Draw text around QPainterPath efficiently

Question

The following code draws text around a QPainterPath:

QString text = "Short text";
QPainterPath rawPath;
// fill path ...

while (...)
{
    double percent = ...;
    double angle = rawPath.angleAtPercent(percent);
    QPointF point = rawPath.pointAtPercent(percent);

    m_painter.save();

    m_painter.translate(point);
    m_painter.rotate(-angle);

    // Version 1:
    //m_painter.drawText(0, 0, text.at(currentChar));

    // Version 2:
    QPainterPath p2;
    p2.addText(0, 0, font, text.at(currentChar));
    m_painter.drawPath(p2);

    m_painter.restore();
}

The graphical result is as expected but the performance of both, Version 1 and Version 2 is very poor. The bottleneck is the QPainter::drawText() respectively the QPainterPath::addText() method. Is there a more efficient way to draw text around a path?

Regards,

Answer 1

There is no built-in method to draw a text following a path.

But, if QPainter is not efficient enough, you can gain some perfs by building a new path with your text and by drawing after your loop (you will not use QPainter::save() and QPainter::restore() in a loop):

void paintEvent(QPaintEvent* event)
    {
        QString text = "Short text";
        text = text.repeated(1000); // Check with a very long text
        QPainterPath rawPath;
        rawPath.addEllipse(QRect(0, 0, 200, 200));

        QPainterPath path;

        double const step = 1.0 / double(text.length());
        double percent = 0.0;
        for(QChar const& c: text)
        {
            double angle = rawPath.angleAtPercent(percent);
            QPointF point = rawPath.pointAtPercent(percent);

            QTransform mat;
            mat.rotate(-angle);
            QPainterPath p2;
            p2.addText(0, 0, font(), c);
            p2 = mat.map(p2);
            path.addPath(p2.translated(point));
            percent += step;
        }

        QPainter painter(this);
        painter.drawPath(path.translated(100, 100));
    }