#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); //去長和寬的小值
 QRect rect((W -side)/2,(H-side)/2,side,side); //viewport矩形區(qū)
 painter.drawRect(rect); //Viewport大小
 // painter.setViewport((W-side)/2, (H-side)/2,side,side);//設(shè)置Viewport，物理坐標范圍
 painter.setViewport(rect); //設(shè)置Viewport
 painter.setWindow(-100,-100,200,200); // 設(shè)置窗口大小，邏輯坐標
 
 painter.setRenderHint(QPainter::Antialiasing); //設(shè)置反走樣
 
 //設(shè)置畫筆
 QPen pen;
 pen.setWidth(1); //線寬
 pen.setColor(Qt::red); //畫線顏色
  //Qt::NoPen,Qt::SolidLine, Qt::DashLine, Qt::DotLine,Qt::DashDotLine,Qt::DashDotDotLine,Qt::CustomDashLine
 pen.setStyle(Qt::SolidLine); //線的類型
 //Qt::FlatCap, Qt::SquareCap,Qt::RoundCap
 pen.setCapStyle(Qt::FlatCap);//線端點樣式
 //Qt::MiterJoin,Qt::BevelJoin,Qt::RoundJoin,Qt::SvgMiterJoin
 pen.setJoinStyle(Qt::BevelJoin);//線的連接點樣式
 painter.setPen(pen);
 //線性漸變
 
 QLinearGradient linearGrad(0,0,100,0);//從左到右，
 linearGrad.setColorAt(0,Qt::yellow); //起點顏色
 linearGrad.setColorAt(1,Qt::green); //終點顏色
 linearGrad.setSpread(QGradient::PadSpread);//展布模式
 painter.setBrush(linearGrad);
 //設(shè)置復合
 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;
}