Qt在QLabel显示图片并绘制矩形框，顶点可修改并且能够拖动

可以使用Qt的QPainter和QGraphicsView来实现这个功能。

首先，创建一个继承自QGraphicsView的类来显示图片和矩形框：

class ImageView : public QGraphicsView
{
public:
    ImageView(QWidget* parent = nullptr);

    void setImage(const QPixmap& image);
    void setRect(const QRectF& rect);

protected:
    void mouseMoveEvent(QMouseEvent* event) override;
    void mousePressEvent(QMouseEvent* event) override;
    void mouseReleaseEvent(QMouseEvent* event) override;
    void wheelEvent(QWheelEvent* event) override;
    void drawBackground(QPainter* painter, const QRectF& rect) override;

private:
    QGraphicsPixmapItem* m_pixmapItem;
    QGraphicsRectItem* m_rectItem;
    QPointF m_lastPos;
};

在构造函数中，我们创建了一个QGraphicsPixmapItem用于显示图片，和一个QGraphicsRectItem用于显示矩形框。在setImage和setRect函数中，我们分别设置了图片和矩形框的位置和大小。

在mouseMoveEvent、mousePressEvent和mouseReleaseEvent函数中，我们实现了矩形框顶点的拖动功能。在wheelEvent函数中，我们实现了缩放图片的功能。在drawBackground函数中，我们绘制了网格线以方便观察。

下面是实现代码：

ImageView::ImageView(QWidget* parent)
    : QGraphicsView(parent)
{
    setDragMode(QGraphicsView::RubberBandDrag);
    setRenderHint(QPainter::Antialiasing);
    setRenderHint(QPainter::SmoothPixmapTransform);

    m_pixmapItem = new QGraphicsPixmapItem;
    m_pixmapItem->setZValue(-1);
    scene()->addItem(m_pixmapItem);

    m_rectItem = new QGraphicsRectItem;
    m_rectItem->setPen(QPen(Qt::red, 2));
    m_rectItem->setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemSendsGeometryChanges);
    scene()->addItem(m_rectItem);
}

void ImageView::setImage(const QPixmap& image)
{
    m_pixmapItem->setPixmap(image);
    setSceneRect(image.rect());
}

void ImageView::setRect(const QRectF& rect)
{
    m_rectItem->setRect(rect);
}

void ImageView::mouseMoveEvent(QMouseEvent* event)
{
    QPointF pos = mapToScene(event->pos());
    QPointF delta = pos - m_lastPos;
    m_lastPos = pos;
    if (m_rectItem->isSelected()) {
        QRectF rect = m_rectItem->rect();
        if (m_rectItem->cornerAt(pos) == QGraphicsItem::TopLeftCorner) {
            rect.translate(delta);
            rect.setWidth(rect.width() - delta.x());
            rect.setHeight(rect.height() - delta.y());
        } else if (m_rectItem->cornerAt(pos) == QGraphicsItem::TopRightCorner) {
            rect.translate(QPointF(0, delta.y()));
            rect.setWidth(rect.width() + delta.x());
            rect.setHeight(rect.height() - delta.y());
        } else if (m_rectItem->cornerAt(pos) == QGraphicsItem::BottomLeftCorner) {
            rect.translate(QPointF(delta.x(), 0));
            rect.setWidth(rect.width() - delta.x());
            rect.setHeight(rect.height() + delta.y());
        } else if (m_rectItem->cornerAt(pos) == QGraphicsItem::BottomRightCorner) {
            rect.setWidth(rect.width() + delta.x());
            rect.setHeight(rect.height() + delta.y());
        } else if (m_rectItem->isUnderMouse()) {
            rect.translate(delta);
        }
        m_rectItem->setRect(rect);
    } else {
        QGraphicsView::mouseMoveEvent(event);
    }
}

void ImageView::mousePressEvent(QMouseEvent* event)
{
    m_lastPos = mapToScene(event->pos());
    QGraphicsView::mousePressEvent(event);
}

void ImageView::mouseReleaseEvent(QMouseEvent* event)
{
    QGraphicsView::mouseReleaseEvent(event);
}

void ImageView::wheelEvent(QWheelEvent* event)
{
    double factor = std::pow(1.2, event->delta() / 240.0);
    scale(factor, factor);
}

void ImageView::drawBackground(QPainter* painter, const QRectF& rect)
{
    QGraphicsView::drawBackground(painter, rect);

    painter->setPen(Qt::lightGray);
    for (int x = int(rect.left()) - (int(rect.left()) % 10); x < rect.right(); x += 10) {
        painter->drawLine(x, rect.top(), x, rect.bottom());
    }
    for (int y = int(rect.top()) - (int(rect.top()) % 10); y < rect.bottom(); y += 10) {
        painter->drawLine(rect.left(), y, rect.right(), y);
    }
}

在主窗口中，我们创建一个ImageView对象，将其添加到布局中，并设置图片和矩形框：

MainWindow::MainWindow(QWidget* parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QPixmap image(":/image.png");
    ui->imageView->setImage(image);
    ui->imageView->setRect(QRectF(100, 100, 200, 200));
}

这样就完成了在QLabel中显示图片并绘制可拖动的矩形框的功能。