Fix rasterization errors in aliased cosmetic drawPolyLine

If a cosmetic polyline contained a 1-pixel segment, the next segment
would be drawn with wrong starting point.

The original fix for QTBUG-26156 had some unwanted side effects
(QTBUG-31579 and now QTBUG-42398). It tried to skip start-point update
if stroke() did not actually draw anything (because the segment was
too small). However, to determine that, it tested for a change in
lastPixel. But that was not failsafe; in some cases (1-pixel segment),
lastPixel could be unchanged even though the segment had been
drawn. With this change, we instead test directly whether stroke()
skipped the segment or not.

Task-number: QTBUG-42398
Change-Id: Id751db69a18cd1af4f45070db9d5698aa532d22a
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>

1 files changed, 37 insertions, 0 deletions

diff --git a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
index 8a0705630d..bf19a04203 100644
--- a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
+++ b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
@@ -290,6 +290,10 @@ private slots:
 
     void RasterOp_NotDestination();
     void drawTextNoHinting();
+
+    void drawPolyline_data();
+    void drawPolyline();
+
 private:
     void fillData();
     void setPenColor(QPainter& p);
@@ -4828,6 +4832,39 @@ void tst_QPainter::drawTextNoHinting()
     QVERIFY(true);
 }
 
+void tst_QPainter::drawPolyline_data()
+{
+    QTest::addColumn< QVector<QPointF> >("points");
+
+    QTest::newRow("basic") << (QVector<QPointF>() << QPointF(10, 10) << QPointF(20, 10) << QPointF(20, 20) << QPointF(10, 20));
+    QTest::newRow("clipped") << (QVector<QPointF>() << QPoint(-10, 100) << QPoint(-1, 100) << QPoint(-1,  -2) << QPoint(100, -2) << QPoint(100, 40)); // QTBUG-31579
+    QTest::newRow("shortsegment") << (QVector<QPointF>() << QPoint(20, 100) << QPoint(20, 99) << QPoint(21, 99) << QPoint(21, 104)); // QTBUG-42398
+}
+
+void tst_QPainter::drawPolyline()
+{
+    QFETCH(QVector<QPointF>, points);
+    QImage images[2];
+
+    for (int r = 0; r < 2; r++) {
+        images[r] = QImage(150, 150, QImage::Format_ARGB32);
+        images[r].fill(Qt::transparent);
+        QPainter p(images + r);
+        QPen pen(Qt::red, 0, Qt::SolidLine, Qt::FlatCap);
+        p.setPen(pen);
+        QVERIFY(p.pen().isCosmetic());
+        if (r) {
+            for (int i = 0; i < points.count()-1; i++) {
+                p.drawLine(points.at(i), points.at(i+1));
+            }
+        } else {
+            p.drawPolyline(points);
+        }
+    }
+
+    QCOMPARE(images[0], images[1]);
+}
+
 QTEST_MAIN(tst_QPainter)
 
 #include "tst_qpainter.moc"