QPainter takes much more time to draw thicker curves?

CJha

Hi, I am drawing a sine wave (as a test) to check the time it takes to draw a curve using either QPainter::drawPolyline() or QPainter::drawPath(). In either case, if I set the pen width to more than 1, then the time it takes to draw the sine wave increases by more than 10 times.

Widget::Widget(QWidget* parent)
    : QWidget(parent)
{
    setAutoFillBackground(true);
    setAttribute(Qt::WA_NoSystemBackground, true);

    for(qsizetype ii = 0; ii < 1000000; ++ii)
        sineWave << (1.0 + sin(2 * M_PI * 3 * ii / 1000000));
}

void Widget::paintEvent(QPaintEvent* evt)
{
    QPainter painter(this);
    painter.fillRect(rect(), QColor(Qt::white));
    painter.setRenderHint(QPainter::Antialiasing);

    QPen pen;
    pen.setWidthF(2); // setting pen width
    painter.setPen(pen);

    QElapsedTimer timer;
    timer.start();

    qDebug() << "-> Paint Event\tPen Width:" << pen.widthF() << "\tWidget WxH:" << width() << "x" << height();

    QList<QPointF> pixPoints;
    for(qsizetype ii = 0; ii < sineWave.length(); ++ii) {
        double yPos = height() * (sineWave[ii] / 2.0);
        double xPos = width() * (ii / 1000000.0);
        pixPoints << QPointF(xPos, yPos);
    }
    qDebug() << "--> 1: Time [ms]:" << (timer.nsecsElapsed() / 1000000.0);
    painter.drawPolyline(pixPoints);
    qDebug() << "--> 2: Time [ms]:" << (timer.nsecsElapsed() / 1000000.0);
}

The results I get:

-> Paint Event	Pen Width: 2 	Widget WxH: 692 x 465
--> 1: Time [ms]: 101.754
--> 2: Time [ms]: 5768.86
-> Paint Event	Pen Width: 2 	Widget WxH: 722 x 494
--> 1: Time [ms]: 109.352
--> 2: Time [ms]: 5887.08

-> Paint Event	Pen Width: 1 	Widget WxH: 720 x 515
--> 1: Time [ms]: 114.286
--> 2: Time [ms]: 253.72
-> Paint Event	Pen Width: 1 	Widget WxH: 741 x 535
--> 1: Time [ms]: 105.882
--> 2: Time [ms]: 233.165

Is there any way I can reduce this time gap?