Support sub pixel positioning of glyphs in raster engine

For the raster engine to provide the same quality of text rendering
as the native engine on Mac Cocoa, we need to support rendering to
different sub pixel positions for each glyph. The number of
subpixel positions is arbitrary and has to be detected, but it's
usually three or four.
Each position will give slightly different coverages inside the
pixel and thus different rasterizations. Other font engines which
support sub pixel positioning of glyphs can provide the same
functionality by implementing supportsSubPixelPositions() to return
true, and then adding the subPixelPosition argument to the x
coordinate used in alphaRGBMapForGlyph().

Task-number: QTBUG-5053
Reviewed-by: Jiang Jiang

13 files changed, 155 insertions, 44 deletions

diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index 0b76898..94d7578 100644
--- a/src/gui/painting/qpaintengine_raster.cpp
+++ b/src/gui/painting/qpaintengine_raster.cpp
@@ -3098,9 +3098,17 @@ void QRasterPaintEngine::drawCachedGlyphs(int numGlyphs, const glyph_t *glyphs,
 
     int margin = cache->glyphMargin();
 
+    bool supportsSubPixelPositions = fontEngine->supportsSubPixelPositions();
+
     const uchar *bits = image.bits();
     for (int i=0; i<numGlyphs; ++i) {
-        const QTextureGlyphCache::Coord &c = cache->coords.value(glyphs[i]);
+
+        QFixed subPixelPosition;
+        if (supportsSubPixelPositions)
+            subPixelPosition = cache->subPixelPositionForX(positions[i].x);
+        QTextureGlyphCache::GlyphAndSubPixelPosition glyph(glyphs[i], subPixelPosition);
+        const QTextureGlyphCache::Coord &c = cache->coords.value(glyph);
+
         int x = qFloor(positions[i].x) + c.baseLineX - margin;
         int y = qFloor(positions[i].y) - c.baseLineY - margin;
 
diff --git a/src/gui/painting/qtextureglyphcache.cpp b/src/gui/painting/qtextureglyphcache.cpp
index 376219b..e992bb2 100644
--- a/src/gui/painting/qtextureglyphcache.cpp
+++ b/src/gui/painting/qtextureglyphcache.cpp
@@ -69,8 +69,60 @@ static inline int qt_next_power_of_two(int v)
     return v;
 }
 
+int QTextureGlyphCache::calculateSubPixelPositionCount(glyph_t glyph) const
+{
+    // Test 12 different subpixel positions since it factors into 3*4 so it gives
+    // the coverage we need.
+
+    QList<QImage> images;
+    for (int i=0; i<12; ++i) {
+        QImage img = textureMapForGlyph(glyph, QFixed::fromReal(i / 12.0));
+
+        if (images.isEmpty()) {
+            QPainterPath path;
+            QFixedPoint point;
+            m_current_fontengine->addGlyphsToPath(&glyph, &point, 1, &path, QTextItem::RenderFlags());
+
+            // Glyph is space, return 0 to indicate that we need to keep trying
+            if (path.isEmpty())
+                break;
+
+            images.append(img);
+        } else {
+            bool found = false;
+            for (int j=0; j<images.size(); ++j) {
+                if (images.at(j) == img) {
+                    found = true;
+                    break;
+                }
+            }
+            if (!found)
+                images.append(img);
+        }
+    }
+
+    return images.size();
+}
+
+QFixed QTextureGlyphCache::subPixelPositionForX(QFixed x) const
+{
+    if (m_subPixelPositionCount == 0)
+        return QFixed();
+
+    QFixed subPixelPosition;
+    if (x != 0) {
+        subPixelPosition = x - x.floor();
+        QFixed fraction = (subPixelPosition / QFixed::fromReal(1.0 / m_subPixelPositionCount)).floor();
+
+        // Compensate for precision loss in fixed point to make sure we are always drawing at a subpixel position over
+        // the lower boundary for the selected rasterization by adding 1/64.
+        subPixelPosition = fraction / QFixed(m_subPixelPositionCount) + QFixed::fromReal(0.015625);
+    }
+    return subPixelPosition;
+}
+
 void QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const glyph_t *glyphs,
-                                  const QFixedPoint *)
+                                                const QFixedPoint *positions)
 {
 #ifdef CACHE_DEBUG
     printf("Populating with %d glyphs\n", numGlyphs);
@@ -81,15 +133,33 @@ void QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
     const int margin = glyphMargin();
     const int paddingDoubled = glyphPadding() * 2;
 
-    QHash<glyph_t, Coord> listItemCoordinates;
+    bool supportsSubPixelPositions = fontEngine->supportsSubPixelPositions();
+    if (m_subPixelPositionCount == 0) {
+        if (!supportsSubPixelPositions) {
+            m_subPixelPositionCount = 1;
+        } else {
+            int i = 0;
+            while (m_subPixelPositionCount == 0 && i < numGlyphs)
+                m_subPixelPositionCount = calculateSubPixelPositionCount(glyphs[i++]);
+        }
+    }
+
+    QHash<GlyphAndSubPixelPosition, Coord> listItemCoordinates;
     int rowHeight = 0;
 
     // check each glyph for its metrics and get the required rowHeight.
     for (int i=0; i < numGlyphs; ++i) {
         const glyph_t glyph = glyphs[i];
-        if (coords.contains(glyph))
+
+        QFixed subPixelPosition;
+        if (supportsSubPixelPositions) {
+            QFixed x = positions != 0 ? positions[i].x : QFixed();
+            subPixelPosition = subPixelPositionForX(x);
+        }
+
+        if (coords.contains(GlyphAndSubPixelPosition(glyph, subPixelPosition)))
             continue;
-        if (listItemCoordinates.contains(glyph))
+        if (listItemCoordinates.contains(GlyphAndSubPixelPosition(glyph, subPixelPosition)))
             continue;
         glyph_metrics_t metrics = fontEngine->boundingBox(glyph, m_transform);
 
@@ -119,7 +189,7 @@ void QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
                     metrics.x.round().truncate(),
                     -metrics.y.truncate() }; // baseline for horizontal scripts
 
-        listItemCoordinates.insert(glyph, c);
+        listItemCoordinates.insert(GlyphAndSubPixelPosition(glyph, subPixelPosition), c);
         rowHeight = qMax(rowHeight, glyph_height);
     }
     if (listItemCoordinates.isEmpty())
@@ -135,7 +205,7 @@ void QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
     }
 
     // now actually use the coords and paint the wanted glyps into cache.
-    QHash<glyph_t, Coord>::iterator iter = listItemCoordinates.begin();
+    QHash<GlyphAndSubPixelPosition, Coord>::iterator iter = listItemCoordinates.begin();
     while (iter != listItemCoordinates.end()) {
         Coord c = iter.value();
 
@@ -166,7 +236,7 @@ void QTextureGlyphCache::fillInPendingGlyphs()
 
     int requiredHeight = 0;
     {
-        QHash<glyph_t, Coord>::iterator iter = m_pendingGlyphs.begin();
+        QHash<GlyphAndSubPixelPosition, Coord>::iterator iter = m_pendingGlyphs.begin();
         while (iter != m_pendingGlyphs.end()) {
             Coord c = iter.value();
             requiredHeight = qMax(requiredHeight, c.y + c.h);
@@ -182,9 +252,10 @@ void QTextureGlyphCache::fillInPendingGlyphs()
     }
 
     {
-        QHash<glyph_t, Coord>::iterator iter = m_pendingGlyphs.begin();
+        QHash<GlyphAndSubPixelPosition, Coord>::iterator iter = m_pendingGlyphs.begin();
         while (iter != m_pendingGlyphs.end()) {
-            fillTexture(iter.value(), iter.key());
+            GlyphAndSubPixelPosition key = iter.key();
+            fillTexture(iter.value(), key.glyph, key.subPixelPosition);
 
             ++iter;
         }
@@ -193,7 +264,7 @@ void QTextureGlyphCache::fillInPendingGlyphs()
     m_pendingGlyphs.clear();
 }
 
-QImage QTextureGlyphCache::textureMapForGlyph(glyph_t g) const
+QImage QTextureGlyphCache::textureMapForGlyph(glyph_t g, QFixed subPixelPosition) const
 {
 #if defined(Q_WS_X11)
     if (m_transform.type() > QTransform::TxTranslate) {
@@ -226,7 +297,7 @@ QImage QTextureGlyphCache::textureMapForGlyph(glyph_t g) const
     } else
 #endif
     if (m_type == QFontEngineGlyphCache::Raster_RGBMask)
-        return m_current_fontengine->alphaRGBMapForGlyph(g, glyphMargin(), m_transform);
+        return m_current_fontengine->alphaRGBMapForGlyph(g, subPixelPosition, glyphMargin(), m_transform);
     else
         return m_current_fontengine->alphaMapForGlyph(g, m_transform);
 
@@ -272,9 +343,9 @@ int QImageTextureGlyphCache::glyphMargin() const
 #endif
 }
 
-void QImageTextureGlyphCache::fillTexture(const Coord &c, glyph_t g)
+void QImageTextureGlyphCache::fillTexture(const Coord &c, glyph_t g, QFixed subPixelPosition)
 {
-    QImage mask = textureMapForGlyph(g);
+    QImage mask = textureMapForGlyph(g, subPixelPosition);
 
 #ifdef CACHE_DEBUG
     printf("fillTexture of %dx%d at %d,%d in the cache of %dx%d\n", c.w, c.h, c.x, c.y, m_image.width(), m_image.height());
diff --git a/src/gui/painting/qtextureglyphcache_p.h b/src/gui/painting/qtextureglyphcache_p.h
index 0770ed4..4131003 100644
--- a/src/gui/painting/qtextureglyphcache_p.h
+++ b/src/gui/painting/qtextureglyphcache_p.h
@@ -77,11 +77,24 @@ class Q_GUI_EXPORT QTextureGlyphCache : public QFontEngineGlyphCache
 public:
     QTextureGlyphCache(QFontEngineGlyphCache::Type type, const QTransform &matrix)
         : QFontEngineGlyphCache(matrix, type), m_current_fontengine(0),
-                                               m_w(0), m_h(0), m_cx(0), m_cy(0), m_currentRowHeight(0)
+                                               m_w(0), m_h(0), m_cx(0), m_cy(0), m_currentRowHeight(0), m_subPixelPositionCount(0)
         { }
 
     virtual ~QTextureGlyphCache() { }
 
+    struct GlyphAndSubPixelPosition
+    {
+        GlyphAndSubPixelPosition(glyph_t g, QFixed spp) : glyph(g), subPixelPosition(spp) {}
+
+        bool operator==(const GlyphAndSubPixelPosition &other) const
+        {
+            return glyph == other.glyph && subPixelPosition == other.subPixelPosition;
+        }
+
+        glyph_t glyph;
+        QFixed subPixelPosition;
+    };
+
     struct Coord {
         int x;
         int y;
@@ -101,7 +114,7 @@ public:
     virtual int glyphMargin() const { return 0; }
     virtual int glyphPadding() const { return 0; }
 
-    virtual void fillTexture(const Coord &coord, glyph_t glyph) = 0;
+    virtual void fillTexture(const Coord &coord, glyph_t glyph, QFixed subPixelPosition) = 0;
 
     inline void createCache(int width, int height) {
         m_w = width;
@@ -118,22 +131,31 @@ public:
 
     inline bool isNull() const { return m_h == 0; }
 
-    QHash<glyph_t, Coord> coords;
+    QHash<GlyphAndSubPixelPosition, Coord> coords;
 
-    QImage textureMapForGlyph(glyph_t g) const;
+    QImage textureMapForGlyph(glyph_t g, QFixed subPixelPosition) const;
+
+    QFixed subPixelPositionForX(QFixed x) const;
 
 protected:
-    QFontEngine *m_current_fontengine;
+    int calculateSubPixelPositionCount(glyph_t) const;
 
-    QHash<glyph_t, Coord> m_pendingGlyphs;
+    QFontEngine *m_current_fontengine;
+    QHash<GlyphAndSubPixelPosition, Coord> m_pendingGlyphs;
 
     int m_w; // image width
     int m_h; // image height
     int m_cx; // current x
     int m_cy; // current y
     int m_currentRowHeight; // Height of last row
+    int m_subPixelPositionCount; // Number of positions within a single pixel for this cache
 };
 
+inline uint qHash(const QTextureGlyphCache::GlyphAndSubPixelPosition &g)
+{
+    return (g.glyph << 8)  | (g.subPixelPosition * 10).round().toInt();
+}
+
 
 class Q_GUI_EXPORT QImageTextureGlyphCache : public QTextureGlyphCache
 {
@@ -143,7 +165,7 @@ public:
     virtual int glyphMargin() const;
     virtual void createTextureData(int width, int height);
     virtual void resizeTextureData(int width, int height);
-    virtual void fillTexture(const Coord &c, glyph_t glyph);
+    virtual void fillTexture(const Coord &c, glyph_t glyph, QFixed subPixelPosition);
 
     inline const QImage &image() const { return m_image; }
 
diff --git a/src/gui/text/qfontengine.cpp b/src/gui/text/qfontengine.cpp
index 1b94915..55d2a83 100644
--- a/src/gui/text/qfontengine.cpp
+++ b/src/gui/text/qfontengine.cpp
@@ -616,7 +616,7 @@ QImage QFontEngine::alphaMapForGlyph(glyph_t glyph, const QTransform &t)
     return i;
 }
 
-QImage QFontEngine::alphaRGBMapForGlyph(glyph_t glyph, int /* margin */, const QTransform &t)
+QImage QFontEngine::alphaRGBMapForGlyph(glyph_t glyph, QFixed /*subPixelPosition*/, int /* margin */, const QTransform &t)
 {
     QImage alphaMask = alphaMapForGlyph(glyph, t);
     QImage rgbMask(alphaMask.width(), alphaMask.height(), QImage::Format_RGB32);
diff --git a/src/gui/text/qfontengine_ft.cpp b/src/gui/text/qfontengine_ft.cpp
index 7a8b1e5..7b6ef0e 100644
--- a/src/gui/text/qfontengine_ft.cpp
+++ b/src/gui/text/qfontengine_ft.cpp
@@ -1873,10 +1873,10 @@ QImage QFontEngineFT::alphaMapForGlyph(glyph_t g)
     return img;
 }
 
-QImage QFontEngineFT::alphaRGBMapForGlyph(glyph_t g, int margin, const QTransform &t)
+QImage QFontEngineFT::alphaRGBMapForGlyph(glyph_t g, QFixed subPixelPosition, int margin, const QTransform &t)
 {
     if (t.type() > QTransform::TxTranslate)
-        return QFontEngine::alphaRGBMapForGlyph(g, margin, t);
+        return QFontEngine::alphaRGBMapForGlyph(g, subPixelPosition, margin, t);
 
     lockFace();
 
@@ -1885,7 +1885,7 @@ QImage QFontEngineFT::alphaRGBMapForGlyph(glyph_t g, int margin, const QTransfor
     Glyph *glyph = defaultGlyphSet.outline_drawing ? 0 : loadGlyph(g, glyph_format);
     if (!glyph) {
         unlockFace();
-        return QFontEngine::alphaRGBMapForGlyph(g, margin, t);
+        return QFontEngine::alphaRGBMapForGlyph(g, subPixelPosition, margin, t);
     }
 
     QImage img(glyph->width, glyph->height, QImage::Format_RGB32);
diff --git a/src/gui/text/qfontengine_ft_p.h b/src/gui/text/qfontengine_ft_p.h
index 2f05a8b..72f7d9f 100644
--- a/src/gui/text/qfontengine_ft_p.h
+++ b/src/gui/text/qfontengine_ft_p.h
@@ -241,7 +241,7 @@ private:
 
     virtual void recalcAdvances(QGlyphLayout *glyphs, QTextEngine::ShaperFlags flags) const;
     virtual QImage alphaMapForGlyph(glyph_t);
-    virtual QImage alphaRGBMapForGlyph(glyph_t, int margin, const QTransform &t);
+    virtual QImage alphaRGBMapForGlyph(glyph_t, QFixed subPixelPosition, int margin, const QTransform &t);
     virtual void removeGlyphFromCache(glyph_t glyph);
 
     virtual int glyphCount() const;
diff --git a/src/gui/text/qfontengine_mac.mm b/src/gui/text/qfontengine_mac.mm
index 2bbf9f2..1ca6ec3 100644
--- a/src/gui/text/qfontengine_mac.mm
+++ b/src/gui/text/qfontengine_mac.mm
@@ -663,7 +663,7 @@ QFont QCoreTextFontEngine::createExplicitFont() const
     return createExplicitFontWithName(familyName);
 }
 
-QImage QCoreTextFontEngine::imageForGlyph(glyph_t glyph, int margin, bool aa)
+QImage QCoreTextFontEngine::imageForGlyph(glyph_t glyph, QFixed subPixelPosition, int /*margin*/, bool aa)
 {
     const glyph_metrics_t br = boundingBox(glyph);
     QImage im(qRound(br.width)+2, qRound(br.height)+2, QImage::Format_RGB32);
@@ -700,7 +700,8 @@ QImage QCoreTextFontEngine::imageForGlyph(glyph_t glyph, int margin, bool aa)
     QCFType<CGFontRef> cgFont = CGFontCreateWithPlatformFont(&atsfont);
     CGContextSetFont(ctx, cgFont);
 
-    qreal pos_x = -br.x.toReal()+1, pos_y = im.height()+br.y.toReal();
+    qreal pos_x = -br.x.toReal() + subPixelPosition.toReal();
+    qreal pos_y = im.height()+br.y.toReal();
     CGContextSetTextPosition(ctx, pos_x, pos_y);
 
     CGSize advance;
@@ -721,7 +722,7 @@ QImage QCoreTextFontEngine::imageForGlyph(glyph_t glyph, int margin, bool aa)
 
 QImage QCoreTextFontEngine::alphaMapForGlyph(glyph_t glyph)
 {
-    QImage im = imageForGlyph(glyph, 0, false);
+    QImage im = imageForGlyph(glyph, QFixed(), 0, false);
 
     QImage indexed(im.width(), im.height(), QImage::Format_Indexed8);
     QVector<QRgb> colors(256);
@@ -742,12 +743,12 @@ QImage QCoreTextFontEngine::alphaMapForGlyph(glyph_t glyph)
     return indexed;
 }
 
-QImage QCoreTextFontEngine::alphaRGBMapForGlyph(glyph_t glyph, int margin, const QTransform &x)
+QImage QCoreTextFontEngine::alphaRGBMapForGlyph(glyph_t glyph, QFixed subPixelPosition, int margin, const QTransform &x)
 {
     if (x.type() >= QTransform::TxScale)
-        return QFontEngine::alphaRGBMapForGlyph(glyph, margin, x);
+        return QFontEngine::alphaRGBMapForGlyph(glyph, subPixelPosition, margin, x);
 
-    QImage im = imageForGlyph(glyph, margin, true);
+    QImage im = imageForGlyph(glyph, subPixelPosition, margin, true);
     qmacfontengine_gamma_correct(&im);
     return im;
 }
@@ -1706,7 +1707,7 @@ QImage QFontEngineMac::alphaMapForGlyph(glyph_t glyph)
     return indexed;
 }
 
-QImage QFontEngineMac::alphaRGBMapForGlyph(glyph_t glyph, int margin, const QTransform &t)
+QImage QFontEngineMac::alphaRGBMapForGlyph(glyph_t glyph, QFixed, int margin, const QTransform &t)
 {
     QImage im = imageForGlyph(glyph, margin, true);
 
diff --git a/src/gui/text/qfontengine_p.h b/src/gui/text/qfontengine_p.h
index 6e9bd0bc..6ebbc1f 100644
--- a/src/gui/text/qfontengine_p.h
+++ b/src/gui/text/qfontengine_p.h
@@ -155,6 +155,7 @@ public:
         SynthesizedStretch = 0x4
     };
     virtual int synthesized() const { return 0; }
+    virtual bool supportsSubPixelPositions() const { return false; }
 
     virtual QFixed emSquareSize() const { return ascent(); }
 
@@ -188,7 +189,7 @@ public:
      */
     virtual QImage alphaMapForGlyph(glyph_t);
     virtual QImage alphaMapForGlyph(glyph_t, const QTransform &t);
-    virtual QImage alphaRGBMapForGlyph(glyph_t, int margin, const QTransform &t);
+    virtual QImage alphaRGBMapForGlyph(glyph_t, QFixed subPixelPosition, int margin, const QTransform &t);
 
     virtual void removeGlyphFromCache(glyph_t);
 
@@ -448,6 +449,7 @@ public:
     virtual bool canRender(const QChar *string, int len);
 
     virtual int synthesized() const { return synthesisFlags; }
+    virtual bool supportsSubPixelPositions() const { return true; }
 
     virtual Type type() const { return QFontEngine::Mac; }
 
@@ -457,13 +459,13 @@ public:
     virtual bool getSfntTableData(uint /*tag*/, uchar * /*buffer*/, uint * /*length*/) const;
     virtual void getUnscaledGlyph(glyph_t glyph, QPainterPath *path, glyph_metrics_t *metrics);
     virtual QImage alphaMapForGlyph(glyph_t);
-    virtual QImage alphaRGBMapForGlyph(glyph_t, int margin, const QTransform &t);
+    virtual QImage alphaRGBMapForGlyph(glyph_t, QFixed subPixelPosition, int margin, const QTransform &t);
     virtual qreal minRightBearing() const;
     virtual qreal minLeftBearing() const;
     virtual QFont createExplicitFont() const;
 
 private:
-    QImage imageForGlyph(glyph_t glyph, int margin, bool colorful);
+    QImage imageForGlyph(glyph_t glyph, QFixed subPixelPosition, int margin, bool colorful);
     CTFontRef ctfont;
     CGFontRef cgFont;
     QCoreTextFontEngineMulti *parentEngine;
@@ -546,7 +548,7 @@ public:
     virtual Properties properties() const;
     virtual void getUnscaledGlyph(glyph_t glyph, QPainterPath *path, glyph_metrics_t *metrics);
     virtual QImage alphaMapForGlyph(glyph_t);
-    virtual QImage alphaRGBMapForGlyph(glyph_t, int margin, const QTransform &t);
+    virtual QImage alphaRGBMapForGlyph(glyph_t, QFixed subPixelPosition, int margin, const QTransform &t);
 
 private:
     QImage imageForGlyph(glyph_t glyph, int margin, bool colorful);
diff --git a/src/gui/text/qfontengine_win.cpp b/src/gui/text/qfontengine_win.cpp
index 4bed2b5..928bca0 100644
--- a/src/gui/text/qfontengine_win.cpp
+++ b/src/gui/text/qfontengine_win.cpp
@@ -1254,7 +1254,7 @@ QImage QFontEngineWin::alphaMapForGlyph(glyph_t glyph, const QTransform &xform)
 #define SPI_GETFONTSMOOTHINGCONTRAST           0x200C
 #define SPI_SETFONTSMOOTHINGCONTRAST           0x200D
 
-QImage QFontEngineWin::alphaRGBMapForGlyph(glyph_t glyph, int margin, const QTransform &t)
+QImage QFontEngineWin::alphaRGBMapForGlyph(glyph_t glyph, QFixed, int margin, const QTransform &t)
 {
     HFONT font = hfont;
 
diff --git a/src/gui/text/qfontengine_win_p.h b/src/gui/text/qfontengine_win_p.h
index 68b53b5..d86f42e 100644
--- a/src/gui/text/qfontengine_win_p.h
+++ b/src/gui/text/qfontengine_win_p.h
@@ -104,7 +104,7 @@ public:
 
     virtual QImage alphaMapForGlyph(glyph_t t) { return alphaMapForGlyph(t, QTransform()); }
     virtual QImage alphaMapForGlyph(glyph_t, const QTransform &xform);
-    virtual QImage alphaRGBMapForGlyph(glyph_t t, int margin, const QTransform &xform);
+    virtual QImage alphaRGBMapForGlyph(glyph_t t, QFixed subPixelPosition, int margin, const QTransform &xform);
 
 #ifndef Q_CC_MINGW
     virtual void getGlyphBearings(glyph_t glyph, qreal *leftBearing = 0, qreal *rightBearing = 0);
diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
index 6e56553..e7ee91c 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
@@ -1519,8 +1519,15 @@ void QGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngineGlyphCache::Type glyp
         vertexCoordinates->clear();
         textureCoordinates->clear();
 
+        bool supportsSubPixelPositions = staticTextItem->fontEngine->supportsSubPixelPositions();
         for (int i=0; i<staticTextItem->numGlyphs; ++i) {
-            const QTextureGlyphCache::Coord &c = cache->coords.value(staticTextItem->glyphs[i]);
+            QFixed subPixelPosition;
+            if (supportsSubPixelPositions)
+                subPixelPosition = cache->subPixelPositionForX(staticTextItem->glyphPositions[i].x);
+
+            QTextureGlyphCache::GlyphAndSubPixelPosition glyph(staticTextItem->glyphs[i], subPixelPosition);
+
+            const QTextureGlyphCache::Coord &c = cache->coords.value(glyph);
             int x = staticTextItem->glyphPositions[i].x.toInt() + c.baseLineX - margin;
             int y = staticTextItem->glyphPositions[i].y.toInt() - c.baseLineY - margin;
 
diff --git a/src/opengl/gl2paintengineex/qtextureglyphcache_gl.cpp b/src/opengl/gl2paintengineex/qtextureglyphcache_gl.cpp
index 53a2f3a..3dc7789 100644
--- a/src/opengl/gl2paintengineex/qtextureglyphcache_gl.cpp
+++ b/src/opengl/gl2paintengineex/qtextureglyphcache_gl.cpp
@@ -216,7 +216,7 @@ void QGLTextureGlyphCache::resizeTextureData(int width, int height)
     pex->updateClipScissorTest();
 }
 
-void QGLTextureGlyphCache::fillTexture(const Coord &c, glyph_t glyph)
+void QGLTextureGlyphCache::fillTexture(const Coord &c, glyph_t glyph, QFixed subPixelPosition)
 {
     if (ctx == 0) {
         qWarning("QGLTextureGlyphCache::fillTexture: Called with no context");
@@ -225,7 +225,7 @@ void QGLTextureGlyphCache::fillTexture(const Coord &c, glyph_t glyph)
 
     QGLGlyphTexture *glyphTexture = m_textureResource.value(ctx);
     if (pex == 0 || ctx->d_ptr->workaround_brokenFBOReadBack) {
-        QImageTextureGlyphCache::fillTexture(c, glyph);
+        QImageTextureGlyphCache::fillTexture(c, glyph, subPixelPosition);
 
         glBindTexture(GL_TEXTURE_2D, glyphTexture->m_texture);
         const QImage &texture = image();
@@ -238,7 +238,7 @@ void QGLTextureGlyphCache::fillTexture(const Coord &c, glyph_t glyph)
         return;
     }
 
-    QImage mask = textureMapForGlyph(glyph);
+    QImage mask = textureMapForGlyph(glyph, subPixelPosition);
     const int maskWidth = mask.width();
     const int maskHeight = mask.height();
 
diff --git a/src/opengl/gl2paintengineex/qtextureglyphcache_gl_p.h b/src/opengl/gl2paintengineex/qtextureglyphcache_gl_p.h
index 2ae3a64..345684d 100644
--- a/src/opengl/gl2paintengineex/qtextureglyphcache_gl_p.h
+++ b/src/opengl/gl2paintengineex/qtextureglyphcache_gl_p.h
@@ -106,7 +106,7 @@ public:
 
     virtual void createTextureData(int width, int height);
     virtual void resizeTextureData(int width, int height);
-    virtual void fillTexture(const Coord &c, glyph_t glyph);
+    virtual void fillTexture(const Coord &c, glyph_t glyph, QFixed subPixelPosition);
     virtual int glyphPadding() const;
 
     inline GLuint texture() const {