High DPI cause wrong scaling of pixmaps

Question

I'm using Qt 6.5 on Windows 10, the pixmaps on my application are scaled differently according to the current DPI.

A minimal reproducible exxample:

Windows scale 100%

scale 125%

scale 150%

class Label : public QLabel
{
    Q_OBJECT
public:
    QIcon icon;
    Label(QWidget* parent = nullptr) : QLabel(parent){}
    void paintEvent(QPaintEvent* event) override
    {
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing);
        painter.setRenderHint(QPainter::SmoothPixmapTransform);
        icon.paint(&painter, rect(), Qt::AlignCenter, QIcon::Normal, QIcon::On);
    }
};

inline QPixmap setRoundness(QPixmap pixmap, int roundness)
{
    QPainterPath path;
    qDebug() << pixmap.width() << pixmap.height();
    // Round the pixmap by 50%
    path.addRoundedRect(QRectF(0, 0, pixmap.width(), pixmap.height()), pixmap.width() / 2, pixmap.height() / 2);

    QPixmap roundedPixmap = QPixmap(pixmap.size());
    roundedPixmap.fill(Qt::transparent);
    QPainter painter(&roundedPixmap);
    painter.setRenderHint(QPainter::Antialiasing, true);
    painter.setRenderHint(QPainter::SmoothPixmapTransform);
    painter.fillPath(path, Qt::black);
    painter.setCompositionMode(QPainter::CompositionMode_SourceIn);
    painter.drawPixmap(0, 0, pixmap);

    pixmap.save("test_1.png");
    roundedPixmap.save("test_2.png");
    return roundedPixmap;
}

class MainWindow : public QMainWindow
{
   Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr) : QMainWindow(parent), ui(new Ui::MainWindowClass())
{
      ui->setupUi(this);
      setStyleSheet("#centralWidget { background-color: rgba(80, 80, 80, 80); }");

      QIcon icon = QIcon("C:/Users/Katia/test.png"); // test.png => https://i.imgur.com/cZfWHkW.png
      QPixmap roundedPixmap = setRoundness(icon.pixmap(QSize(55, 55)), 50);

      Label* label = new Label;
      label->icon = roundedPixmap;
      QLabel* label_2 = new QLabel;
      label_2->setPixmap(roundedPixmap);

      QWidget* widget = new QWidget;
      QHBoxLayout* hLayout = new QHBoxLayout(widget);
      hLayout->addWidget(label);
      hLayout->addWidget(label_2);
      hLayout->setContentsMargins(0, 0, 0, 0);
      hLayout->setSpacing(30);

      setCentralWidget(widget);
      setContentsMargins(32, 32, 32, 32);
      return;
   }
}

Its a problem in the QPixmap and not in QPaintEvent because when i save roundedPixmap.save("test_2.png"); this is what i get:

(DPI 150%)

(DPI100%)

How i could fix this dpi problem?

Answer 1

try this:

label_2->setPixmap(roundedPixmap.scaled(label2->size()));

Anyway I don't like to use QLabels to show images, because QLabel has properties you dont like: try myLabel.setEnabled(false)