QWidgets/drawutils: Handle device pixel ratio != 1

The panel drawing helpers have code drawing shadows that relies
on working with integer coordinates and one pixel lines, which causes
artifacts when Qt High DPI scaling is in effect.

Add code that checks for device pixel ratio != 1 and in that
case reverts out the Qt High DPI scaling transformation and scales
the parameters so that the drawing code used device
pixels.

Task-number: QTBUG-58611
Task-number: QTBUG-59116
Change-Id: I8402044f3fd4dfcd349b31c573dcad12ae1f609f
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

1 files changed, 85 insertions, 0 deletions

diff --git a/src/widgets/styles/qdrawutil.cpp b/src/widgets/styles/qdrawutil.cpp
index 0b0583ea94..299dbb9f82 100644
--- a/src/widgets/styles/qdrawutil.cpp
+++ b/src/widgets/styles/qdrawutil.cpp
@@ -49,6 +49,35 @@
 
 QT_BEGIN_NAMESPACE
 
+namespace {
+class PainterStateGuard {
+    Q_DISABLE_COPY(PainterStateGuard)
+public:
+    explicit PainterStateGuard(QPainter *p) : m_painter(p) {}
+    ~PainterStateGuard()
+    {
+        for ( ; m_level > 0; --m_level)
+            m_painter->restore();
+    }
+
+    void save()
+    {
+        m_painter->save();
+        ++m_level;
+    }
+
+    void restore()
+    {
+        m_painter->restore();
+        --m_level;
+    }
+
+private:
+    QPainter *m_painter;
+    int m_level= 0;
+};
+} // namespace
+
 /*!
     \headerfile <qdrawutil.h>
     \title Drawing Utility Functions
@@ -213,6 +242,21 @@ void qDrawShadeRect(QPainter *p, int x, int y, int w, int h,
         qWarning("qDrawShadeRect: Invalid parameters");
         return;
     }
+
+    PainterStateGuard painterGuard(p);
+    const qreal devicePixelRatio = p->device()->devicePixelRatioF();
+    if (!qFuzzyCompare(devicePixelRatio, qreal(1))) {
+        painterGuard.save();
+        const qreal inverseScale = qreal(1) / devicePixelRatio;
+        p->scale(inverseScale, inverseScale);
+        x = qRound(devicePixelRatio * x);
+        y = qRound(devicePixelRatio * y);
+        w = qRound(devicePixelRatio * w);
+        h = qRound(devicePixelRatio * h);
+        lineWidth = qRound(devicePixelRatio * lineWidth);
+        midLineWidth = qRound(devicePixelRatio * midLineWidth);
+    }
+
     QPen oldPen = p->pen();
     if (sunken)
         p->setPen(pal.dark().color());
@@ -312,6 +356,20 @@ void qDrawShadePanel(QPainter *p, int x, int y, int w, int h,
     if (Q_UNLIKELY(w < 0 || h < 0 || lineWidth < 0)) {
         qWarning("qDrawShadePanel: Invalid parameters");
     }
+
+    PainterStateGuard painterGuard(p);
+    const qreal devicePixelRatio = p->device()->devicePixelRatioF();
+    if (!qFuzzyCompare(devicePixelRatio, qreal(1))) {
+        painterGuard.save();
+        const qreal inverseScale = qreal(1) / devicePixelRatio;
+        p->scale(inverseScale, inverseScale);
+        x = qRound(devicePixelRatio * x);
+        y = qRound(devicePixelRatio * y);
+        w = qRound(devicePixelRatio * w);
+        h = qRound(devicePixelRatio * h);
+        lineWidth = qRound(devicePixelRatio * lineWidth);
+    }
+
     QColor shade = pal.dark().color();
     QColor light = pal.light().color();
     if (fill) {
@@ -389,6 +447,19 @@ static void qDrawWinShades(QPainter *p,
 {
     if (w < 2 || h < 2)                        // can't do anything with that
         return;
+
+    PainterStateGuard painterGuard(p);
+    const qreal devicePixelRatio = p->device()->devicePixelRatioF();
+    if (!qFuzzyCompare(devicePixelRatio, qreal(1))) {
+        painterGuard.save();
+        const qreal inverseScale = qreal(1) / devicePixelRatio;
+        p->scale(inverseScale, inverseScale);
+        x = qRound(devicePixelRatio * x);
+        y = qRound(devicePixelRatio * y);
+        w = qRound(devicePixelRatio * w);
+        h = qRound(devicePixelRatio * h);
+    }
+
     QPen oldPen = p->pen();
     QPoint a[3] = { QPoint(x, y+h-2), QPoint(x, y), QPoint(x+w-2, y) };
     p->setPen(c1);
@@ -518,6 +589,20 @@ void qDrawPlainRect(QPainter *p, int x, int y, int w, int h, const QColor &c,
     if (Q_UNLIKELY(w < 0 || h < 0 || lineWidth < 0)) {
         qWarning("qDrawPlainRect: Invalid parameters");
     }
+
+    PainterStateGuard painterGuard(p);
+    const qreal devicePixelRatio = p->device()->devicePixelRatioF();
+    if (!qFuzzyCompare(devicePixelRatio, qreal(1))) {
+        painterGuard.save();
+        const qreal inverseScale = qreal(1) / devicePixelRatio;
+        p->scale(inverseScale, inverseScale);
+        x = qRound(devicePixelRatio * x);
+        y = qRound(devicePixelRatio * y);
+        w = qRound(devicePixelRatio * w);
+        h = qRound(devicePixelRatio * h);
+        lineWidth = qRound(devicePixelRatio * lineWidth);
+    }
+
     QPen   oldPen   = p->pen();
     QBrush oldBrush = p->brush();
     p->setPen(c);