c++ - Qt 覆盖 QLabel PaintEvent

Question

过去几天我一直在为这个问题苦苦挣扎。当用户调整窗口大小时，我希望能够扩大和缩小 QLabel 中分配的任何 Pixmap。问题是保持纵横比和图像质量。这里的另一位用户建议我重新实现标签的绘画事件 - 但我仍然很迷茫。我什至不确定我是否正确覆盖了 paintEvent。我会在这里寻找一些示例代码。

这是我所在的地方:

void MyLabel::paintEvent(QPaintEvent * event)
{
    //if this widget is assigned a pixmap
    //paint that pixmap at the size of the parent, aspect ratio preserved
    //otherwise, nothing
}

Answer 1

这是 QLabel 子类的一个可能实现，它在保持纵横比不变的同时缩放其像素图内容。它是基于QLabel::paintEvent的方式实现的 is implemented .

如果您在布局中使用它，您还可以将其大小策略设置为 QSizePolicy::Expanding , 以便 QLabel 占用布局中的额外空间，以更大地显示像素图内容。

#include <QApplication>
#include <QtWidgets>

class PixmapLabel : public QLabel{
public:
    explicit PixmapLabel(QWidget* parent=nullptr):QLabel(parent){
        //By default, this class scales the pixmap according to the label's size
        setScaledContents(true);
    }
    ~PixmapLabel(){}

protected:
    void paintEvent(QPaintEvent* event);
private:
    //QImage to cache the pixmap()
    //to avoid constructing a new QImage on every scale operation
    QImage cachedImage;
    //used to cache the last scaled pixmap
    //to avoid calling scale again when the size is still at the same
    QPixmap scaledPixmap;
    //key for the currently cached QImage and QPixmap
    //used to make sure the label was not set to another QPixmap
    qint64 cacheKey{0};
};

//based on the implementation of QLabel::paintEvent
void PixmapLabel::paintEvent(QPaintEvent *event){
    //if this is assigned to a pixmap
    if(pixmap() && !pixmap()->isNull()){
        QStyle* style= PixmapLabel::style();
        QPainter painter(this);
        drawFrame(&painter);
        QRect cr = contentsRect();
        cr.adjust(margin(), margin(), -margin(), -margin());
        int align= QStyle::visualAlignment(layoutDirection(), alignment());
        QPixmap pix;
        if(hasScaledContents()){ //if scaling is enabled
            QSize scaledSize= cr.size() * devicePixelRatioF();
            //if scaledPixmap is invalid
            if(scaledPixmap.isNull() || scaledPixmap.size()!=scaledSize
                    || pixmap()->cacheKey()!=cacheKey){
                //if cachedImage is also invalid
                if(pixmap()->cacheKey() != cacheKey){
                    //reconstruct cachedImage
                    cachedImage= pixmap()->toImage();
                }
                QImage scaledImage= cachedImage.scaled(
                            scaledSize, Qt::KeepAspectRatio, Qt::SmoothTransformation);
                scaledPixmap= QPixmap::fromImage(scaledImage);
                scaledPixmap.setDevicePixelRatio(devicePixelRatioF());
            }
            pix= scaledPixmap;
        } else { // no scaling, Just use pixmap()
            pix= *pixmap();
        }
        QStyleOption opt;
        opt.initFrom(this);
        if(!isEnabled())
            pix= style->generatedIconPixmap(QIcon::Disabled, pix, &opt);
        style->drawItemPixmap(&painter, cr, align, pix);
    } else { //otherwise (if the label is not assigned to a pixmap)
        //call base paintEvent
        QLabel::paintEvent(event);
    }
}

//DEMO program
QPixmap generatePixmap(QSize size) {
    QPixmap pixmap(size);
    pixmap.fill(Qt::white);
    QPainter p(&pixmap);
    p.setRenderHint(QPainter::Antialiasing);
    p.setPen(QPen(Qt::black, 10));
    p.drawEllipse(pixmap.rect());
    p.setPen(QPen(Qt::red, 2));
    p.drawLine(pixmap.rect().topLeft(), pixmap.rect().bottomRight());
    p.drawLine(pixmap.rect().topRight(), pixmap.rect().bottomLeft());
    return pixmap;
}


int main(int argc, char *argv[])

{
    QApplication a(argc, argv);


    QPixmap pixmap= generatePixmap(QSize(1280, 960));
    PixmapLabel label;
    label.setPixmap(pixmap);
    label.setAlignment(Qt::AlignCenter);
    label.setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    label.setMinimumSize(320, 240);
    label.show();

    return a.exec();
}

我认为这比 this answer 中的解决方案更好，因为这里的 QLabel 负责调整其像素图的大小。因此，每次调整父窗口小部件的大小时以及每次在其上设置新的像素图时都不需要手动调整它的大小。