Disable unneeded ligatures when letter spacing is set

For writing systems where glyph substitutions are purely
cosmetic, we should disable them when letter spacing is set,
otherwise we get ligatures where the spacing is not applied.

To avoid changing Harfbuzz-NG upstream, we detect this
case when fetching the GSUB table and return an empty
blob instead.

Task-number: QTBUG-44393
Change-Id: Ie5f6b2d795d7fecbba0ece3941fb70ba7f04c395
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

1 files changed, 16 insertions, 3 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index 1c924175e2..7dc8e8fadb 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -1059,7 +1059,7 @@ void QTextEngine::shapeText(int item) const
 
 #ifdef QT_ENABLE_HARFBUZZ_NG
     if (Q_LIKELY(qt_useHarfbuzzNG()))
-        si.num_glyphs = shapeTextWithHarfbuzzNG(si, string, itemLength, fontEngine, itemBoundaries, kerningEnabled);
+        si.num_glyphs = shapeTextWithHarfbuzzNG(si, string, itemLength, fontEngine, itemBoundaries, kerningEnabled, letterSpacing != 0);
     else
 #endif
     si.num_glyphs = shapeTextWithHarfbuzz(si, string, itemLength, fontEngine, itemBoundaries, kerningEnabled);
@@ -1121,7 +1121,13 @@ QT_BEGIN_INCLUDE_NAMESPACE
 
 QT_END_INCLUDE_NAMESPACE
 
-int QTextEngine::shapeTextWithHarfbuzzNG(const QScriptItem &si, const ushort *string, int itemLength, QFontEngine *fontEngine, const QVector<uint> &itemBoundaries, bool kerningEnabled) const
+int QTextEngine::shapeTextWithHarfbuzzNG(const QScriptItem &si,
+                                         const ushort *string,
+                                         int itemLength,
+                                         QFontEngine *fontEngine,
+                                         const QVector<uint> &itemBoundaries,
+                                         bool kerningEnabled,
+                                         bool hasLetterSpacing) const
 {
     uint glyphs_shaped = 0;
 
@@ -1135,7 +1141,8 @@ int QTextEngine::shapeTextWithHarfbuzzNG(const QScriptItem &si, const ushort *st
 
     hb_segment_properties_t props = HB_SEGMENT_PROPERTIES_DEFAULT;
     props.direction = si.analysis.bidiLevel % 2 ? HB_DIRECTION_RTL : HB_DIRECTION_LTR;
-    props.script = hb_qt_script_to_script(QChar::Script(si.analysis.script));
+    QChar::Script script = QChar::Script(si.analysis.script);
+    props.script = hb_qt_script_to_script(script);
     // ### props.language = hb_language_get_default_for_script(props.script);
 
     for (int k = 0; k < itemBoundaries.size(); k += 3) {
@@ -1168,6 +1175,12 @@ int QTextEngine::shapeTextWithHarfbuzzNG(const QScriptItem &si, const ushort *st
             Q_ASSERT(hb_font);
             hb_qt_font_set_use_design_metrics(hb_font, option.useDesignMetrics() ? uint(QFontEngine::DesignMetrics) : 0); // ###
 
+            bool scriptRequiresOpenType = ((script >= QChar::Script_Syriac && script <= QChar::Script_Sinhala)
+                                         || script == QChar::Script_Khmer || script == QChar::Script_Nko);
+            hb_face_t *hb_face = hb_font_get_face(hb_font);
+            Q_ASSERT(hb_face);
+            hb_qt_face_set_ignore_gsub(hb_face, hasLetterSpacing && !scriptRequiresOpenType);
+
             const hb_feature_t features[1] = {
                 { HB_TAG('k','e','r','n'), !!kerningEnabled, 0, uint(-1) }
             };