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

1 files changed, 6 insertions, 4 deletions

diff --git a/src/gui/text/qfontengine_p.h b/src/gui/text/qfontengine_p.h
index 6e9bd0bc2e..6ebbc1fd78 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);