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::setAttribute(Qt::AA_UseHighDpiPixmaps, true);
    QApplication app(argc, argv);
    ...
}

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 (the disadvantage is that it may not work well in multi-display set up with different DPR):

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.

Note: you don't need to scale x, y, width, height values.

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. But we noticed that only some applications are affected, like Kate, Qt Creator, VirtualBox, Spectacle. Other applications look fine.

In Spectacle, we found the cause is that when we set widget position to (0, 0) and the size is too big, Qt/X11 will move it to a position like (-640, 0), (960, -640). The positive value doesn't affect, but the negative value brings weird offset. To solve it, we simply detect position and change negative value to 0. Note this can only be done in paintEvent() rather than moveEvent(). Check the patch here.

In Kate, the cause is KonsolePart widget embedded in project plugin. When calling a specific function, the rendering is messed. Change to another function solved the issue. Check the patch here.

We haven't find a common solution for those issues. But here might be different hacks we can do.

Examples

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