Add API to disable text shaping on fonts

In the past, we had an undocumented text flag that worked with
one of the QPainter::drawText() overloads. This was never intended
as public API and served a specific cause in Qt WebKit at one point.

But there is a general need for such API, as disabling shaping features
easily gives 25% performance improvement on text rendering even for
fairly short strings.

This patch adds a new style strategy flag to disable shaping and
will just uses the CMAP and HDMX tables to get glyph indices and advances
for the characters. In Qt 6, the TextBypassShaping flag can be removed
completely and be replaced by the style strategy.

[ChangeLog][QtGui][Text] Added QFont::PreferNoShaping style strategy to support
improvements to performance at the expense of some cosmetic font features.

Task-number: QTBUG-56728
Change-Id: I48e025dcc06afe02824bf5b5011702a7e0036f6d
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 60 insertions, 0 deletions

diff --git a/tests/auto/gui/text/qtextscriptengine/tst_qtextscriptengine.cpp b/tests/auto/gui/text/qtextscriptengine/tst_qtextscriptengine.cpp
index ee50b98733..0371f51961 100644
--- a/tests/auto/gui/text/qtextscriptengine/tst_qtextscriptengine.cpp
+++ b/tests/auto/gui/text/qtextscriptengine/tst_qtextscriptengine.cpp
@@ -78,6 +78,9 @@ private slots:
     void thaiIsolatedSaraAm();
     void thaiWithZWJ();
     void thaiMultipleVowels();
+
+    void shapingDisabledDevanagari();
+    void shapingDisabledLatin();
 private:
     bool haveTestFonts;
 };
@@ -1280,5 +1283,62 @@ void tst_QTextScriptEngine::thaiMultipleVowels()
     // If we haven't crashed at this point, then the test has passed.
 }
 
+void tst_QTextScriptEngine::shapingDisabledLatin()
+{
+    QString s("fi");
+
+    QFont font("Calibri");
+    font.setStyleStrategy(QFont::PreferNoShaping);
+
+    QTextLayout layout(s);
+    layout.setFont(font);
+    layout.beginLayout();
+    layout.createLine();
+    layout.endLayout();
+
+    QList<QGlyphRun> runs = layout.glyphRuns();
+
+    QCOMPARE(runs.size(), 1);
+    QCOMPARE(runs.first().glyphIndexes().size(), 2);
+}
+
+void tst_QTextScriptEngine::shapingDisabledDevanagari()
+{
+    QString s;
+    s += QChar(0x0915); // KA
+    s += QChar(0x094D); // VIRAMA
+    s += QChar(0x0915); // KA
+
+
+    QList<QGlyphRun> normalRuns;
+    {
+        QTextLayout layout(s);
+        layout.beginLayout();
+        layout.createLine();
+        layout.endLayout();
+
+        normalRuns = layout.glyphRuns();
+    }
+
+    QFont font;
+    font.setStyleStrategy(QFont::PreferNoShaping);
+
+    QList<QGlyphRun> noShapingRuns;
+    {
+        QTextLayout layout(s);
+        layout.setFont(font);
+        layout.beginLayout();
+        layout.createLine();
+        layout.endLayout();
+
+        noShapingRuns = layout.glyphRuns();
+    }
+
+    // Even though shaping is disabled, Devanagari requires it, so the flag should be ignored.
+    QCOMPARE(normalRuns.size(), 1);
+    QCOMPARE(noShapingRuns.size(), 1);
+    QCOMPARE(noShapingRuns.first().glyphIndexes().size(), normalRuns.first().glyphIndexes().size());
+}
+
 QTEST_MAIN(tst_QTextScriptEngine)
 #include "tst_qtextscriptengine.moc"