Fixed performance regression in curve stroking.

Change c46688b8a88da made us use m_curve_threshold for both
QBezier::shifted and QBezier::toPolygon, and adjusted the threshold
dynamically based on the painter scale. Since the threshold in shifted
was already relative to the pen width, it is independent from the
painter scale. Instead, we need to set a separate threshold for dashing.

Also, in several places we were calling setCurveThresholdForTransform
with the painter matrix even though we were transforming the points into
device coordinate space before stroking.

Task-number: QTBUG-13894
Reviewed-by: Gunnar Sletta
(cherry picked from commit d60dc7cba21794866c9382f83080fab1a129eb08)

3 files changed, 8 insertions, 6 deletions

diff --git a/src/gui/painting/qpaintengineex.cpp b/src/gui/painting/qpaintengineex.cpp
index 1e857e4..c1e3d66 100644
--- a/src/gui/painting/qpaintengineex.cpp
+++ b/src/gui/painting/qpaintengineex.cpp
@@ -517,7 +517,7 @@ void QPaintEngineEx::stroke(const QVectorPath &path, const QPen &pen)
             QPainterPath painterPath = state()->matrix.map(path.convertToPainterPath());
             d->activeStroker->strokePath(painterPath, d->strokeHandler, QTransform());
         } else {
-            d->activeStroker->setCurveThresholdFromTransform(state()->matrix);
+            d->activeStroker->setCurveThresholdFromTransform(QTransform());
             d->activeStroker->begin(d->strokeHandler);
             if (types) {
                 while (points < lastPoint) {
diff --git a/src/gui/painting/qstroker.cpp b/src/gui/painting/qstroker.cpp
index 9cff339..9decf41 100644
--- a/src/gui/painting/qstroker.cpp
+++ b/src/gui/painting/qstroker.cpp
@@ -190,6 +190,7 @@ static inline qreal adapted_angle_on_x(const QLineF &line)
 QStrokerOps::QStrokerOps()
     : m_elements(0)
     , m_curveThreshold(qt_real_to_fixed(0.25))
+    , m_dashThreshold(qt_real_to_fixed(0.25))
     , m_customData(0)
     , m_moveTo(0)
     , m_lineTo(0)
@@ -243,7 +244,7 @@ void QStrokerOps::strokePath(const QPainterPath &path, void *customData, const Q
     if (path.isEmpty())
         return;
 
-    setCurveThresholdFromTransform(matrix);
+    setCurveThresholdFromTransform(QTransform());
     begin(customData);
     int count = path.elementCount();
     if (matrix.isIdentity()) {
@@ -315,7 +316,7 @@ void QStrokerOps::strokePolygon(const QPointF *points, int pointCount, bool impl
     if (!pointCount)
         return;
 
-    setCurveThresholdFromTransform(matrix);
+    setCurveThresholdFromTransform(QTransform());
     begin(data);
     if (matrix.isIdentity()) {
         moveTo(qt_real_to_fixed(points[0].x()), qt_real_to_fixed(points[0].y()));
@@ -356,7 +357,7 @@ void QStrokerOps::strokeEllipse(const QRectF &rect, void *data, const QTransform
         }
     }
 
-    setCurveThresholdFromTransform(matrix);
+    setCurveThresholdFromTransform(QTransform());
     begin(data);
     moveTo(qt_real_to_fixed(start.x()), qt_real_to_fixed(start.y()));
     for (int i=0; i<12; i+=3) {
@@ -1142,7 +1143,7 @@ void QDashStroker::processCurrentSubpath()
 
     QPainterPath dashPath;
 
-    QSubpathFlatIterator it(&m_elements, m_curveThreshold);
+    QSubpathFlatIterator it(&m_elements, m_dashThreshold);
     qfixed2d prev = it.next();
 
     bool clipping = !m_clip_rect.isEmpty();
diff --git a/src/gui/painting/qstroker_p.h b/src/gui/painting/qstroker_p.h
index d646135..5607a8e 100644
--- a/src/gui/painting/qstroker_p.h
+++ b/src/gui/painting/qstroker_p.h
@@ -168,7 +168,7 @@ public:
     {
         qreal scale;
         qt_scaleForTransform(transform, &scale);
-        setCurveThreshold(scale == 0 ? qreal(0.5) : (qreal(0.5) / scale));
+        m_dashThreshold = scale == 0 ? qreal(0.5) : (qreal(0.5) / scale);
     }
 
     void setCurveThreshold(qfixed threshold) { m_curveThreshold = threshold; }
@@ -184,6 +184,7 @@ protected:
 
     QRectF m_clip_rect;
     qfixed m_curveThreshold;
+    qfixed m_dashThreshold;
 
     void *m_customData;
     qStrokerMoveToHook m_moveTo;