Fix transformations on DirectWrite rasterized text

There were a few bugs in the DirectWrite font engine that caused
transformed text to break.

First of all, alphaMapForGlyph() ignored the transform, so no gray
antialiased text would be transformed.

Second of all, the imageForGlyph() function would use the wrong
bounding box for the rasterized glyph, causing its positioning to
become a little bit off when rotating. The fix is to get the bounding
box from the system and add a margin to this instead of trying to predict
how it will appear after the vertical hinting etc. has been applied.

So that the positioning metrics are in sync with the actual metrics used
by the alphaMap* functions, we also need to implement the
alphaMapBoundingBox() function.

Task-number: QTBUG-19829
Reviewed-by: Jiang Jiang

1 files changed, 5 insertions, 1 deletions

diff --git a/src/gui/text/qfontenginedirectwrite_p.h b/src/gui/text/qfontenginedirectwrite_p.h
index d0086fc..edf1e6a 100644
--- a/src/gui/text/qfontenginedirectwrite_p.h
+++ b/src/gui/text/qfontenginedirectwrite_p.h
@@ -86,6 +86,10 @@ public:
 
     glyph_metrics_t boundingBox(const QGlyphLayout &glyphs);
     glyph_metrics_t boundingBox(glyph_t g);
+    glyph_metrics_t alphaMapBoundingBox(glyph_t glyph,
+                                        QFixed subPixelPosition,
+                                        const QTransform &matrix,
+                                        GlyphFormat format);
 
     QFixed ascent() const;
     QFixed descent() const;
@@ -97,7 +101,7 @@ public:
 
     bool supportsSubPixelPositions() const;
 
-    QImage alphaMapForGlyph(glyph_t glyph, QFixed subPixelPosition);
+    QImage alphaMapForGlyph(glyph_t, QFixed subPixelPosition, const QTransform &t);
     QImage alphaRGBMapForGlyph(glyph_t t, QFixed subPixelPosition, int margin,
                                const QTransform &xform);