From 07fb176b103dc6c4dc963ddd4551fee7e8cb80b7 Mon Sep 17 00:00:00 2001
From: Eirik Aavitsland <eirik.aavitsland@qt.io>
Date: Fri, 30 Jul 2021 13:03:49 +0200
Subject: Refix for avoiding huge number of tiny dashes

Previous fix hit too widely so some valid horizontal and vertical
lines were affected; the root problem being that such lines have an
empty control point rect (width or height is 0). Fix by caculating in
the pen width.

Change-Id: I7a436e873f6d485028f6759d0e2c6456f07eebdc
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
(cherry picked from commit 84aba80944a2e1c3058d7a1372e0e66676411884)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/gui/painting/qpaintengineex.cpp               |  8 +++---
 tests/auto/gui/painting/qpainter/tst_qpainter.cpp | 31 +++++++++++++++++++++++
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/src/gui/painting/qpaintengineex.cpp b/src/gui/painting/qpaintengineex.cpp
index 22e9a621c3..d85c2a951c 100644
--- a/src/gui/painting/qpaintengineex.cpp
+++ b/src/gui/painting/qpaintengineex.cpp
@@ -415,18 +415,18 @@ void QPaintEngineEx::stroke(const QVectorPath &path, const QPen &inPen)
             clipRect = xf.inverted().mapRect(QRectF(d->exDeviceRect));
         }
         // Check to avoid generating unwieldy amount of dashes that will not be visible anyway
-        QRectF extentRect = cpRect & clipRect;
+        qreal pw = pen.widthF() ? pen.widthF() : 1;
+        QRectF extentRect = cpRect.adjusted(-pw, -pw, pw, pw) & clipRect;
         qreal extent = qMax(extentRect.width(), extentRect.height());
         qreal patternLength = 0;
         const QList<qreal> pattern = pen.dashPattern();
         const int patternSize = qMin(pattern.size(), 32);
         for (int i = 0; i < patternSize; i++)
             patternLength += qMax(pattern.at(i), qreal(0));
-        if (pen.widthF())
-            patternLength *= pen.widthF();
+        patternLength *= pw;
         if (qFuzzyIsNull(patternLength)) {
             pen.setStyle(Qt::NoPen);
-        } else if (qFuzzyIsNull(extent) || extent / patternLength > 10000) {
+        } else if (extent / patternLength > 10000) {
             // approximate stream of tiny dashes with semi-transparent solid line
             pen.setStyle(Qt::SolidLine);
             QColor color(pen.color());
diff --git a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
index 1679d5b8df..f0cc06a2e4 100644
--- a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
+++ b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
@@ -300,6 +300,7 @@ private slots:
     void fillPolygon();
 
     void drawImageAtPointF();
+    void scaledDashes();
 
 private:
     void fillData();
@@ -5362,6 +5363,36 @@ void tst_QPainter::drawImageAtPointF()
     paint.end();
 }
 
+void tst_QPainter::scaledDashes()
+{
+    // Test that we do not hit the limit-huge-number-of-dashes path
+    QRgb fore = qRgb(0, 0, 0xff);
+    QRgb back = qRgb(0xff, 0xff, 0);
+    QImage image(5, 32, QImage::Format_RGB32);
+    image.fill(back);
+    QPainter p(&image);
+    QPen pen(QColor(fore), 3, Qt::DotLine);
+    p.setPen(pen);
+    p.scale(1, 2);
+    p.drawLine(2, 0, 2, 16);
+    p.end();
+
+    bool foreFound = false;
+    bool backFound = false;
+    int i = 0;
+    while (i < 32 && (!foreFound || !backFound)) {
+        QRgb pix = image.pixel(3, i);
+        if (pix == fore)
+            foreFound = true;
+        else if (pix == back)
+            backFound = true;
+        i++;
+    }
+
+    QVERIFY(foreFound);
+    QVERIFY(backFound);
+}
+
 QTEST_MAIN(tst_QPainter)
 
 #include "tst_qpainter.moc"
-- 
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