Introduce optional smarter font merging with ContextFontMerging

This introduces an optional, slightly more expensive approach
to font merging which takes the full string into account,
instead of just going character by character.

This addresses the issue that you may sometimes get multiple fonts
to cover one string of text in a single language. With Chinese,
this is especially an issue because many fonts will only support
parts of the very large character set.

The new algorithm detects if the string was incompletely covered
by the font and tries the fallback fonts in order to find the best
match. This is obviously more expensive, especially if no perfect
match is found and we have to check all the fallbacks in the list,
but it is opt-in and only enabled if the ContextFontMerging flag
is set.

Task-number: QTBUG-121131
Change-Id: I8c7874d0918640bd83418e3c4726c89f43a220a3
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

16 files changed, 152 insertions, 66 deletions

diff --git a/src/gui/text/coretext/qfontengine_coretext.mm b/src/gui/text/coretext/qfontengine_coretext.mm
index b6fef2fecb..1050c03d75 100644
--- a/src/gui/text/coretext/qfontengine_coretext.mm
+++ b/src/gui/text/coretext/qfontengine_coretext.mm
@@ -13,6 +13,7 @@
 #include <private/qcoregraphics_p.h>
 #include <private/qimage_p.h>
 #include <private/qguiapplication_p.h>
+#include <private/qstringiterator_p.h>
 #include <qpa/qplatformtheme.h>
 
 #include <cmath>
@@ -274,29 +275,30 @@ glyph_t QCoreTextFontEngine::glyphIndex(uint ucs4) const
     return glyphIndices[0];
 }
 
-bool QCoreTextFontEngine::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs,
-                                       int *nglyphs, QFontEngine::ShaperFlags flags) const
+int QCoreTextFontEngine::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs,
+                                      int *nglyphs, QFontEngine::ShaperFlags flags) const
 {
     Q_ASSERT(glyphs->numGlyphs >= *nglyphs);
     if (*nglyphs < len) {
         *nglyphs = len;
-        return false;
+        return -1;
     }
 
     QVarLengthArray<CGGlyph> cgGlyphs(len);
     CTFontGetGlyphsForCharacters(ctfont, (const UniChar*)str, cgGlyphs.data(), len);
 
     int glyph_pos = 0;
-    for (int i = 0; i < len; ++i) {
-        glyphs->glyphs[glyph_pos] = cgGlyphs[i];
-        if (glyph_pos < i)
-            cgGlyphs[glyph_pos] = cgGlyphs[i];
-        glyph_pos++;
-
-        // If it's a non-BMP char, skip the lower part of surrogate pair and go
-        // directly to the next char without increasing glyph_pos
-        if (str[i].isHighSurrogate() && i < len-1 && str[i+1].isLowSurrogate())
-            ++i;
+    int mappedGlyphs = 0;
+    QStringIterator it(str, str + len);
+    while (it.hasNext()) {
+      qsizetype idx = it.index();
+      char32_t ucs4 = it.next();
+      glyphs->glyphs[glyph_pos] = cgGlyphs[idx];
+      if (glyph_pos < idx)
+          cgGlyphs[glyph_pos] = cgGlyphs[idx];
+      if (glyphs->glyphs[glyph_pos] != 0 || isIgnorableChar(ucs4))
+          mappedGlyphs++;
+      glyph_pos++;
     }
 
     *nglyphs = glyph_pos;
@@ -305,7 +307,7 @@ bool QCoreTextFontEngine::stringToCMap(const QChar *str, int len, QGlyphLayout *
     if (!(flags & GlyphIndicesOnly))
         loadAdvancesForGlyphs(cgGlyphs, glyphs);
 
-    return true;
+    return mappedGlyphs;
 }
 
 glyph_metrics_t QCoreTextFontEngine::boundingBox(glyph_t glyph)
diff --git a/src/gui/text/coretext/qfontengine_coretext_p.h b/src/gui/text/coretext/qfontengine_coretext_p.h
index af87f5f6f3..2f388c32bc 100644
--- a/src/gui/text/coretext/qfontengine_coretext_p.h
+++ b/src/gui/text/coretext/qfontengine_coretext_p.h
@@ -38,7 +38,7 @@ public:
     ~QCoreTextFontEngine();
 
     glyph_t glyphIndex(uint ucs4) const override;
-    bool stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
+    int stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
     void recalcAdvances(QGlyphLayout *, ShaperFlags) const override;
 
     glyph_metrics_t boundingBox(glyph_t glyph) override;
diff --git a/src/gui/text/freetype/qfontengine_ft.cpp b/src/gui/text/freetype/qfontengine_ft.cpp
index 72d2c72fe3..d3791f1f6e 100644
--- a/src/gui/text/freetype/qfontengine_ft.cpp
+++ b/src/gui/text/freetype/qfontengine_ft.cpp
@@ -1678,15 +1678,16 @@ glyph_t QFontEngineFT::glyphIndex(uint ucs4) const
     return glyph;
 }
 
-bool QFontEngineFT::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs,
+int QFontEngineFT::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs,
                                  QFontEngine::ShaperFlags flags) const
 {
     Q_ASSERT(glyphs->numGlyphs >= *nglyphs);
     if (*nglyphs < len) {
         *nglyphs = len;
-        return false;
+        return -1;
     }
 
+    int mappedGlyphs = 0;
     int glyph_pos = 0;
     if (freetype->symbol_map) {
         FT_Face face = freetype->face;
@@ -1719,6 +1720,8 @@ bool QFontEngineFT::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs
                 if (uc < QFreetypeFace::cmapCacheSize)
                     freetype->cmapCache[uc] = glyph;
             }
+            if (glyphs->glyphs[glyph_pos] || isIgnorableChar(uc))
+                mappedGlyphs++;
             ++glyph_pos;
         }
     } else {
@@ -1740,6 +1743,8 @@ bool QFontEngineFT::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs
                         freetype->cmapCache[uc] = glyph;
                 }
             }
+            if (glyphs->glyphs[glyph_pos] || isIgnorableChar(uc))
+                mappedGlyphs++;
             ++glyph_pos;
         }
     }
@@ -1750,7 +1755,7 @@ bool QFontEngineFT::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs
     if (!(flags & GlyphIndicesOnly))
         recalcAdvances(glyphs, flags);
 
-    return true;
+    return mappedGlyphs;
 }
 
 bool QFontEngineFT::shouldUseDesignMetrics(QFontEngine::ShaperFlags flags) const
diff --git a/src/gui/text/freetype/qfontengine_ft_p.h b/src/gui/text/freetype/qfontengine_ft_p.h
index c8e5e20d37..bdd4549827 100644
--- a/src/gui/text/freetype/qfontengine_ft_p.h
+++ b/src/gui/text/freetype/qfontengine_ft_p.h
@@ -186,7 +186,7 @@ private:
     void addOutlineToPath(qreal x, qreal y, const QGlyphLayout &glyphs,
                   QPainterPath *path, QTextItem::RenderFlags flags) override;
 
-    bool stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
+    int stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
 
     glyph_metrics_t boundingBox(const QGlyphLayout &glyphs) override;
     glyph_metrics_t boundingBox(glyph_t glyph) override;
diff --git a/src/gui/text/qfont.cpp b/src/gui/text/qfont.cpp
index 7fd590c355..f3b861957e 100644
--- a/src/gui/text/qfont.cpp
+++ b/src/gui/text/qfont.cpp
@@ -1462,6 +1462,14 @@ QFont::StyleHint QFont::styleHint() const
     \value NoAntialias don't antialias the fonts.
     \value NoSubpixelAntialias avoid subpixel antialiasing on the fonts if possible.
     \value PreferAntialias antialias if possible.
+    \value [since 6.8] ContextFontMerging If the selected font does not contain a certain character,
+           then Qt automatically chooses a similar-looking fallback font that contains the
+           character. By default this is done on a character-by-character basis. This means that in
+           certain uncommon cases, multiple fonts may be used to represent one string of text even
+           if it's in the same script. Setting \c ContextFontMerging will try finding the fallback
+           font that matches the largest subset of the input string instead. This will be more
+           expensive for strings where missing glyphs occur, but may give more consistent results.
+           If \c NoFontMerging is set, then \c ContextFontMerging will have no effect.
     \value NoFontMerging If the font selected for a certain writing system
            does not contain a character requested to draw, then Qt automatically chooses a similar
            looking font that contains the character. The NoFontMerging flag disables this feature.
@@ -3266,7 +3274,7 @@ bool QFontInfo::fixedPitch() const
         QChar ch[2] = { u'i', u'm' };
         QGlyphLayoutArray<2> g;
         int l = 2;
-        if (!engine->stringToCMap(ch, 2, &g, &l, {}))
+        if (engine->stringToCMap(ch, 2, &g, &l, {}) < 0)
             Q_UNREACHABLE();
         Q_ASSERT(l == 2);
         engine->fontDef.fixedPitch = g.advances[0] == g.advances[1];
diff --git a/src/gui/text/qfont.h b/src/gui/text/qfont.h
index ace07780b5..66a5f7c155 100644
--- a/src/gui/text/qfont.h
+++ b/src/gui/text/qfont.h
@@ -47,6 +47,7 @@ public:
         NoAntialias         = 0x0100,
         NoSubpixelAntialias = 0x0800,
         PreferNoShaping     = 0x1000,
+        ContextFontMerging  = 0x2000,
         NoFontMerging       = 0x8000
     };
     Q_ENUM(StyleStrategy)
diff --git a/src/gui/text/qfontengine.cpp b/src/gui/text/qfontengine.cpp
index dff400c18b..4e78aaac2e 100644
--- a/src/gui/text/qfontengine.cpp
+++ b/src/gui/text/qfontengine.cpp
@@ -13,6 +13,7 @@
 #include "qpainter.h"
 #include "qpainterpath.h"
 #include "qvarlengtharray.h"
+#include "qtextengine_p.h"
 #include <qmath.h>
 #include <qendian.h>
 #include <private/qstringiterator_p.h>
@@ -1542,12 +1543,12 @@ glyph_t QFontEngineBox::glyphIndex(uint ucs4) const
     return 1;
 }
 
-bool QFontEngineBox::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, QFontEngine::ShaperFlags flags) const
+int QFontEngineBox::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, QFontEngine::ShaperFlags flags) const
 {
     Q_ASSERT(glyphs->numGlyphs >= *nglyphs);
     if (*nglyphs < len) {
         *nglyphs = len;
-        return false;
+        return -1;
     }
 
     int ucs4Length = 0;
@@ -1563,7 +1564,7 @@ bool QFontEngineBox::stringToCMap(const QChar *str, int len, QGlyphLayout *glyph
     if (!(flags & GlyphIndicesOnly))
         recalcAdvances(glyphs, flags);
 
-    return true;
+    return *nglyphs;
 }
 
 void QFontEngineBox::recalcAdvances(QGlyphLayout *glyphs, QFontEngine::ShaperFlags) const
@@ -1793,11 +1794,7 @@ QFontEngine *QFontEngineMulti::loadEngine(int at)
 glyph_t QFontEngineMulti::glyphIndex(uint ucs4) const
 {
     glyph_t glyph = engine(0)->glyphIndex(ucs4);
-    if (glyph == 0
-            && ucs4 != QChar::LineSeparator
-            && ucs4 != QChar::LineFeed
-            && ucs4 != QChar::CarriageReturn
-            && ucs4 != QChar::ParagraphSeparator) {
+    if (glyph == 0 && !isIgnorableChar(ucs4)) {
         if (!m_fallbackFamiliesQueried)
             const_cast<QFontEngineMulti *>(this)->ensureFallbackFamiliesQueried();
         for (int x = 1, n = qMin(m_engines.size(), 256); x < n; ++x) {
@@ -1824,13 +1821,55 @@ glyph_t QFontEngineMulti::glyphIndex(uint ucs4) const
     return glyph;
 }
 
-bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
-                                    QGlyphLayout *glyphs, int *nglyphs,
-                                    QFontEngine::ShaperFlags flags) const
+int QFontEngineMulti::stringToCMap(const QChar *str, int len,
+                                   QGlyphLayout *glyphs, int *nglyphs,
+                                   QFontEngine::ShaperFlags flags) const
 {
-    if (!engine(0)->stringToCMap(str, len, glyphs, nglyphs, flags))
-        return false;
+    const int originalNumGlyphs = glyphs->numGlyphs;
+    int mappedGlyphCount = engine(0)->stringToCMap(str, len, glyphs, nglyphs, flags);
+    if (mappedGlyphCount < 0)
+        return -1;
+
+    // If ContextFontMerging is set and the match for the string was incomplete, we try all
+    // fallbacks on the full string until we find the best match.
+    bool contextFontMerging = mappedGlyphCount < *nglyphs && (fontDef.styleStrategy & QFont::ContextFontMerging);
+    if (contextFontMerging) {
+        QVarLengthGlyphLayoutArray tempLayout(len);
+        if (!m_fallbackFamiliesQueried)
+            const_cast<QFontEngineMulti *>(this)->ensureFallbackFamiliesQueried();
 
+        int maxGlyphCount = 0;
+        uchar engineIndex = 0;
+        for (int x = 1, n = qMin(m_engines.size(), 256); x < n; ++x) {
+            int numGlyphs = len;
+            const_cast<QFontEngineMulti *>(this)->ensureEngineAt(x);
+            maxGlyphCount = engine(x)->stringToCMap(str, len, &tempLayout, &numGlyphs, flags);
+
+            // If we found a better match, we copy data into the main QGlyphLayout
+            if (maxGlyphCount > mappedGlyphCount) {
+                *nglyphs = numGlyphs;
+                glyphs->numGlyphs = originalNumGlyphs;
+                glyphs->copy(&tempLayout);
+                engineIndex = x;
+                if (maxGlyphCount == numGlyphs)
+                    break;
+            }
+        }
+
+        if (engineIndex > 0) {
+            for (int y = 0; y < glyphs->numGlyphs; ++y) {
+                if (glyphs->glyphs[y] != 0)
+                    glyphs->glyphs[y] |= (engineIndex << 24);
+            }
+        } else {
+            contextFontMerging = false;
+        }
+
+        mappedGlyphCount = maxGlyphCount;
+    }
+
+    // Fill in missing glyphs by going through string one character at the time and finding
+    // the first viable fallback.
     int glyph_pos = 0;
     QStringIterator it(str, str + len);
 
@@ -1861,15 +1900,10 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
             lastFallback = -1;
         }
 
-        if (glyphs->glyphs[glyph_pos] == 0
-                && ucs4 != QChar::LineSeparator
-                && ucs4 != QChar::LineFeed
-                && ucs4 != QChar::CarriageReturn
-                && ucs4 != QChar::ParagraphSeparator
-                && QChar::category(ucs4) != QChar::Other_Control) {
+        if (glyphs->glyphs[glyph_pos] == 0 && !isIgnorableChar(ucs4)) {
             if (!m_fallbackFamiliesQueried)
                 const_cast<QFontEngineMulti *>(this)->ensureFallbackFamiliesQueried();
-            for (int x = 1, n = qMin(m_engines.size(), 256); x < n; ++x) {
+            for (int x = contextFontMerging ? 0 : 1, n = qMin(m_engines.size(), 256); x < n; ++x) {
                 QFontEngine *engine = m_engines.at(x);
                 if (!engine) {
                     if (!shouldLoadFontEngineForCharacter(x, ucs4))
@@ -1946,8 +1980,7 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
 
     *nglyphs = glyph_pos;
     glyphs->numGlyphs = glyph_pos;
-
-    return true;
+    return mappedGlyphCount;
 }
 
 bool QFontEngineMulti::shouldLoadFontEngineForCharacter(int at, uint ucs4) const
@@ -2239,7 +2272,7 @@ bool QFontEngineMulti::canRender(const QChar *string, int len) const
     QGlyphLayout g;
     g.numGlyphs = nglyphs;
     g.glyphs = glyphs.data();
-    if (!stringToCMap(string, len, &g, &nglyphs, GlyphIndicesOnly))
+    if (stringToCMap(string, len, &g, &nglyphs, GlyphIndicesOnly) < 0)
         Q_UNREACHABLE();
 
     for (int i = 0; i < nglyphs; i++) {
diff --git a/src/gui/text/qfontengine_p.h b/src/gui/text/qfontengine_p.h
index af44eeab3f..a0e0801354 100644
--- a/src/gui/text/qfontengine_p.h
+++ b/src/gui/text/qfontengine_p.h
@@ -76,7 +76,8 @@ public:
 
     enum ShaperFlag {
         DesignMetrics = 0x0002,
-        GlyphIndicesOnly = 0x0004
+        GlyphIndicesOnly = 0x0004,
+        FullStringFallback = 0x008
     };
     Q_DECLARE_FLAGS(ShaperFlags, ShaperFlag)
 
@@ -148,12 +149,20 @@ public:
     }
 
     bool isColorFont() const { return glyphFormat == Format_ARGB; }
+    static bool isIgnorableChar(char32_t ucs4)
+    {
+        return ucs4 == QChar::LineSeparator
+               || ucs4 == QChar::LineFeed
+               || ucs4 == QChar::CarriageReturn
+               || ucs4 == QChar::ParagraphSeparator
+               || QChar::category(ucs4) == QChar::Other_Control;
+    }
 
     virtual QFixed emSquareSize() const { return ascent(); }
 
     /* returns 0 as glyph index for non existent glyphs */
     virtual glyph_t glyphIndex(uint ucs4) const = 0;
-    virtual bool stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const = 0;
+    virtual int stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const = 0;
     virtual void recalcAdvances(QGlyphLayout *, ShaperFlags) const {}
     virtual void doKerning(QGlyphLayout *, ShaperFlags) const;
 
@@ -393,7 +402,7 @@ public:
     ~QFontEngineBox();
 
     virtual glyph_t glyphIndex(uint ucs4) const override;
-    virtual bool stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
+    virtual int stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
     virtual void recalcAdvances(QGlyphLayout *, ShaperFlags) const override;
 
     void draw(QPaintEngine *p, qreal x, qreal y, const QTextItemInt &si);
@@ -431,7 +440,7 @@ public:
     ~QFontEngineMulti();
 
     virtual glyph_t glyphIndex(uint ucs4) const override;
-    virtual bool stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
+    virtual int stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
 
     virtual glyph_metrics_t boundingBox(const QGlyphLayout &glyphs) override;
     virtual glyph_metrics_t boundingBox(glyph_t glyph) override;
diff --git a/src/gui/text/qrawfont.cpp b/src/gui/text/qrawfont.cpp
index c5a92f95cb..54676b3560 100644
--- a/src/gui/text/qrawfont.cpp
+++ b/src/gui/text/qrawfont.cpp
@@ -498,7 +498,7 @@ QList<quint32> QRawFont::glyphIndexesForString(const QString &text) const
     QGlyphLayout glyphs;
     glyphs.numGlyphs = numGlyphs;
     glyphs.glyphs = glyphIndexes.data();
-    if (!d->fontEngine->stringToCMap(text.data(), text.size(), &glyphs, &numGlyphs, QFontEngine::GlyphIndicesOnly))
+    if (d->fontEngine->stringToCMap(text.data(), text.size(), &glyphs, &numGlyphs, QFontEngine::GlyphIndicesOnly) < 0)
         Q_UNREACHABLE();
 
     glyphIndexes.resize(numGlyphs);
@@ -531,7 +531,7 @@ bool QRawFont::glyphIndexesForChars(const QChar *chars, int numChars, quint32 *g
     QGlyphLayout glyphs;
     glyphs.numGlyphs = *numGlyphs;
     glyphs.glyphs = glyphIndexes;
-    return d->fontEngine->stringToCMap(chars, numChars, &glyphs, numGlyphs, QFontEngine::GlyphIndicesOnly);
+    return d->fontEngine->stringToCMap(chars, numChars, &glyphs, numGlyphs, QFontEngine::GlyphIndicesOnly) >= 0;
 }
 
 /*!
diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index cb945b73ce..a18157ab9b 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -1445,7 +1445,7 @@ void QTextEngine::shapeText(int item) const
                 shapingEnabled
                     ? QFontEngine::GlyphIndicesOnly
                     : QFontEngine::ShaperFlag(0);
-        if (!fontEngine->stringToCMap(reinterpret_cast<const QChar *>(string), itemLength, &initialGlyphs, &nGlyphs, shaperFlags))
+        if (fontEngine->stringToCMap(reinterpret_cast<const QChar *>(string), itemLength, &initialGlyphs, &nGlyphs, shaperFlags) < 0)
             Q_UNREACHABLE();
     }
 
@@ -2741,6 +2741,21 @@ bool QTextEngine::LayoutData::reallocate(int totalGlyphs)
     return true;
 }
 
+void QGlyphLayout::copy(QGlyphLayout *oldLayout)
+{
+    Q_ASSERT(offsets != oldLayout->offsets);
+
+    int n = std::min(numGlyphs, oldLayout->numGlyphs);
+
+    memcpy(offsets, oldLayout->offsets, n * sizeof(QFixedPoint));
+    memcpy(attributes, oldLayout->attributes, n * sizeof(QGlyphAttributes));
+    memcpy(justifications, oldLayout->justifications, n * sizeof(QGlyphJustification));
+    memcpy(advances, oldLayout->advances, n * sizeof(QFixed));
+    memcpy(glyphs, oldLayout->glyphs, n * sizeof(glyph_t));
+
+    numGlyphs = n;
+}
+
 // grow to the new size, copying the existing data to the new layout
 void QGlyphLayout::grow(char *address, int totalGlyphs)
 {
diff --git a/src/gui/text/qtextengine_p.h b/src/gui/text/qtextengine_p.h
index a829265a22..c01d3a0711 100644
--- a/src/gui/text/qtextengine_p.h
+++ b/src/gui/text/qtextengine_p.h
@@ -224,6 +224,7 @@ struct QGlyphLayout
         return reinterpret_cast<char *>(offsets);
     }
 
+    void copy(QGlyphLayout *other);
     void grow(char *address, int totalGlyphs);
 };
 
diff --git a/src/gui/text/windows/qwindowsfontengine.cpp b/src/gui/text/windows/qwindowsfontengine.cpp
index fe07897369..5de80dc8a3 100644
--- a/src/gui/text/windows/qwindowsfontengine.cpp
+++ b/src/gui/text/windows/qwindowsfontengine.cpp
@@ -132,8 +132,9 @@ void QWindowsFontEngine::getCMap()
     }
 }
 
-int QWindowsFontEngine::getGlyphIndexes(const QChar *str, int numChars, QGlyphLayout *glyphs) const
+int QWindowsFontEngine::getGlyphIndexes(const QChar *str, int numChars, QGlyphLayout *glyphs, int *mappedGlyphs) const
 {
+    *mappedGlyphs = 0;
     int glyph_pos = 0;
     {
         if (symbol) {
@@ -143,6 +144,8 @@ int QWindowsFontEngine::getGlyphIndexes(const QChar *str, int numChars, QGlyphLa
                 glyphs->glyphs[glyph_pos] = getTrueTypeGlyphIndex(cmap, cmapSize, uc);
                 if (!glyphs->glyphs[glyph_pos] && uc < 0x100)
                     glyphs->glyphs[glyph_pos] = getTrueTypeGlyphIndex(cmap, cmapSize, uc + 0xf000);
+                if (glyphs->glyphs[glyph_pos] || isIgnorableChar(uc))
+                    (*mappedGlyphs)++;
                 ++glyph_pos;
             }
         } else if (ttf) {
@@ -150,6 +153,8 @@ int QWindowsFontEngine::getGlyphIndexes(const QChar *str, int numChars, QGlyphLa
             while (it.hasNext()) {
                 const uint uc = it.next();
                 glyphs->glyphs[glyph_pos] = getTrueTypeGlyphIndex(cmap, cmapSize, uc);
+                if (glyphs->glyphs[glyph_pos] || isIgnorableChar(uc))
+                    (*mappedGlyphs)++;
                 ++glyph_pos;
             }
         } else {
@@ -160,6 +165,8 @@ int QWindowsFontEngine::getGlyphIndexes(const QChar *str, int numChars, QGlyphLa
                     glyphs->glyphs[glyph_pos] = uc;
                 else
                     glyphs->glyphs[glyph_pos] = 0;
+                if (glyphs->glyphs[glyph_pos] || isIgnorableChar(uc))
+                    (*mappedGlyphs)++;
                 ++glyph_pos;
             }
         }
@@ -259,7 +266,7 @@ HGDIOBJ QWindowsFontEngine::selectDesignFont() const
     return SelectObject(m_fontEngineData->hdc, designFont);
 }
 
-bool QWindowsFontEngine::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, QFontEngine::ShaperFlags flags) const
+int QWindowsFontEngine::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, QFontEngine::ShaperFlags flags) const
 {
     Q_ASSERT(glyphs->numGlyphs >= *nglyphs);
     if (*nglyphs < len) {
@@ -268,12 +275,13 @@ bool QWindowsFontEngine::stringToCMap(const QChar *str, int len, QGlyphLayout *g
     }
 
     glyphs->numGlyphs = *nglyphs;
-    *nglyphs = getGlyphIndexes(str, len, glyphs);
+    int mappedGlyphs;
+    *nglyphs = getGlyphIndexes(str, len, glyphs, &mappedGlyphs);
 
     if (!(flags & GlyphIndicesOnly))
         recalcAdvances(glyphs, flags);
 
-    return true;
+    return mappedGlyphs;
 }
 
 inline void calculateTTFGlyphWidth(HDC hdc, UINT glyph, int &width)
diff --git a/src/gui/text/windows/qwindowsfontengine_p.h b/src/gui/text/windows/qwindowsfontengine_p.h
index afe8ee4ca5..07f4db3c4a 100644
--- a/src/gui/text/windows/qwindowsfontengine_p.h
+++ b/src/gui/text/windows/qwindowsfontengine_p.h
@@ -48,7 +48,7 @@ public:
     QFixed emSquareSize() const override;
 
     glyph_t glyphIndex(uint ucs4) const override;
-    bool stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
+    int stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs, ShaperFlags flags) const override;
     void recalcAdvances(QGlyphLayout *glyphs, ShaperFlags) const override;
 
     void addOutlineToPath(qreal x, qreal y, const QGlyphLayout &glyphs, QPainterPath *path, QTextItem::RenderFlags flags) override;
@@ -88,7 +88,7 @@ public:
 
     bool hasUnreliableGlyphOutline() const override;
 
-    int getGlyphIndexes(const QChar *ch, int numChars, QGlyphLayout *glyphs) const;
+    int getGlyphIndexes(const QChar *ch, int numChars, QGlyphLayout *glyphs, int *mappedGlyphs) const;
     void getCMap();
 
     bool getOutlineMetrics(glyph_t glyph, const QTransform &t, glyph_metrics_t *metrics) const;
diff --git a/src/gui/text/windows/qwindowsfontenginedirectwrite.cpp b/src/gui/text/windows/qwindowsfontenginedirectwrite.cpp
index 2070deb296..47b8a7ee3c 100644
--- a/src/gui/text/windows/qwindowsfontenginedirectwrite.cpp
+++ b/src/gui/text/windows/qwindowsfontenginedirectwrite.cpp
@@ -435,13 +435,13 @@ glyph_t QWindowsFontEngineDirectWrite::glyphIndex(uint ucs4) const
     return glyphIndex;
 }
 
-bool QWindowsFontEngineDirectWrite::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs,
-                                                 int *nglyphs, QFontEngine::ShaperFlags flags) const
+int QWindowsFontEngineDirectWrite::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs,
+                                                int *nglyphs, QFontEngine::ShaperFlags flags) const
 {
     Q_ASSERT(glyphs->numGlyphs >= *nglyphs);
     if (*nglyphs < len) {
         *nglyphs = len;
-        return false;
+        return -1;
     }
 
     QVarLengthArray<UINT32> codePoints(len);
@@ -455,11 +455,15 @@ bool QWindowsFontEngineDirectWrite::stringToCMap(const QChar *str, int len, QGly
                                                          glyphIndices.data());
     if (FAILED(hr)) {
         qErrnoWarning("%s: GetGlyphIndicesW failed", __FUNCTION__);
-        return false;
+        return -1;
     }
 
-    for (int i = 0; i < actualLength; ++i)
+    int mappedGlyphs = 0;
+    for (int i = 0; i < actualLength; ++i) {
         glyphs->glyphs[i] = glyphIndices.at(i);
+        if (glyphs->glyphs[i] != 0 || isIgnorableChar(codePoints.at(i)))
+            mappedGlyphs++;
+    }
 
     *nglyphs = actualLength;
     glyphs->numGlyphs = actualLength;
@@ -467,7 +471,7 @@ bool QWindowsFontEngineDirectWrite::stringToCMap(const QChar *str, int len, QGly
     if (!(flags & GlyphIndicesOnly))
         recalcAdvances(glyphs, {});
 
-    return true;
+    return mappedGlyphs;
 }
 
 QFontEngine::FaceId QWindowsFontEngineDirectWrite::faceId() const
diff --git a/src/gui/text/windows/qwindowsfontenginedirectwrite_p.h b/src/gui/text/windows/qwindowsfontenginedirectwrite_p.h
index 44e466789c..d7c9a79267 100644
--- a/src/gui/text/windows/qwindowsfontenginedirectwrite_p.h
+++ b/src/gui/text/windows/qwindowsfontenginedirectwrite_p.h
@@ -53,8 +53,8 @@ public:
     QFixed emSquareSize() const override;
 
     glyph_t glyphIndex(uint ucs4) const override;
-    bool stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs,
-                      ShaperFlags flags) const override;
+    int stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs, int *nglyphs,
+                     ShaperFlags flags) const override;
     void recalcAdvances(QGlyphLayout *glyphs, ShaperFlags) const override;
 
     void addGlyphsToPath(glyph_t *glyphs, QFixedPoint *positions, int nglyphs,
diff --git a/tests/auto/gui/text/qfontmetrics/tst_qfontmetrics.cpp b/tests/auto/gui/text/qfontmetrics/tst_qfontmetrics.cpp
index 678eb0393f..9471c1d93f 100644
--- a/tests/auto/gui/text/qfontmetrics/tst_qfontmetrics.cpp
+++ b/tests/auto/gui/text/qfontmetrics/tst_qfontmetrics.cpp
@@ -263,7 +263,7 @@ void tst_QFontMetrics::inFontUcs4()
             glyphs.glyphs[0] = 0;
             QVERIFY(engine->stringToCMap(string.constData(), string.size(),
                                          &glyphs, &glyphs.numGlyphs,
-                                         QFontEngine::GlyphIndicesOnly));
+                                         QFontEngine::GlyphIndicesOnly) > 0);
             QCOMPARE(glyphs.numGlyphs, 1);
             QCOMPARE(glyphs.glyphs[0], uint(1));
         }
@@ -275,7 +275,7 @@ void tst_QFontMetrics::inFontUcs4()
             glyphs.glyphs[0] = 0;
             QVERIFY(engine->stringToCMap(string.constData(), string.size(),
                                          &glyphs, &glyphs.numGlyphs,
-                                         QFontEngine::GlyphIndicesOnly));
+                                         QFontEngine::GlyphIndicesOnly) >= 0);
             QVERIFY(glyphs.glyphs[0] != 1);
         }
     }