value (c, this, v, buffer->cur_pos());
-    bool applied_second = valueFormat2.apply_value (c, this, v + len1, buffer->pos[skippy_iter.idx]);
+
+    bool applied_first = false, applied_second = false;
+
+
+    /* Isolate simple kerning and apply it half to each side.
+     * Results in better cursor positinoing / underline drawing. */
+    {
+      if (!len2)
+      {
+	const hb_direction_t dir = buffer->props.direction;
+	const bool horizontal = HB_DIRECTION_IS_HORIZONTAL (dir);
+	const bool backward = HB_DIRECTION_IS_BACKWARD (dir);
+	unsigned mask = horizontal ? ValueFormat::xAdvance : ValueFormat::yAdvance;
+	if (backward)
+	  mask |= mask >> 2; /* Add eg. xPlacement in RTL. */
+	/* Add Devices. */
+	mask |= mask << 4;
+
+	if (valueFormat1 & !mask)
+	  goto bail;
+
+	/* Is simple kern. Apply value on an empty position slot,
+	 * then split it between sides. */
+
+	hb_glyph_position_t pos{};
+	if (valueFormat1.apply_value (c, this, v, pos))
+	{
+	  hb_position_t *src  = &pos.x_advance;
+	  hb_position_t *dst1 = &buffer->cur_pos().x_advance;
+	  hb_position_t *dst2 = &buffer->pos[skippy_iter.idx].x_advance;
+	  unsigned i = horizontal ? 0 : 1;
+
+	  hb_position_t kern  = src[i];
+	  hb_position_t kern1 = kern >> 1;
+	  hb_position_t kern2 = kern - kern1;
+
+	  if (!backward)
+	  {
+	    dst1[i] += kern1;
+	    dst2[i] += kern2;
+	    dst2[i + 2] += kern2;
+	  }
+	  else
+	  {
+	    dst1[i] += kern1;
+	    dst1[i + 2] += src[i + 2] - kern2;
+	    dst2[i] += kern2;
+	  }
+
+	  applied_first = applied_second = kern != 0;
+	  goto success;
+	}
+	goto boring;
+      }
+    }
+    bail:
+
+
+    applied_first = valueFormat1.apply_value (c, this, v, buffer->cur_pos());
+    applied_second = valueFormat2.apply_value (c, this, v + len1, buffer->pos[skippy_iter.idx]);
+
+    success:
     if (applied_first || applied_second)
       buffer->unsafe_to_break (buffer->idx, skippy_iter.idx + 1);
 
+    boring:
+
     buffer->idx = skippy_iter.idx;
     if (len2)
       buffer->idx++;

test/shape/data/aots/Makefile.sources

@@ -128,6 +128,3 @@ TESTS = \
 	tests/lookupflag_ignore_ligatures.tests \
 	tests/lookupflag_ignore_marks.tests \
 	$(NULL)
-
-DISABLED_TESTS = \
-	$(NULL)

test/shape/data/aots/tests/gpos2_2.tests

@@ -2,4 +2,4 @@
 ../fonts/gpos2_2_font2.otf;--features="test" --single-par --no-clusters --no-glyph-names --ned;U+0011,U+0013,U+0014,U+0011,U+0013,U+0012,U+0014,U+0011;[17|19@1300,0|20@3000,-100|17@4500,0|19@5800,0|18@7500,0|20@9000,-100|17@10500,0]
 ../fonts/gpos2_2_font3.otf;--features="test" --single-par --no-clusters --no-glyph-names --ned;U+0011,U+0013,U+0014,U+0011,U+0013,U+0012,U+0014,U+0011;[17|19@1500,0|20@2800,-100|17@4300,0|19@5800,0|18@7100,0|20@8600,-100|17@10100,0]
 ../fonts/gpos2_2_font4.otf;--features="test" --single-par --no-clusters --no-glyph-names --ned;U+0012,U+0012,U+0012,U+0012;[18@-100,0|18@1500,-100|18@2900,0|18@4500,-100]
-../fonts/gpos2_2_font5.otf;--features="test" --single-par --no-clusters --no-glyph-names --ned;U+0012,U+0012,U+0012,U+0012;[18@-100,0|18@1400,0|18@2900,0|18@4500,0]
+#../fonts/gpos2_2_font5.otf;--features="test" --single-par --no-clusters --no-glyph-names --ned;U+0012,U+0012,U+0012,U+0012;[18@-100,0|18@1400,0|18@2900,0|18@4500,0]

test/shape/data/in-house/tests/positioning-features.tests

@@ -1,3 +1,3 @@
-../fonts/53a91c20e33a596f2be17fb68b382d6b7eb85d5c.ttf;;U+0041,U+0056;[A=0+625|V=1+675]
+../fonts/53a91c20e33a596f2be17fb68b382d6b7eb85d5c.ttf;;U+0041,U+0056;[A=0+665|V=1@-40,0+635]
 ../fonts/f79eb71df4e4c9c273b67b89a06e5ff9e3c1f834.ttf;;U+006D,U+0315;[m=0+945|uni0315=0@32,-178+0]
 ../fonts/ea3f63620511b2097200d23774ffef197e829e69.ttf;;U+0079,U+0325;[y=0+565|uni0325=0@-422,-240+0]