Implement subpixel positioning with FreeType

QFontEngineFT are used in raster/OpenGL paint engine and QGLWidget,
also in Symbian, etc. We want to make sure it works well in raster
and QGLWidget first. Regardless subpixel antialiasing (LCD filtering)
is enabled or not (though it does look better when subpixel
antialiasing is on). We also need to support transformations.

The tricky part here is that, under X11, we have a different code
path for QFontEngineFT and other font engines in raster engine,
which uses QFontEngineFT's own glyph cache system. While in other
platforms (Windows and Mac) and QGLWidget, we will use the generic
QTextureGlyphCache.

The generic QTextureGlyphCache already has support for subpixel
positions, this solution is ported to QFontEngineFT for its
QGlyphSet: the key for QGlyphSet hash table has been extended from
glyph_t to <glyph_t, QFixed subPixelPosition> pair.

The real work to enable subpixel positioning with FreeType is in
fact really simple, we just set the horizontal translation to the
subpixel position * 64 (FreeType uses a coordinate system of 1/64
of a pixel resolution internally) immediately before loading the
glyph. A slight tweek to bitmap width is applied when we are
getting the bitmap ourselves instead of using FT_Render_Glyph to
accommodate the subpixel translation, which will only be used in
non-LCD filtering cases (grayscale antialiasing).

From what we have observed, FreeType can generate different bitmaps
for at least 12 different subpixel positions (each with a slight
difference). To limit the memory consumption, we restrict the
number of subpixel positions to be 4 (hardcoded), which should be
good enough for most low resolution displays.

Subpixel positioning (and fractional glyph advances) will only be
enabled when the hintingPreference for the font being used is
PreferNoHinting or PreferVerticalHinting. We will use fontconfig
hintstyle setting by default when no hintingPreference is set for
the font.

Task-number: QTBUG-12279
Reviewed-by: Eskil

1 files changed, 92 insertions, 83 deletions

diff --git a/src/gui/text/qfontengine_ft.cpp b/src/gui/text/qfontengine_ft.cpp
index f5e9e1c..a4211a9 100644
--- a/src/gui/text/qfontengine_ft.cpp
+++ b/src/gui/text/qfontengine_ft.cpp
@@ -174,9 +174,7 @@ int QFreetypeFace::fsType() const
 
 HB_Error QFreetypeFace::getPointInOutline(HB_Glyph glyph, int flags, hb_uint32 point, HB_Fixed *xpos, HB_Fixed *ypos, hb_uint32 *nPoints)
 {
-    int load_flags = (flags & HB_ShaperFlag_UseDesignMetrics) ? FT_LOAD_NO_HINTING : FT_LOAD_DEFAULT;
-
-    if (HB_Error error = (HB_Error)FT_Load_Glyph(face, glyph, load_flags))
+    if (HB_Error error = (HB_Error)FT_Load_Glyph(face, glyph, flags))
         return error;
 
     if (face->glyph->format != FT_GLYPH_FORMAT_OUTLINE)
@@ -760,12 +758,9 @@ void QFontEngineFT::setDefaultHintStyle(HintStyle style)
     default_hint_style = style;
 }
 
-QFontEngineFT::Glyph *QFontEngineFT::loadGlyphMetrics(QGlyphSet *set, uint glyph, GlyphFormat format) const
+int QFontEngineFT::loadFlags(QGlyphSet *set, GlyphFormat format, int flags,
+                             bool &hsubpixel, int &vfactor) const
 {
-    Glyph *g = set->getGlyph(glyph);
-    if (g && g->format == format)
-        return g;
-
     int load_flags = FT_LOAD_DEFAULT | default_load_flags;
     int load_target = default_hint_style == HintLight
                       ? FT_LOAD_TARGET_LIGHT
@@ -777,20 +772,35 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyphMetrics(QGlyphSet *set, uint glyph
         if (subpixelType == QFontEngineFT::Subpixel_RGB || subpixelType == QFontEngineFT::Subpixel_BGR) {
             if (default_hint_style == HintFull)
                 load_target = FT_LOAD_TARGET_LCD;
+            hsubpixel = true;
         } else if (subpixelType == QFontEngineFT::Subpixel_VRGB || subpixelType == QFontEngineFT::Subpixel_VBGR) {
             if (default_hint_style == HintFull)
                 load_target = FT_LOAD_TARGET_LCD_V;
+            vfactor = 3;
         }
     }
 
-    if (set->outline_drawing)
+    if (set && set->outline_drawing)
         load_flags = FT_LOAD_NO_BITMAP;
 
-    if (default_hint_style == HintNone)
+    if (default_hint_style == HintNone || (flags & HB_ShaperFlag_UseDesignMetrics))
         load_flags |= FT_LOAD_NO_HINTING;
     else
         load_flags |= load_target;
 
+    return load_flags;
+}
+
+QFontEngineFT::Glyph *QFontEngineFT::loadGlyphMetrics(QGlyphSet *set, uint glyph, GlyphFormat format) const
+{
+    Glyph *g = set->getGlyph(glyph);
+    if (g && g->format == format)
+        return g;
+
+    bool hsubpixel = false;
+    int vfactor = 1;
+    int load_flags = loadFlags(set, format, 0, hsubpixel, vfactor);
+
     // apply our matrix to this, but note that the metrics will not be affected by this.
     FT_Face face = lockFace();
     FT_Matrix matrix = this->matrix;
@@ -881,7 +891,10 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyphMetrics(QGlyphSet *set, uint glyph
     return g;
 }
 
-QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph, GlyphFormat format, bool fetchMetricsOnly) const
+QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph,
+                                               QFixed subPixelPosition,
+                                               GlyphFormat format,
+                                               bool fetchMetricsOnly) const
 {
 //     Q_ASSERT(freetype->lock == 1);
 
@@ -896,10 +909,10 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph, Glyph
         }
     }
 
-    Glyph *g = set->getGlyph(glyph);
+    Glyph *g = set->getGlyph(glyph, subPixelPosition);
     if (g && g->format == format) {
         if (uploadToServer && !g->uploadedToServer) {
-            set->setGlyph(glyph, 0);
+            set->setGlyph(glyph, subPixelPosition, 0);
             delete g;
             g = 0;
         } else {
@@ -912,33 +925,7 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph, Glyph
     Q_ASSERT(format != Format_None);
     bool hsubpixel = false;
     int vfactor = 1;
-    int load_flags = FT_LOAD_DEFAULT | default_load_flags;
-
-    int load_target = default_hint_style == HintLight
-                      ? FT_LOAD_TARGET_LIGHT
-                      : FT_LOAD_TARGET_NORMAL;
-
-    if (set->outline_drawing)
-        load_flags |= FT_LOAD_NO_BITMAP;
-
-    if (format == Format_Mono) {
-        load_target = FT_LOAD_TARGET_MONO;
-    } else if (format == Format_A32) {
-        if (subpixelType == QFontEngineFT::Subpixel_RGB || subpixelType == QFontEngineFT::Subpixel_BGR) {
-            if (default_hint_style == HintFull)
-                load_target = FT_LOAD_TARGET_LCD;
-            hsubpixel = true;
-        } else if (subpixelType == QFontEngineFT::Subpixel_VRGB || subpixelType == QFontEngineFT::Subpixel_VBGR) {
-            if (default_hint_style == HintFull)
-                load_target = FT_LOAD_TARGET_LCD_V;
-            vfactor = 3;
-        }
-    }
-
-    if (default_hint_style == HintNone)
-        load_flags |= FT_LOAD_NO_HINTING;
-    else
-        load_flags |= load_target;
+    int load_flags = loadFlags(set, format, 0, hsubpixel, vfactor);
 
 #ifndef Q_WS_QWS
     if (format != Format_Mono && !embeddedbitmap)
@@ -955,6 +942,12 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph, Glyph
         load_flags |= FT_LOAD_NO_BITMAP;
 
     FT_Face face = freetype->face;
+
+    FT_Vector v;
+    v.x = FT_Pos((subPixelPosition).toReal() * 64);
+    v.y = 0;
+    FT_Set_Transform(face, &freetype->matrix, &v);
+
     FT_Error err = FT_Load_Glyph(face, glyph, load_flags);
     if (err && (load_flags & FT_LOAD_NO_BITMAP)) {
         load_flags &= ~FT_LOAD_NO_BITMAP;
@@ -1055,6 +1048,10 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph, Glyph
     top = CEIL(top);
 
     int hpixels = TRUNC(right - left);
+    // subpixel position requires one more pixel
+    if (subPixelPosition > 0)
+        hpixels++;
+
     if (hsubpixel)
         hpixels = hpixels*3 + 8;
     info.width = hpixels;
@@ -1197,7 +1194,7 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph, Glyph
         uploadGlyphToServer(set, glyph, g, &info, glyph_buffer_size);
     }
 
-    set->setGlyph(glyph, g);
+    set->setGlyph(glyph, subPixelPosition, g);
 
     return g;
 }
@@ -1430,12 +1427,28 @@ QFontEngineFT::QGlyphSet *QFontEngineFT::loadTransformedGlyphSet(const QTransfor
     return gs;
 }
 
-bool QFontEngineFT::loadGlyphs(QGlyphSet *gs, glyph_t *glyphs, int num_glyphs, GlyphFormat format)
+QFixed QFontEngineFT::subPixelPositionForX(QFixed x)
+{
+    int m_subPixelPositionCount = 4;
+
+    QFixed subPixelPosition;
+    if (x != 0) {
+        subPixelPosition = x - x.floor();
+        QFixed fraction = (subPixelPosition / QFixed::fromReal(1.0 / m_subPixelPositionCount)).floor();
+        subPixelPosition = fraction / QFixed(m_subPixelPositionCount);
+    }
+    return subPixelPosition;
+}
+
+bool QFontEngineFT::loadGlyphs(QGlyphSet *gs, glyph_t *glyphs, int num_glyphs,
+                               QVarLengthArray<QFixedPoint> &positions,
+                               GlyphFormat format)
 {
     FT_Face face = 0;
 
     for (int i = 0; i < num_glyphs; ++i) {
-        Glyph *glyph = gs->getGlyph(glyphs[i]);
+        QFixed spp = subPixelPositionForX(positions[i].x);
+        Glyph *glyph = gs->getGlyph(glyphs[i], spp);
         if (glyph == 0 || glyph->format != format) {
             if (!face) {
                 face = lockFace();
@@ -1444,7 +1457,7 @@ bool QFontEngineFT::loadGlyphs(QGlyphSet *gs, glyph_t *glyphs, int num_glyphs, G
                 FT_Set_Transform(face, &m, 0);
                 freetype->matrix = m;
             }
-            if (!loadGlyph(gs, glyphs[i], format)) {
+            if (!loadGlyph(gs, glyphs[i], spp, format)) {
                 unlockFace();
                 return false;
             }
@@ -1668,35 +1681,26 @@ bool QFontEngineFT::stringToCMap(const QChar *str, int len, QGlyphLayout *glyphs
 void QFontEngineFT::recalcAdvances(QGlyphLayout *glyphs, QTextEngine::ShaperFlags flags) const
 {
     FT_Face face = 0;
-    if (flags & QTextEngine::DesignMetrics) {
-        for (int i = 0; i < glyphs->numGlyphs; i++) {
-            Glyph *g = defaultGlyphSet.getGlyph(glyphs->glyphs[i]);
-            if (g) {
-                glyphs->advances_x[i] = QFixed::fromFixed(g->linearAdvance);
-            } else {
-                if (!face)
-                    face = lockFace();
-                g = loadGlyph(glyphs->glyphs[i], Format_None, true);
-                glyphs->advances_x[i] = QFixed::fromFixed(face->glyph->linearHoriAdvance >> 10);
-            }
-            glyphs->advances_y[i] = 0;
-        }
-    } else {
-        for (int i = 0; i < glyphs->numGlyphs; i++) {
-            Glyph *g = defaultGlyphSet.getGlyph(glyphs->glyphs[i]);
-            if (g) {
-                glyphs->advances_x[i] = QFixed(g->advance);
-            } else {
-                if (!face)
-                    face = lockFace();
-                g = loadGlyph(glyphs->glyphs[i], Format_None, true);
-                glyphs->advances_x[i] = QFixed::fromFixed(face->glyph->metrics.horiAdvance).round();
-            }
-            glyphs->advances_y[i] = 0;
+    bool design = (default_hint_style == HintNone ||
+                   default_hint_style == HintLight ||
+                   (flags & HB_ShaperFlag_UseDesignMetrics));
+    for (int i = 0; i < glyphs->numGlyphs; i++) {
+        Glyph *g = defaultGlyphSet.getGlyph(glyphs->glyphs[i]);
+        if (g) {
+            glyphs->advances_x[i] = design ? QFixed::fromFixed(g->linearAdvance) : QFixed(g->advance);
+        } else {
+            if (!face)
+                face = lockFace();
+            g = loadGlyph(glyphs->glyphs[i], 0, Format_None, true);
+            glyphs->advances_x[i] = design ? QFixed::fromFixed(face->glyph->linearHoriAdvance >> 10)
+                                           : QFixed::fromFixed(face->glyph->metrics.horiAdvance).round();
+            if (face)
+                unlockFace();
         }
+        if (fontDef.styleStrategy & QFont::ForceIntegerMetrics)
+            glyphs->advances_x[i] = glyphs->advances_x[i].round();
+        glyphs->advances_y[i] = 0;
     }
-    if (face)
-        unlockFace();
 }
 
 glyph_metrics_t QFontEngineFT::boundingBox(const QGlyphLayout &glyphs)
@@ -1716,7 +1720,7 @@ glyph_metrics_t QFontEngineFT::boundingBox(const QGlyphLayout &glyphs)
         if (!g) {
             if (!face)
                 face = lockFace();
-            g = loadGlyph(glyphs.glyphs[i], Format_None, true);
+            g = loadGlyph(glyphs.glyphs[i], 0, Format_None, true);
         }
         if (g) {
             QFixed x = overall.xoff + glyphs.offsets[i].x + g->x;
@@ -1757,7 +1761,7 @@ glyph_metrics_t QFontEngineFT::boundingBox(glyph_t glyph)
     Glyph *g = defaultGlyphSet.getGlyph(glyph);
     if (!g) {
         face = lockFace();
-        g = loadGlyph(glyph, Format_None, true);
+        g = loadGlyph(glyph, 0, Format_None, true);
     }
     if (g) {
         overall.x = g->x;
@@ -1765,6 +1769,8 @@ glyph_metrics_t QFontEngineFT::boundingBox(glyph_t glyph)
         overall.width = g->width;
         overall.height = g->height;
         overall.xoff = g->advance;
+        if (fontDef.styleStrategy & QFont::ForceIntegerMetrics)
+            overall.xoff = overall.xoff.round();
     } else {
         int left  = FLOOR(face->glyph->metrics.horiBearingX);
         int right = CEIL(face->glyph->metrics.horiBearingX + face->glyph->metrics.width);
@@ -1866,7 +1872,7 @@ QImage QFontEngineFT::alphaMapForGlyph(glyph_t g)
 
     GlyphFormat glyph_format = antialias ? Format_A8 : Format_Mono;
 
-    Glyph *glyph = defaultGlyphSet.outline_drawing ? 0 : loadGlyph(g, glyph_format);
+    Glyph *glyph = defaultGlyphSet.outline_drawing ? 0 : loadGlyph(g, 0, glyph_format);
     if (!glyph) {
         unlockFace();
         return QFontEngine::alphaMapForGlyph(g);
@@ -1905,7 +1911,7 @@ QImage QFontEngineFT::alphaRGBMapForGlyph(glyph_t g, QFixed subPixelPosition, in
 
     GlyphFormat glyph_format = Format_A32;
 
-    Glyph *glyph = defaultGlyphSet.outline_drawing ? 0 : loadGlyph(g, glyph_format);
+    Glyph *glyph = defaultGlyphSet.outline_drawing ? 0 : loadGlyph(g, subPixelPosition, glyph_format);
     if (!glyph) {
         unlockFace();
         return QFontEngine::alphaRGBMapForGlyph(g, subPixelPosition, margin, t);
@@ -1920,7 +1926,7 @@ QImage QFontEngineFT::alphaRGBMapForGlyph(glyph_t g, QFixed subPixelPosition, in
 
 void QFontEngineFT::removeGlyphFromCache(glyph_t glyph)
 {
-    defaultGlyphSet.removeGlyphFromCache(glyph);
+    defaultGlyphSet.removeGlyphFromCache(glyph, 0);
 }
 
 int QFontEngineFT::glyphCount() const
@@ -2000,9 +2006,9 @@ void QFontEngineFT::QGlyphSet::clear()
     glyph_data.clear();
 }
 
-void QFontEngineFT::QGlyphSet::removeGlyphFromCache(int index)
+void QFontEngineFT::QGlyphSet::removeGlyphFromCache(glyph_t index, QFixed subPixelPosition)
 {
-    if (index < 256) {
+    if (useFastGlyphData(index, subPixelPosition)) {
         if (fast_glyph_data[index]) {
             delete fast_glyph_data[index];
             fast_glyph_data[index] = 0;
@@ -2010,18 +2016,18 @@ void QFontEngineFT::QGlyphSet::removeGlyphFromCache(int index)
                 --fast_glyph_count;
         }
     } else {
-        delete glyph_data.take(index);
+        delete glyph_data.take(GlyphAndSubPixelPosition(index, subPixelPosition));
     }
 }
 
-void QFontEngineFT::QGlyphSet::setGlyph(int index, Glyph *glyph)
+void QFontEngineFT::QGlyphSet::setGlyph(glyph_t index, QFixed subPixelPosition, Glyph *glyph)
 {
-    if (index < 256) {
+    if (useFastGlyphData(index, subPixelPosition)) {
         if (!fast_glyph_data[index])
             ++fast_glyph_count;
         fast_glyph_data[index] = glyph;
     } else {
-        glyph_data.insert(index, glyph);
+        glyph_data.insert(GlyphAndSubPixelPosition(index, subPixelPosition), glyph);
     }
 }
 
@@ -2038,7 +2044,10 @@ void QFontEngineFT::freeServerGlyphSet(unsigned long id)
 HB_Error QFontEngineFT::getPointInOutline(HB_Glyph glyph, int flags, hb_uint32 point, HB_Fixed *xpos, HB_Fixed *ypos, hb_uint32 *nPoints)
 {
     lockFace();
-    HB_Error result = freetype->getPointInOutline(glyph, flags, point, xpos, ypos, nPoints);
+    bool hsubpixel = true;
+    int vfactor = 1;
+    int load_flags = loadFlags(0, Format_A8, flags, hsubpixel, vfactor);
+    HB_Error result = freetype->getPointInOutline(glyph, load_flags, point, xpos, ypos, nPoints);
     unlockFace();
     return result;
 }