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

3 files changed, 12 insertions, 5 deletions

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 {