helper.cpp Example File
opengl/2dpainting/helper.cpp

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 #include <QtGui>
 #include "helper.h"

 Helper::Helper()
 {
     QLinearGradient gradient(QPointF(50, -20), QPointF(80, 20));
     gradient.setColorAt(0.0, Qt::white);
     gradient.setColorAt(1.0, QColor(0xa6, 0xce, 0x39));

     background = QBrush(QColor(64, 32, 64));
     circleBrush = QBrush(gradient);
     circlePen = QPen(Qt::black);
     circlePen.setWidth(1);
     textPen = QPen(Qt::white);
     textFont.setPixelSize(50);
 }

 void Helper::paint(QPainter *painter, QPaintEvent *event, int elapsed)
 {
     painter->fillRect(event->rect(), background);
     painter->translate(100, 100);

     painter->save();
     painter->setBrush(circleBrush);
     painter->setPen(circlePen);
     painter->rotate(elapsed * 0.030);

     qreal r = elapsed/1000.0;
     int n = 30;
     for (int i = 0; i < n; ++i) {
         painter->rotate(30);
         qreal radius = 0 + 120.0*((i+r)/n);
         qreal circleRadius = 1 + ((i+r)/n)*20;
         painter->drawEllipse(QRectF(radius, -circleRadius,
                                     circleRadius*2, circleRadius*2));
     }
     painter->restore();

     painter->setPen(textPen);
     painter->setFont(textFont);
     painter->drawText(QRect(-50, -50, 100, 100), Qt::AlignCenter, "Qt");
 }