Qt5.9实现简单复合图形

#ifndef WIDGET_H
#define WIDGET_H
#include <QWidget>
namespace Ui {
class Widget;
}
class Widget : public QWidget
{
 Q_OBJECT
protected:
 void paintEvent(QPaintEvent *event);
public:
 explicit Widget(QWidget *parent = nullptr);
 ~Widget();
private:
 Ui::Widget *ui;
};
#endif // WIDGET_H

#include "widget.h"
#include "ui_widget.h"
#include <QPainter>
void Widget::paintEvent(QPaintEvent *event)
{
 Q_UNUSED(event)
 QPainter painter(this);
 int W = width();
 int H = height();
 int side = qMin(W,H); //去长和宽的小值
 QRect rect((W -side)/2,(H-side)/2,side,side); //viewport矩形区
 painter.drawRect(rect); //Viewport大小
 // painter.setViewport((W-side)/2, (H-side)/2,side,side);//设置Viewport，物理坐标范围
 painter.setViewport(rect); //设置Viewport
 painter.setWindow(-100,-100,200,200); // 设置窗口大小，逻辑坐标
 painter.setRenderHint(QPainter::Antialiasing); //设置反走样
 //设置画笔
 QPen pen;
 pen.setWidth(1); //线宽
 pen.setColor(Qt::red); //画线颜色
  //Qt::NoPen,Qt::SolidLine, Qt::DashLine, Qt::DotLine,Qt::DashDotLine,Qt::DashDotDotLine,Qt::CustomDashLine
 pen.setStyle(Qt::SolidLine); //线的类型
 //Qt::FlatCap, Qt::SquareCap,Qt::RoundCap
 pen.setCapStyle(Qt::FlatCap);//线端点样式
 //Qt::MiterJoin,Qt::BevelJoin,Qt::RoundJoin,Qt::SvgMiterJoin
 pen.setJoinStyle(Qt::BevelJoin);//线的连接点样式
 painter.setPen(pen);
 //线性渐变
 QLinearGradient linearGrad(0,0,100,0);//从左到右，
 linearGrad.setColorAt(0,Qt::yellow); //起点颜色
 linearGrad.setColorAt(1,Qt::green); //终点颜色
 linearGrad.setSpread(QGradient::PadSpread);//展布模式
 painter.setBrush(linearGrad);
 //设置复合
 painter.setCompositionMode(QPainter::CompositionMode_Difference);
 for (int i = 0;i < 36;i++) {
  painter.drawEllipse(QPoint(50,0),50,50);
  painter.rotate(10);
 }
}
Widget::Widget(QWidget *parent) :
 QWidget(parent),
 ui(new Ui::Widget)
{
 ui->setupUi(this);
 setPalette(QPalette(Qt::white));
 setAutoFillBackground(true);
 this->resize(300,300);
}
Widget::~Widget()
{
 delete ui;
}