Direct2D QPA: Match raster engine line output

For whatever reason direct2d and the raster engine disagree by one pixel
about the positioning of positively sloping aliased lines. To get the same
output, we shift such lines by one pixel.

Change-Id: I4b20319770c02e3fdd212b3535ccae3c27ca9f2f
Reviewed-by: Risto Avila <risto.avila@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/src/plugins/platforms/direct2d/qwindowsdirect2dpaintengine.cpp b/src/plugins/platforms/direct2d/qwindowsdirect2dpaintengine.cpp
index a1c405067c..4c39560cbe 100644
--- a/src/plugins/platforms/direct2d/qwindowsdirect2dpaintengine.cpp
+++ b/src/plugins/platforms/direct2d/qwindowsdirect2dpaintengine.cpp
@@ -1160,6 +1160,25 @@ void QWindowsDirect2DPaintEngine::drawRects(const QRectF *rects, int rectCount)
     }
 }
 
+static bool isLinePositivelySloped(const QPointF &p1, const QPointF &p2)
+{
+    if (p2.x() > p1.x())
+        return p2.y() < p1.y();
+
+    if (p1.x() > p2.x())
+        return p1.y() < p2.y();
+
+    return false;
+}
+
+static void adjustLine(QPointF *p1, QPointF *p2)
+{
+    if (isLinePositivelySloped(*p1, *p2)) {
+        p1->ry() -= qreal(1.0);
+        p2->ry() -= qreal(1.0);
+    }
+}
+
 void QWindowsDirect2DPaintEngine::drawLines(const QLine *lines, int lineCount)
 {
     Q_D(QWindowsDirect2DPaintEngine);
@@ -1181,6 +1200,10 @@ void QWindowsDirect2DPaintEngine::drawLines(const QLine *lines, int lineCount)
                 continue;
             }
 
+            // Match raster engine output
+            if (!antiAliasingEnabled())
+                adjustLine(&p1, &p2);
+
             adjustForAliasing(&p1);
             adjustForAliasing(&p2);
 
@@ -1213,6 +1236,10 @@ void QWindowsDirect2DPaintEngine::drawLines(const QLineF *lines, int lineCount)
                 continue;
             }
 
+            // Match raster engine output
+            if (!antiAliasingEnabled())
+                adjustLine(&p1, &p2);
+
             adjustForAliasing(&p1);
             adjustForAliasing(&p2);