Improve widget painting under dpr>1 by enabling smooth pixmap scaling

Smooth scaling of icons etc. give far better visual results,
particularly with fractional dpr scaling. So enable this generally in
QStylePainter, and make more of the relevant widgets use QStylePainter
instead of QPainter directly.

Differences can be seen in the widgets examples, e.g. textedit,
gallery, stylesheet (Pagefold), mdi.

Pick-to: 6.4 6.3 6.2
Task-number: QTBUG-96223
Fixes: QTBUG-101058
Change-Id: I3c34a455d097e5f6a6a09d3b020528b4fbda4d85
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

7 files changed, 25 insertions, 20 deletions

diff --git a/src/widgets/dialogs/qwizard.cpp b/src/widgets/dialogs/qwizard.cpp
index 79a83da33a..5b95b62772 100644
--- a/src/widgets/dialogs/qwizard.cpp
+++ b/src/widgets/dialogs/qwizard.cpp
@@ -18,7 +18,7 @@
 #include "qlineedit.h"
 #endif
 #include <qpointer.h>
-#include "qpainter.h"
+#include "qstylepainter.h"
 #include "qwindow.h"
 #include "qpushbutton.h"
 #include "qset.h"
@@ -386,7 +386,7 @@ void QWizardHeader::setup(const QWizardLayoutInfo &info, const QString &title,
 
 void QWizardHeader::paintEvent(QPaintEvent * /* event */)
 {
-    QPainter painter(this);
+    QStylePainter painter(this);
     painter.drawPixmap(0, 0, bannerPixmap);
 
     int x = width() - 2;
@@ -3231,7 +3231,7 @@ void QWizard::paintEvent(QPaintEvent * event)
         if (backgroundPixmap.isNull())
             return;
 
-        QPainter painter(this);
+        QStylePainter painter(this);
         painter.drawPixmap(0, (height() - backgroundPixmap.height()) / 2, backgroundPixmap);
     }
 #if QT_CONFIG(style_windowsvista)
diff --git a/src/widgets/itemviews/qlistview.cpp b/src/widgets/itemviews/qlistview.cpp
index 7f8f7a20fe..30bf327af7 100644
--- a/src/widgets/itemviews/qlistview.cpp
+++ b/src/widgets/itemviews/qlistview.cpp
@@ -9,7 +9,7 @@
 #include <qaccessible.h>
 #endif
 #include <qapplication.h>
-#include <qpainter.h>
+#include <qstylepainter.h>
 #include <qbitmap.h>
 #include <qdebug.h>
 #if QT_CONFIG(draganddrop)
@@ -988,7 +988,7 @@ void QListView::paintEvent(QPaintEvent *e)
         return;
     QStyleOptionViewItem option;
     initViewItemOption(&option);
-    QPainter painter(d->viewport);
+    QStylePainter painter(d->viewport);
 
     const QList<QModelIndex> toBeRendered =
             d->intersectingSet(e->rect().translated(horizontalOffset(), verticalOffset()), false);
@@ -1059,7 +1059,7 @@ void QListView::paintEvent(QPaintEvent *e)
             // is provided by the delegate
             QStyle::State oldState = option.state;
             option.state &= ~QStyle::State_Selected;
-            style()->drawPrimitive(QStyle::PE_PanelItemViewRow, &option, &painter, this);
+            painter.drawPrimitive(QStyle::PE_PanelItemViewRow, option);
             option.state = oldState;
 
             alternateBase = !alternateBase;
@@ -1083,7 +1083,7 @@ void QListView::paintEvent(QPaintEvent *e)
         opt.rect = d->mapToViewport(d->elasticBand, false).intersected(
             d->viewport->rect().adjusted(-16, -16, 16, 16));
         painter.save();
-        style()->drawControl(QStyle::CE_RubberBand, &opt, &painter);
+        painter.drawControl(QStyle::CE_RubberBand, opt);
         painter.restore();
     }
 #endif
diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index f0da19f9c8..ab129b73e7 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -5529,6 +5529,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                     && !q->testAttribute(Qt::WA_OpaquePaintEvent) && !q->testAttribute(Qt::WA_NoSystemBackground)) {
                     beginBackingStorePainting();
                     QPainter p(q);
+                    p.setRenderHint(QPainter::SmoothPixmapTransform);
                     paintBackground(&p, toBePainted, (asRoot || onScreen) ? (flags | DrawAsRoot) : DrawWidgetFlags());
                     endBackingStorePainting();
                 }
diff --git a/src/widgets/styles/qstylepainter.h b/src/widgets/styles/qstylepainter.h
index 8243ff5f39..65664b776e 100644
--- a/src/widgets/styles/qstylepainter.h
+++ b/src/widgets/styles/qstylepainter.h
@@ -23,7 +23,9 @@ public:
         Q_ASSERT_X(w, "QStylePainter::QStylePainter", "Widget must be non-zero");
         widget = w;
         wstyle = w->style();
-        return QPainter::begin(pd);
+        const bool res = QPainter::begin(pd);
+        setRenderHint(QPainter::SmoothPixmapTransform);
+        return res;
     };
     inline void drawPrimitive(QStyle::PrimitiveElement pe, const QStyleOption &opt);
     inline void drawControl(QStyle::ControlElement ce, const QStyleOption &opt);
diff --git a/src/widgets/widgets/qlabel.cpp b/src/widgets/widgets/qlabel.cpp
index 3e06360920..4d1c4ebb74 100644
--- a/src/widgets/widgets/qlabel.cpp
+++ b/src/widgets/widgets/qlabel.cpp
@@ -1091,8 +1091,10 @@ void QLabel::paintEvent(QPaintEvent *)
 #endif
     if (d->pixmap && !d->pixmap->isNull()) {
         QPixmap pix;
-        if (d->scaledcontents) {
-            QSize scaledSize = cr.size() * devicePixelRatio();
+        const qreal dpr = devicePixelRatio();
+        if (d->scaledcontents || dpr != d->pixmap->devicePixelRatio()) {
+            QSize scaledSize = d->scaledcontents ? (cr.size() * dpr)
+                               : (d->pixmap->size() * (dpr / d->pixmap->devicePixelRatio()));
             if (!d->scaledpixmap || d->scaledpixmap->size() != scaledSize) {
                 if (!d->cachedimage)
                     d->cachedimage = d->pixmap->toImage();
@@ -1101,7 +1103,7 @@ void QLabel::paintEvent(QPaintEvent *)
                     d->cachedimage->scaled(scaledSize,
                                            Qt::IgnoreAspectRatio, Qt::SmoothTransformation);
                 d->scaledpixmap = QPixmap::fromImage(std::move(scaledImage));
-                d->scaledpixmap->setDevicePixelRatio(devicePixelRatio());
+                d->scaledpixmap->setDevicePixelRatio(dpr);
             }
             pix = *d->scaledpixmap;
         } else
diff --git a/src/widgets/widgets/qmenu.cpp b/src/widgets/widgets/qmenu.cpp
index 0c493b3f67..6e4c39e03f 100644
--- a/src/widgets/widgets/qmenu.cpp
+++ b/src/widgets/widgets/qmenu.cpp
@@ -12,7 +12,7 @@
 #include "qevent.h"
 #include "qtimer.h"
 #include "qlayout.h"
-#include "qpainter.h"
+#include "qstylepainter.h"
 #include <qpa/qplatformtheme.h>
 #include "qapplication.h"
 #if QT_CONFIG(accessibility)
@@ -2695,7 +2695,7 @@ void QMenu::paintEvent(QPaintEvent *e)
 {
     Q_D(QMenu);
     d->updateActionRects();
-    QPainter p(this);
+    QStylePainter p(this);
     QRegion emptyArea = QRegion(rect());
 
     QStyleOptionMenuItem menuOpt;
@@ -2704,7 +2704,7 @@ void QMenu::paintEvent(QPaintEvent *e)
     menuOpt.checkType = QStyleOptionMenuItem::NotCheckable;
     menuOpt.maxIconWidth = 0;
     menuOpt.reservedShortcutWidth = 0;
-    style()->drawPrimitive(QStyle::PE_PanelMenu, &menuOpt, &p, this);
+    p.drawPrimitive(QStyle::PE_PanelMenu, menuOpt);
 
     //calculate the scroll up / down rect
     const int fw = style()->pixelMetric(QStyle::PM_MenuPanelWidth, nullptr, this);
@@ -2772,7 +2772,7 @@ void QMenu::paintEvent(QPaintEvent *e)
         QStyleOptionMenuItem opt;
         initStyleOption(&opt, action);
         opt.rect = actionRect;
-        style()->drawControl(QStyle::CE_MenuItem, &opt, &p, this);
+        p.drawControl(QStyle::CE_MenuItem, opt);
     }
 
     emptyArea -= QRegion(scrollUpTearOffRect);
@@ -2806,7 +2806,7 @@ void QMenu::paintEvent(QPaintEvent *e)
         frame.state = QStyle::State_None;
         frame.lineWidth = style()->pixelMetric(QStyle::PM_MenuPanelWidth, &frame);
         frame.midLineWidth = 0;
-        style()->drawPrimitive(QStyle::PE_FrameMenu, &frame, &p, this);
+        p.drawPrimitive(QStyle::PE_FrameMenu, frame);
     }
 
     //finally the rest of the spaces
@@ -2816,7 +2816,7 @@ void QMenu::paintEvent(QPaintEvent *e)
     menuOpt.checkType = QStyleOptionMenuItem::NotCheckable;
     menuOpt.rect = rect();
     menuOpt.menuRect = rect();
-    style()->drawControl(QStyle::CE_MenuEmptyArea, &menuOpt, &p, this);
+    p.drawControl(QStyle::CE_MenuEmptyArea, menuOpt);
 }
 
 #if QT_CONFIG(wheelevent)
diff --git a/src/widgets/widgets/qsizegrip.cpp b/src/widgets/widgets/qsizegrip.cpp
index 0e6ac5332a..8ffcb0081b 100644
--- a/src/widgets/widgets/qsizegrip.cpp
+++ b/src/widgets/widgets/qsizegrip.cpp
@@ -5,7 +5,7 @@
 
 #include "qapplication.h"
 #include "qevent.h"
-#include "qpainter.h"
+#include "qstylepainter.h"
 #include "qwindow.h"
 #include <qpa/qplatformwindow.h>
 #include "qstyle.h"
@@ -212,11 +212,11 @@ void QSizeGrip::paintEvent(QPaintEvent *event)
 {
     Q_UNUSED(event);
     Q_D(QSizeGrip);
-    QPainter painter(this);
+    QStylePainter painter(this);
     QStyleOptionSizeGrip opt;
     opt.initFrom(this);
     opt.corner = d->m_corner;
-    style()->drawControl(QStyle::CE_SizeGrip, &opt, &painter, this);
+    painter.drawControl(QStyle::CE_SizeGrip, opt);
 }
 
 /*!