Avoid blurry text with OpenVG on Symbian

Qt's graphics system uses qreal as measurement unit.

The established paint engines (such as "raster") implicitly
round textitem coordinates for technical reasons, e.g. for
optimized blitting.

Some Qt based Ui libraries (such as QWidgets) can in some
cases cause textitems to be drawn on non-integer coordinates.
In particular, this happens when centering text.

Since these libraries have been developed against the
established paint engines with implicit rounding, these
non-integer coordinates were never an issue.

The new OpenVG paintengine took these coordinates seriously
without rounding them. On some OpenVG implementations (e.g.
that of some Symbian phones), this led to blurry text
rendering.

This patch adds coordinate rounding for unscaled, unrotated
textitems to the OpenVG paintengine. So that it does the
same as the raster paint engine.

Task-number: QT-3071
Reviewed-By: Jason Barron

1 files changed, 7 insertions, 1 deletions

diff --git a/src/openvg/qpaintengine_vg.cpp b/src/openvg/qpaintengine_vg.cpp
index cfc481e..515197a 100644
--- a/src/openvg/qpaintengine_vg.cpp
+++ b/src/openvg/qpaintengine_vg.cpp
@@ -3424,7 +3424,13 @@ void QVGPaintEngine::drawTextItem(const QPointF &p, const QTextItem &textItem)
 
     // Set the transformation to use for drawing the current glyphs.
     QTransform glyphTransform(d->pathTransform);
-    glyphTransform.translate(p.x(), p.y());
+    if (d->transform.type() <= QTransform::TxTranslate) {
+        // Prevent blurriness of unscaled, unrotated text by using integer coordinates.
+        // Using ceil(x-0.5) instead of qRound() or int-cast, behave like other paint engines.
+        glyphTransform.translate(ceil(p.x() - 0.5), ceil(p.y() - 0.5));
+    } else {
+        glyphTransform.translate(p.x(), p.y());
+    }
 #if defined(QVG_NO_IMAGE_GLYPHS)
     glyphTransform.scale(glyphCache->scaleX, glyphCache->scaleY);
 #endif