如何使图像大小缩放到Qt？ [英] How do I make an image resize to scale in Qt?

问题描述

我创建了一个名为ImageLabel的类来扩展QLabel。我想要它保持它正在显示的图像的大小比，无论它是多么伸展。它工作正常，当你使窗口更大。问题出现在当你试图使窗口变小时：它不调整高度，它使它伸展。如何解决这个问题？

I made a class called ImageLabel which extends QLabel. I want it to keep the size ratio of the image that it is displaying no matter how stretched out it is. It works fine when you make the window larger. The problem comes in when you try to make the window smaller: it doesnt resize the height, it leaves it stretched out. How do I fix this?

int ImageLabel::heightForWidth(int width) const {
    int height = (this->size.height()*width)/this->size.width();
    return height;
}

QSize ImageLabel::sizeHint() const {
    return this->size;
}

QSize ImageLabel::minimumSizeHint() const {
    return QSize(0, 0);
}

void ImageLabel::setSizePolicy(){
    QSizePolicy policy = QSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    policy.setHeightForWidth(true);
    QLabel::setSizePolicy(policy);
    QLabel::setScaledContents(true);
}

void ImageLabel::setPixmap ( const QPixmap &pixmap ){
    this->size = pixmap.size();
    QLabel::setPixmap(pixmap);
}

int main(int argc, char *argv[]){
    QApplication a(argc, argv);

    QFrame *frame = new QFrame;
    QVBoxLayout *layout = new QVBoxLayout;
    frame->setLayout(layout);

    QPixmap map;
    map.load("test.png");
    ImageLabel *label = new ImageLabel;
    label->setSizePolicy();
    label->setPixmap(map);
    layout->addWidget(label);
    frame->show();

    return a.exec();
}

推荐答案

一对夫妇的方法来做到这一点，但最重要的是我会建议也许不打架布局系统，试图暗示方面。正如你所看到的，你必须尝试并实现一些方法来帮助布局。

There are a couple ways to do this, but most of all I would recommend maybe not fighting against the layout system by trying to hint the aspect. As you can see, you are having to try and implement a number methods trying to help the layout.

我可以提供两个例子。它们都不使用布局...

I can offer two examples. Neither of them use layouts...

第一个使用一个子 QLabel 来显示图像，固定大小关闭resize事件：

The first uses a child QLabel to show the image, and drives its fixed size off of the resize event:

// imagelabel.h

class ImageLabel : public QWidget
{
    Q_OBJECT

public:
    explicit ImageLabel(QWidget *parent = 0);
    const QPixmap* pixmap() const;

public slots:
    void setPixmap(const QPixmap&);

protected:
    void resizeEvent(QResizeEvent *);

private slots:
    void resizeImage();

private:
    QLabel *label;
};

// imagelabel.cpp

ImageLabel::ImageLabel(QWidget *parent) :
    QWidget(parent)
{
    label = new QLabel(this);
    label->setScaledContents(true);
    label->setFixedSize(0,0);
}

void ImageLabel::resizeEvent(QResizeEvent *event) {
    QWidget::resizeEvent(event);
    resizeImage();
}

const QPixmap* ImageLabel::pixmap() const {
    return label->pixmap();
}

void ImageLabel::setPixmap (const QPixmap &pixmap){
    label->setPixmap(pixmap);
    resizeImage();
}

void ImageLabel::resizeImage() {
    QSize pixSize = label->pixmap()->size();
    pixSize.scale(size(), Qt::KeepAspectRatio);
    label->setFixedSize(pixSize);
}

第二个示例基于@Arnold_Spence给出的答案。它甚至更短，因为它不使用孩子QLabel。它只是在绘制事件中绘制像素图：

The second example is based off of the answer given by @Arnold_Spence. It is even shorter as it doesn't use a child QLabel. It just draws the pixmap in the paint event:

// imagelabel2.h

class ImageLabel2 : public QWidget
{
    Q_OBJECT

public:
    explicit ImageLabel2(QWidget *parent = 0);
    const QPixmap* pixmap() const;

public slots:
    void setPixmap(const QPixmap&);

protected:
    void paintEvent(QPaintEvent *);

private:
    QPixmap pix;
};

// imagelabel2.cpp

ImageLabel2::ImageLabel2(QWidget *parent) :
    QWidget(parent)
{
}

void ImageLabel2::paintEvent(QPaintEvent *event) {
    QWidget::paintEvent(event);

    if (pix.isNull())
        return;

    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);

    QSize pixSize = pix.size();
    pixSize.scale(event->rect().size(), Qt::KeepAspectRatio);

    QPixmap scaledPix = pix.scaled(pixSize,
                                   Qt::KeepAspectRatio,
                                   Qt::SmoothTransformation
                                   );

    painter.drawPixmap(QPoint(), scaledPix);

}

const QPixmap* ImageLabel2::pixmap() const {
    return &pix;
}

void ImageLabel2::setPixmap (const QPixmap &pixmap){
    pix = pixmap;
}

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