Development/Tutorials/Graphics/HiDPI

Introduction

HiDPI devices are very common today. Users need to scale up UI to make them looks like normal size. Common scale factors are: 1.25, 1.5, 1.75, 2, 3. This means UI dimensions are not integers anymore. Qt provides classes and functions that accept and return qreal instead of int. They usually end with F.

• QRect --> QRectF
• QPoint --> QPointF
• QWidget::devicePixelRatio() --> QWidget::devicePixelRatioF()

KDE and Qt applications may render blur icons and graphics. This guide shows how to make everything sharp and clear in HiDPI devices. Both Qt 5 Widgets and Qt 5 Quick applications are supported.

Migrate to Qt 5 and KF5

Qt 4 doesn't support HiDPI rendering. Please migrate your application to Qt 5 and KF5.

Texts

You don't need to do anything special. Text rendering should support HiDPI out of box.

Icons

In your application's main function, add the following lines at the very beginning:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    app.setAttribute(Qt::AA_UseHighDpiPixmaps, true);
    ...
}

Then all icons in your application should look sharp.

QPixmap

Change all QPixmap from

pixmap = QPixmap( width, height );

to

pixmap = QPixmap( width * dpr, height * dpr );
pixmap.setDevicePixelRatio(dpr);

So where is the dpr variable from? You can get it from any QWidget objects with function devicePixelRatioF(). F means the function return a float number (qreal). If you are not in a QWidget class context, you can get the value via:

const qreal dpr = qApp->devicePixelRatio();

And don't forget to change width and height from int to qreal.

QPainter

QPainter has various drawing functions. You need to change parameters of those functions from integer types:

• QRect
• QPolygon
• QPoint
• int

to floating types:

• QRectF
• QPolygonF
• QPointF
• qreal

For example, you need to change this code:

QPainter p;
p->drawText(0, 0, 12, 50, Qt::AlignLeft | Qt::AlignTop, text);

to:

QPainter p;
p->drawText(QRectF(0, 0, 12, 50), Qt::AlignLeft | Qt::AlignTop, text);

Similarly, You should set float number width for QPen. Please change:

QPen pen(Qt::green, 3);

to:

QPen pen(Qt::green, qreal(3));

After all these changes, your QPainter object should be able to render HiDPI graphics.

Qt Quick Controls 1

If the QML contains Qt Quick Controls 1, all text components will be blur and small in HiDPI display. This is likely a Qt bug.

To solve this, we suggest you to migrate to Qt Quick Controls 2.

Dual-screen setup

If you have dual screen setup with 1080p and 4K, set scale factor to 1.5, and put them side by side, running Qt Widgets applications in such setup will cause wrong rendering. This is likely a Qt bug. We cannot do anything.

Examples

• KSysGuard patch #1 (Icons)
• Font Manager patch (QPainer, QPixmap, QImage)
• Filelight patch (QPainer)