#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

protected:
    void paintEvent(QPaintEvent *event)override;

private:
    enum class AspectRatioMode
    {
        zoomWithWindow = 0,//随窗口缩放
        keepAspectRatio,//保持原图
        scaleAspectFit, //保持纵横比并缩放
        scaleAspectFill //保持纵横比并填充
    };
    AspectRatioMode mode{AspectRatioMode::scaleAspectFill};
    void drewBg(QPainter * painter,const QRect & rect,const QPixmap & pixmap);
    QPixmap bgImage;
};
#endif // WIDGET_H

#include "widget.h"
#include <QPainter>
#include <QPaintEvent>
#include <QDebug>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    bgImage.load(":/backgroundImage1.jpg");
}

Widget::~Widget()
{
}

void Widget::drewBg(QPainter * painter,const QRect & rect,const QPixmap & pixmap)
{
    if(mode == AspectRatioMode::zoomWithWindow)
    {
        painter->drawPixmap(rect,pixmap);
    }
    else if(mode == AspectRatioMode::keepAspectRatio)
    {
        painter->drawPixmap(rect.topLeft(),pixmap);
    }
    else if(mode == AspectRatioMode::scaleAspectFit)
    {
        auto tempPximap = pixmap.scaled(rect.size(),Qt::KeepAspectRatio);
        if(rect.width() == tempPximap.width())
        {
            painter->drawPixmap(QPoint(0,(rect.height() - tempPximap.height()) / 2),tempPximap);
        }
        else
        {
            painter->drawPixmap(QPoint((rect.width() - tempPximap.width())/2,0),tempPximap);
        }
    }
    else if(mode == AspectRatioMode::scaleAspectFill)
    {
        if(rect.width() > rect.height())
        {
            auto y2 = pixmap.height() * rect.width() / pixmap.width();
            QPixmap tempPximap = pixmap.scaled(rect.width(),y2,Qt::KeepAspectRatio);
            painter->drawPixmap(rect,tempPximap.copy(0,(y2 - rect.height()) / 2,rect.width(),rect.height()));
        }
        else
        {
            auto x2 = pixmap.width() * rect.height() / pixmap.height();
            QPixmap tempPximap = pixmap.scaled(x2,rect.height(),Qt::KeepAspectRatio);
            painter->drawPixmap(rect,tempPximap.copy((x2 - rect.width()) / 2,0,rect.width(),rect.height()));
        }
    }
}

void Widget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing,true);  //反走样开启
    const auto rect = event->rect();

    painter.fillRect(rect,Qt::cyan);
    drewBg(&painter,rect,bgImage);
}

1、随窗口释放

2、 保持原图

3、保持纵横比并缩放

4、保持纵横比并填充

示意图：

效果图：