How can I implement a style sheet rounded border in a paintEvent?

[l3u_]

Hi all,

I'm working on a custom QPushButton. Currently, I set a style sheet:

setStyleSheet(QStringLiteral("QPushButton { border: 1px solid palette(highlight); "
                                           "border-radius: 4px; }"));

which makes the button appear e.g. like so:

Now, I want to do this exactly in a custom paintEvent. I tried the following:

painter.setPen(QPen(palette().color(QPalette::Highlight), 1));
painter.drawRoundedRect(rect().adjusted(0, 0, -1, -1), 4, 4);

which gives me

The corners are not rounded in the same way, and they are not smooth like in the style sheet version. So I tried to enable anti-aliasing:

painter.setRenderHint(QPainter::Antialiasing, true);

which then causes the button to be rendered like this:

still very different from the style sheet version.

So: How can I get the exact same result like when setting a style sheet?

Thanks for all help in advance!

[l3u_]

It actually can be achieved way easier. Shamelessly stolen from Breeze's drawFocusFrame method and a lot simplified:

void PopupButton::paintEvent(QPaintEvent *)
{
    static const int margin = 1;
    static const int radius = 5;

    QPainterPath painterPath;
    painterPath.setFillRule(Qt::OddEvenFill);
    painterPath.addRoundedRect(rect().adjusted(margin, margin, -margin, -margin), radius, radius);
    painterPath.addRoundedRect(rect(), radius + margin, radius + margin);

    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.fillPath(painterPath, palette().color(QPalette::Highlight));
}

Result:

Of course lacks the text, hover indicator and all that – but the border looks fine.

[SGaist]

Hi,

The most simple would be to read the style sheet's QStyle implementation in Qt's source code.

[l3u_]

I tried to do so, but it doesn't seem to be too easy to figure out what all happens under the hood :-(

[Bonnie]

@l3u_ It is in QRenderRule::drawBorder of Src/qtbase/src/widgets/styles/qstylesheetstyle.cpp and qDrawBorder of Src/qtbase/src/gui/painting/qcssutil.cpp.
From the qDrawBorder source you can see that the edges and corners are drawn separately.
An easier way is to call this function directly, but this need to add the private header module to your project.
If you are using qmake, just add QT += gui-private to pro file. If using cmake you can google :).
Then in the code

#include <private/qcssutil_p.h>

void Form::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing, true);
    const QVector<QCss::BorderStyle> styles(4, QCss::BorderStyle_Solid);
    const QVector<int> borders(4, 1);
    const QVector<QBrush> colors(4, palette().color(QPalette::Highlight));
    const QVector<QSize> radii(4, {4, 4});
    qDrawBorder(&painter, rect().adjusted(0, 0, -1, -1), styles.data(), borders.data(), colors.data(), radii.data());
}

[l3u_]

I don't really want to include private headers … everything will break once they change something about this. This is really hard to get right.

I'm just wondering why the drawRoundedRect result looks so odd in comparison …

[Christian Ehrlicher]

Simply look in the code of this function. It's open source...

[Bonnie]

@l3u_ Then you already know where the source code is. You can check by yourself.

P.S. I won't worry much about the private header.
First, the APIs in this header file haven't been changed from Qt5.0 till current Qt6.9.
Second, even the public headers have also been changed a lot, so what if the private header changes .

[l3u_]

Yeah, of course. I only somehow would have expected to simply get the same result by using QPainer::drawRoundedRect …

However thanks a lot for showing me the location of the code that does the work!

[l3u_]

It actually can be achieved way easier. Shamelessly stolen from Breeze's drawFocusFrame method and a lot simplified:

void PopupButton::paintEvent(QPaintEvent *)
{
    static const int margin = 1;
    static const int radius = 5;

    QPainterPath painterPath;
    painterPath.setFillRule(Qt::OddEvenFill);
    painterPath.addRoundedRect(rect().adjusted(margin, margin, -margin, -margin), radius, radius);
    painterPath.addRoundedRect(rect(), radius + margin, radius + margin);

    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.fillPath(painterPath, palette().color(QPalette::Highlight));
}

Result:

Of course lacks the text, hover indicator and all that – but the border looks fine.