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;
}