Fix antialiasing with transformed text in OpenGL2 paint engine

Since the paint engine now transforms the prerendered glyphs instead of
rendering transformed glyphs as paths, we need to turn on texture
filtering to avoid antialiasing artifacts. In order to do this, we also
need to pad the glyphs in the glyph cache, otherwise you will get
artifacts when sampling the area around the glyph's bounding rect (where
there might be other glyphs.) This done by adding a glyphPadding()
function to the cache which returns the number of pixels to pad between
each glyph.

Task-number: QTBUG-9706
Reviewed-by: Tom

2 files changed, 5 insertions, 3 deletions

diff --git a/src/gui/painting/qtextureglyphcache.cpp b/src/gui/painting/qtextureglyphcache.cpp
index 9eda0ef..631a9cf 100644
--- a/src/gui/painting/qtextureglyphcache.cpp
+++ b/src/gui/painting/qtextureglyphcache.cpp
@@ -79,6 +79,7 @@ void QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
 
     m_current_fontengine = fontEngine;
     const int margin = glyphMargin();
+    const int paddingDoubled = glyphPadding() * 2;
 
     QHash<glyph_t, Coord> listItemCoordinates;
     int rowHeight = 0;
@@ -124,7 +125,7 @@ void QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
     if (listItemCoordinates.isEmpty())
         return;
 
-    rowHeight += margin * 2;
+    rowHeight += margin * 2 + paddingDoubled;
     if (isNull())
         createCache(QT_DEFAULT_TEXTURE_GLYPH_CACHE_WIDTH, qt_next_power_of_two(rowHeight));
 
@@ -138,7 +139,7 @@ void QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
         if (m_cx + c.w > m_w) {
             // no room on the current line, start new glyph strip
             m_cx = 0;
-            m_cy += m_currentRowHeight;
+            m_cy += m_currentRowHeight + paddingDoubled;
             m_currentRowHeight = 0; // New row
         }
         if (m_cy + c.h > m_h) {
@@ -156,7 +157,7 @@ void QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
         fillTexture(c, iter.key());
         coords.insert(iter.key(), c);
 
-        m_cx += c.w;
+        m_cx += c.w + paddingDoubled;
         ++iter;
     }
 
diff --git a/src/gui/painting/qtextureglyphcache_p.h b/src/gui/painting/qtextureglyphcache_p.h
index 8c2f5b4..390fe51 100644
--- a/src/gui/painting/qtextureglyphcache_p.h
+++ b/src/gui/painting/qtextureglyphcache_p.h
@@ -98,6 +98,7 @@ public:
     virtual void createTextureData(int width, int height) = 0;
     virtual void resizeTextureData(int width, int height) = 0;
     virtual int glyphMargin() const { return 0; }
+    virtual int glyphPadding() const { return 0; }
 
     virtual void fillTexture(const Coord &coord, glyph_t glyph) = 0;