Fixed artifacts when drawing same line with different clips.

Expanding on the change fixing QTBUG-24762 with the realization that any
line needs to be drawn in a consistent way regardless of system or
painter clip, not just dashed lines.

Task-number: QTBUG-25036
Change-Id: Ief7ef19cc92c52e7d792500a581a072ba032767e
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
(cherry picked from commit c7191d3e214b9cdb17ec383b61f7e21e784f80d0)

2 files changed, 53 insertions, 17 deletions

diff --git a/src/gui/painting/qcosmeticstroker.cpp b/src/gui/painting/qcosmeticstroker.cpp
index 192b021634..9fddaed168 100644
--- a/src/gui/painting/qcosmeticstroker.cpp
+++ b/src/gui/painting/qcosmeticstroker.cpp
@@ -290,14 +290,14 @@ void QCosmeticStroker::setup()
         ppl = buffer->bytesPerLine()>>2;
     }
 
-    // dashes are sensitive to clips, so we need to clip consistently when painting to the same device
-    QRect clipRect = strokeSelection & Dashed ? deviceRect : clip;
+    // line drawing produces different results with different clips, so
+    // we need to clip consistently when painting to the same device
 
     // setup FP clip bounds
-    xmin = clipRect.left() - 1;
-    xmax = clipRect.right() + 2;
-    ymin = clipRect.top() - 1;
-    ymax = clipRect.bottom() + 2;
+    xmin = deviceRect.left() - 1;
+    xmax = deviceRect.right() + 2;
+    ymin = deviceRect.top() - 1;
+    ymax = deviceRect.bottom() + 2;
 
     lastPixel.x = -1;
 }
diff --git a/tests/auto/qpainter/tst_qpainter.cpp b/tests/auto/qpainter/tst_qpainter.cpp
index c6bf2ec972..e224522232 100644
--- a/tests/auto/qpainter/tst_qpainter.cpp
+++ b/tests/auto/qpainter/tst_qpainter.cpp
@@ -272,7 +272,9 @@ private slots:
 
     void QTBUG26013_squareCapStroke();
     void QTBUG25153_drawLine();
-    void dashing_systemClip();
+
+    void cosmeticStrokerClipping_data();
+    void cosmeticStrokerClipping();
 
 private:
     void fillData();
@@ -4867,22 +4869,54 @@ void tst_QPainter::QTBUG25153_drawLine()
     }
 }
 
-static void dashing_systemClip_paint(QPainter *p)
+enum CosmeticStrokerPaint
+{
+    Antialiasing,
+    Dashing
+};
+
+static void paint_func(QPainter *p, CosmeticStrokerPaint type)
+{
+    p->save();
+    switch (type) {
+    case Antialiasing:
+        p->setPen(Qt::black);
+        p->setRenderHint(QPainter::Antialiasing);
+        p->drawLine(4, 8, 42, 42);
+        break;
+    case Dashing:
+        p->setPen(QPen(Qt::black, 1, Qt::DashLine, Qt::RoundCap, Qt::MiterJoin));
+        p->drawLine(8, 8, 42, 8);
+        p->drawLine(42, 8, 42, 42);
+        p->drawLine(42, 42, 8, 42);
+        p->drawLine(8, 42, 8, 8);
+        break;
+    default:
+        Q_ASSERT(false);
+        break;
+    }
+    p->restore();
+}
+
+Q_DECLARE_METATYPE(CosmeticStrokerPaint)
+
+void tst_QPainter::cosmeticStrokerClipping_data()
 {
-    p->setPen(QPen(Qt::black, 1, Qt::DashLine, Qt::RoundCap, Qt::MiterJoin));
-    p->drawLine(8, 8, 42, 8);
-    p->drawLine(42, 8, 42, 42);
-    p->drawLine(42, 42, 8, 42);
-    p->drawLine(8, 42, 8, 8);
+    QTest::addColumn<CosmeticStrokerPaint>("paint");
+
+    QTest::newRow("antialiasing_paint") << Antialiasing;
+    QTest::newRow("dashing_paint") << Dashing;
 }
 
-void tst_QPainter::dashing_systemClip()
+void tst_QPainter::cosmeticStrokerClipping()
 {
+    QFETCH(CosmeticStrokerPaint, paint);
+
     QImage image(50, 50, QImage::Format_RGB32);
     image.fill(Qt::white);
 
     QPainter p(&image);
-    dashing_systemClip_paint(&p);
+    paint_func(&p, paint);
     p.end();
 
     QImage old = image.copy();
@@ -4890,7 +4924,8 @@ void tst_QPainter::dashing_systemClip()
     image.paintEngine()->setSystemClip(QRect(10, 0, image.width() - 10, image.height()));
 
     p.begin(&image);
-    dashing_systemClip_paint(&p);
+    p.fillRect(image.rect(), Qt::white);
+    paint_func(&p, paint);
 
     // doing same paint operation again with different system clip should not change the image
     QCOMPARE(old, image);
@@ -4898,7 +4933,8 @@ void tst_QPainter::dashing_systemClip()
     old = image;
 
     p.setClipRect(QRect(20, 20, 30, 30));
-    dashing_systemClip_paint(&p);
+    p.fillRect(image.rect(), Qt::white);
+    paint_func(&p, paint);
 
     // ditto for regular clips
     QCOMPARE(old, image);